Overpopulation is a strangely difficult topic to study and understand. It’s easy to find books, articles, and videos that are eager to explain it to you, but almost impossible to find one whose goal is to satisfy your curiosity. Everything written about overpopulation is a polemic. Those who believe overpopulation is a real and urgent danger write to close your mind to the siren song of blind optimists and greedy, short-sighted fools leading humanity off a cliff. Those who believe it’s a fake concern write to stop you from taking dangerous, unnecessary action to stop a disaster that was never really coming. All of them are attempting to decisively inoculate you against the opposing position and slam the door on the conversation.
The nature of overpopulation as a question already lends itself to overconfidence. It collects the complicated worlds of human societies, the global economy, and the biosphere into a set of simple variables — population, consumption, resource stocks and flows — that feel tractable with simple math and foster a sense of logical necessity. Adding a political dimension makes it that much worse. Every book or article or video on overpopulation casts the alternatives to its position as dangerously foolish or openly malicious. And if you believe the position you’re arguing against is both wrong and evil, it’s easy to misrepresent it. This is very much not a conversation that fosters persistent and open inquiry.
So it’s not surprising that for the past few decades, the overpopulation debate has been at a stalemate. At worst, it’s treated as a problem that has been decisively solved in favor of one faction or another. At best, it’s treated as a shallow question that overlooks everything interesting about our situation.
Neither of these are quite accurate. The overpopulation debate came to a stalemate because it was a badly phrased question. But seriously studying other questions about humanity — our evolution, ecology, economy, culture, science, and practically anything else — will inevitably bring up population. While most of it has gone overlooked by incurious ideologues content with their entrenched positions, recent work has made great strides in developing a causal model of how and why the human population has changed.
The urge to end the overpopulation debate with a sense of decisive closure has led both sides to seek models that brook no uncertainty. For overpopulationists, that’s carrying capacity: an eternal law of population ecology that no species can escape. For growth optimists, it’s markets: true scarcity is impossible because rising prices always reduce use and spur innovation. The frightening (or reassuring) truth is that ecology provides no such guarantees. The fate of our species is genuinely open-ended.
Whether the number of humans on the planet continues to increase or abruptly crashes hinges not on our moral capacity to restrain our consumption nor our faith in the wisdom of the market, but on evolution. Overpopulationists, by focusing on resource consumption, have fallen for an unintended sleight of hand. Our current ecological situation, with all its harms and risks and benefits, came about not through some unique form of consumption but through the same evolutionary process that has transformed the raw stuff of the universe into “resources” for every species that has ever lived. Growth optimists, conversely, do appreciate the innovative capacity driving the unprecedented scale of this process in humans, but, failing to perceive it as an extension of evolution, underestimate the odds of failure and dead ends— temporary or permanent.
For those without a strong stake in the debate, overpopulation can seem like an intellectual relic, an ignorant (and often malicious) idea that turned out to be plainly wrong. As we’ll see, it’s true that most of the participants in the debate over the last 200 years have misunderstood their subject in important ways. With the advantage of hindsight, though, it’s easy to make light of their failed predictions without actually correcting their mistakes. Whatever their faults, overpopulationists are right to think that population is significant. It is both the metric of the success of our species as an evolutionary experiment and our bare minimum target in protecting human well-being. It’s not something we can afford to ignore.
The Myth of Carrying Capacity
Overpopulation has been a fertile topic for satire for centuries, and the overpopulation doomsayer has emerged as a familiar trope in pop culture. The most famous and recent of these is Thanos, villain of Avengers: Infinity War. Thanos is in some ways a classic incarnation of the archetype. He’s a prophet of the apocalypse, painting the heroes a vivid picture of the fate that will soon befall the Earth if population and consumption growth aren’t halted or reversed. He proposes a drastic, violent solution that could only seem ethical in comparison with the disaster it’s meant to avert. And, implicitly, he’s wrong. The imminent disaster he’s predicting isn’t a real concern, and the semi-omnicidal steps he takes to prevent it are unjustifiable. In the public eye, this is what it means to be an overpopulationist: to prophesy an apocalypse that will never come.
After Infinity War, dozens of pop-science YouTubers made video essays about overpopulation, usually framed around whether “Thanos was wrong.” Almost universally, they painted Thanos as the latest in a long tradition of such failed prophets, starting with Thomas Malthus in 1798 and followed more recently by Paul Ehrlich in the 1960s. But while there are certain similarities between these figures, they are distinct enough that no one-size-fits-all answer can rebut them all. To treat them as interchangeable means giving up the pretense of convincingly answering any of them.
Thanos is a better doomsayer than any human who ever wore the name in two ways. His prophesy isn’t speculative. He has empirical proof for his prediction. He warned his home planet, Titan, that overpopulation was going to make them go extinct, and begged them to adopt his solution. They didn’t, and they went extinct. He also seems to have no real ulterior motive (outside of a certain level of self-aggrandizement). He doesn’t fearmonger about overpopulation to manipulate people into making the universe the way he wants it to be. He believes it because it’s a tragedy he already experienced, and he wishes to spare others from the same fate. No one who’s raised the alarm about overpopulation on Earth can say the same.
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| Thanos using magic to show the Avengers a before/after illustration of overpopulation |
Still, there was a real overpopulationist whose beliefs resemble Thanos’ in all other respects: Garrett Hardin. Hardin is well known today for his theory of the Tragedy of the Commons. In fact, of all the thinkers mentioned in this article, Hardin is probably the most widely read today. The Tragedy of the Commons is a controversial but widely applied framework for the management of shared resources. But what’s less often remembered is that there was only one Tragedy of the Commons that Hardin cared about: overpopulation. Every single example in his original paper is an analogy for human population growth.
In “The Tragedy of the Commons” and its sister article, “Lifeboat Ethics,” Hardin employs three interchangeable metaphors for overpopulation. The Earth is a spaceship: an enclosed system with limited space and supplies designed to support a limited crew. The Earth is a lifeboat: if we allow too many people to board, it will capsize. Everyone will drown as surely as if they had never boarded in the first place. And the Earth is a pasture: each herder gains from putting more cows out to graze, but too many cattle will ruin the pasture for everyone. Like Thanos, Hardin had a sharply apocalyptic vision of overpopulation. The Earth has some tolerance, but exceeding it by too much will cause a sudden and catastrophic failure. Harsh measures, solutions that take away the freedom and bodily autonomy or even the lives of innocent people, are thus not only justified but ethically imperative.
Hardin, who was a professor of ecology at UC Santa Barbara, presented himself as an objective scientist raising the alarm about worrying findings from his field. He encouraged his readers to believe that the harsh solution he advocated — mandatory sterilization — only occurred to him as the least-worst option in an impossible situation. As it turns out, though, this wasn’t the case. It would be more accurate to say that the problem of overpopulation appealed to Hardin because it created such an iron-clad case for a solution to a very different problem: “white genocide.” He was a white supremacist. He wanted to sterilize black and brown people, and overpopulation was the perfect pretense to justify it.
For many people, Hardin’s hidden motivation has been enough to discredit the idea of overpopulation completely. An ethical violation of the scale Hardin advocated is the sort of thing that can only be sold by someone who truly loathes his conclusion, who would check his work again and again to find some way to avoid it. Hardin’s eagerness to come to the conclusion he did makes you wonder if he ever even did the work in the first place. But while that caution is very much warranted, rejecting overpopulation as a racist ploy in general is not tenable. For one thing, there are credibly anti-racist groups who make similar arguments. For another, it invites racists to take the intellectual high ground claimed by Thanos: they’re the only ones willing to face the truth.
What stands out to me, though, is that it isn’t obvious where Hardin’s racist overreach ends and valid ecological theory begins. Is there anything valuable we’d lose by rejecting this argument whole-hog? And if not, what does that say about ecology?
Like Garrett Hardin, Paul Ehrlich and William Catton were academics who used the presumption of expertise and objectivity that came with their roles to proselytize about the threat of overpopulation. Ehrlich and Catton weren’t quite as histrionic as Hardin. They didn’t lead readers to believe the planet had a population-triggered kill switch, as Hardin’s Spaceship Earth and Lifeboat Earth analogies imply. But when it came to Hardin’s third analogy, there was no disagreement. All three men took it as an iron law of ecology that just as a pasture could only sustainably support a limited number of cattle, the Earth could only sustainably support a limited number of humans.
Of the three, Catton was the only non-ecologist (he was trained as a sociologist). But he was also the one who leaned hardest on the law-like character of ecological theory. The cover of his book Overshoot is a terse glossary of ecological terms, which together frame a stark, scientific picture of a human future ending in “die-off.” The most important term gets pride of place below the title: “carrying capacity: maximum permanently supportable load.” Carrying capacity is the line which population is over in “overpopulation.” It is the single idea on which this whole conversation rests. And it’s based entirely on an analogy with bulls shitting in a field.
The term “carrying capacity” came to ecology through conservation pioneer Aldo Leopold (see Sayre 2008 for a full history of the concept). Working in the Forest Service’s Office of Grazing, his role was to manage wild game using the techniques of scientific range management. In other words, to grow deer with the same mindset as ranchers grew cattle. That meant flooding or burning land to modify habitat, growing crops for supplemental feed, and relocating or releasing captive-bred animals, but most of all it meant killing wolves. Carrying capacity, in this context, was a property of a particular habitat, determined by its climate and vegetation, which could be raised by management or lowered by overgrazing.
As Leopold and his colleagues eradicated wolves from more and more of the Grand Canyon Game Preserve, they got exactly what they wanted: more deer. The population grew so quickly that the game hunters who came to hunt them and the game wardens there to prevent overgrazing were simply unable to keep up. The deer stripped the hills bare and then they starved. Worse, they took the rest of the ecosystem down with them, starving other herbivores and causing catastrophic erosion.
The lesson Leopold took from this, which made him famous and has been passed on to generations of ecologists and conservationists since, is that ecosystems have mechanisms to maintain their internal stability. Populations are limited to a “characteristic number” at which they cause no lasting damage to their habitat’s capacity to sustain them. To exceed this characteristic number was to be in “overshoot,” a temporary state that always ended in a crash, usually a famine. If overshoot was prolonged and severe, the ecosystem could be altered in a way that permanently lowered carrying capacity. For Leopold, the primary cause of such imbalances was human disturbance. Locally, within a particular watershed or game refuge, they could be avoided by wise management. But globally, they were an inevitable consequence of the overshoot of humanity itself. Humans, according to Leopold, had “suspended the laws of carrying capacity by inventing tools,” but in doing so raised the specter of two “new horsemen”: ecological imbalance and the “genetical deterioration of the human species.”
Like Hardin, Leopold understood this lesson to apply to humans in explicitly racial terms. “The mechanisms for limitation,” he wrote, should be understood as “fixed attributes” of populations, “as immutable as the color, form, and habits of the individual creature.” The “characteristic number of Indians in virgin America was small,” but when European settlers “arrived on the scene we raised the carrying capacity for man by means of tools.” Human carrying capacity changed through the replacement of races with a low “characteristic number” by others with a higher “characteristic number.”
By the time Paul Ehrlich learned about carrying capacity, the term had been integrated into the mathematical model of population growth. The basic logistic equation for changes in the population of any species had been figured out centuries earlier. In its original form, the equation combined the biology of the species and the characteristics of the environment to determine an equilibrium value, to which it would grow if below or shrink if above. But when carrying capacity was integrated as “K,” the equation was modified — subtly, almost imperceptibly, but in a way that fundamentally alters its implications. If you’re interested in the math, see this footnote[1] and the paper linked there, but in short, the equation was modified to represent carrying capacity as a single, fixed value external to the population itself.
It’s easy to see why it felt intuitive to conflate these concepts. Leopold developed the insight that in the absence of disturbance, species maintained a consistent population. Lab studies of population growth in fruit flies and flour beetles found the same result. The classic mathematical model of population growth seemed to put this field-tested and experimentally validated insight in the quantitative terms needed to make ecology a formal science. The asymptote of the graph was simply taken to represent the characteristic abundance of the species. Apart from a few math-savvy nerds, most ecologists never realized this formulation differed from the original at all. It is still widely taught that way to this day.
In the original equation, though, K was neither a single value nor external to the species. It was a ratio of two terms, one of which was entirely defined by the biology of the organism itself. Each of those terms in turn was a composite of dozens of specific aspects of the species, its environment, and the relationship between the two. A change to any of these factors could change the equilibrium abundance of the population. Or it could have no effect at all — the model is fully agnostic as to whether any of these factors are stable or variable and to whether they are independent or deeply intertwined.
In the study and management of dynamic systems, these distinctions are crucial. In range and game management, carrying capacity was believed to be both a calculable value managers could use to avoid overgrazing and a metric to gauge the impact of management techniques. In the original population model, equilibrium abundance can’t be calculated even in principle. It can only be broadly inferred in retrospect. It is a mathematically ironclad model because it is true by definition: the abundance of any given species in any given moment is the collective outcome of all the factors that affect its rates of births and deaths. For many species, equilibrium abundance is a purely theoretical notion. It changes faster than the time it would take them to reach it; they’re always chasing a moving target.
The contrast between these two models exposes the paradox that was always inherent in the notion of carrying capacity. It is the characteristic number of a given species in a given habitat, but populations are normally prevented from reaching it by limiting forces like predation. Managers could raise it by adding resources and removing predators, but had to temper the effect by culling to maintain a safe margin below it. It was defined as a “sustainable level,” but on some ranges it had been chronically exceeded for decades. Humans are subject to the law of carrying capacity like every other animal, but we have “suspended the law” by inventing tools. The purpose of “carrying capacity” as a concept was to categorize the mess of terms in the logistic population equation: some natural factors were consistent and characteristic, others were temporary fluctuations; some human interventions were habitat improvements while others were destabilizing disturbances. But the equation is agnostic to these things for a reason. Such categories cannot be imposed without sacrificing its universal, necessary, lawlike character.
The starkest illustration of overshoot and its paradoxes is the reindeer of St Matthew Island. During World War II, the US Coast Guard introduced 29 reindeer to the island to serve as an emergency food supply for the crewmen stationed there. The island, located far from Alaska in the middle of the Bering Sea, had been without grazing mammals for centuries, and had thus accumulated an old-growth community of shrubby plants and lichens. For 29 reindeer, this was an endless supply of food. With plenty of food — and, since the Coast Guard left a year later, no predators — their population ballooned from 29 to 6000 in less than 30 years. Arctic plants and lichens, which grow slowly in the best circumstances, were unable to keep up. In the winter of 1968, the reindeer population crashed to 42 individuals, and within a few years later they were gone too.
According to the carrying capacity model, the reindeer exceeded their “permanently supportable load” almost immediately. There is some number of reindeer whose grazing and browsing would have allowed plants and lichens to keep up. But that number was very low, and the reindeer were utterly oblivious to it. In the process, they made the carrying capacity even lower. Then at some point, the temporary abundance provided by all that old-growth lichen ran out, and the new, lower carrying capacity made itself felt. In this story, carrying capacity was always there, biding its time. Chance gave the reindeer a brief escape, but that only made carrying capacity’s eventual victory all the more brutal. Natural law was suspended, then reinstated.
The equilibrium abundance model sees the situation very differently. It tells us that when the reindeer arrived on the island, high birth rates and low death rates made their equilibrium abundance extremely high, causing rapid population growth. This is a tricky distinction, one that reveals the limits of this terminology. “Equilibrium” in this equation doesn’t necessarily imply a physically plausible reality that could persist for even a moment in a real habitat. You could say that, in this case, the initial equilibrium abundance was the number of reindeer that could survive on St Matthew Island if the food supply neither grew nor shrank as they consumed it. But really it might be better to just think of it as a mathematical artifact.
Environmentalists have made the St Matthew Island overshoot a parable for human overpopulation. The slow-growing lichens are analogous to the coal, gas, and oil that have fueled the recent rise in the human population. They provide an “exemption” from the rules of population ecology, which must sooner or later reassert themselves as the accumulated resource is spent. Except, as it turns out, population ecology doesn’t say that at all. It isn’t a patient, lurking doom, a predestined death that if thwarted or evaded comes back again in an even more gruesome form, Final Destination-style. Quite the opposite: population dynamics are short-sighted, even reactive, simply responding to any changes in their environment. Sometimes, as on St Matthew Island, those changes fit the narrative of carrying capacity: a crash triggered by changes in the environment caused by population growth that result in a permanent decrease in abundance. Other times, as in predator-prey cycles, a crash caused by population growth is not associated with any permanent change in the environment. In many cases booms and busts can be caused entirely by external changes.
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| In the
classic Lotka-Volterra predator-prey model, hare and lynx populations
spike and crash in response to changes in each others’ abundance
triggered by prior changes in their own abundance, etc |
The aura of scientific, objective inevitability cultivated by Hardin, Ehrlich, and Catton, therefore, simply does not exist in population ecology. It sets only one constraint on the future abundance of our species: a maximum potential growth rate. Every outcome between that and extinction is equally consistent with its laws. It is even possible that we could experience the opposite of the carrying capacity narrative: population growth could cause changes in the environment that permanently increase our abundance.
In its translation from pasture to ecological theory, “carrying capacity” generalized the worst stereotypes of livestock to life itself, and especially to humans. Animals are meat-making machines, consuming any resources that are put in front of them, tearing up the soil with sharp hooves and polluting water with piss and shit. Population growth exhausts resources and destroys ecosystems. And because humans have systemically disabled or exterminated every predator and disease that limited us, we have been able to consume and destroy at an inconceivable scale.
There’s something paradoxical about this framing, though. If we’re the livestock consuming fossil fuels, who are the ranchers dumping them in our pen? There is, after all, a reason there were no reindeer on St Matthew Island to eat all the luxurious food growing there. It’s the same reason no other species in the last 300 million years has taken advantage of the colossal supply of energy accumulating underground as coal. To call something a resource, you need more than just the urge to consume it. You need the capacity to access it.
The entire evolutionary history of life on Earth has been driven by the creation of new resources. In the absence of life, the universe is an undifferentiated span of moving particles. Only from the perspective of a living organism does it contain hazards to avoid and resources to collect. And even then, only a narrow slice of the universe is relevant. This fact is so fundamental that it would be very reasonable to say that the conversion of inert matter and energy into resources is the defining characteristic of life. And the way organisms achieve this conversion is by combining molecules according to instructions stored in their DNA. In a sense, recipes for molecules. It is the breadth and quality of this molecular recipe book that determines the scale of the biosphere, far more so than the Earth’s physical properties.
We know this is the case because the biosphere has grown enormously over time, and each expansion was marked by the discovery of new recipes. Arguably the most dramatic of these occurred in the Archaean. Photosynthetic cyanobacteria, which formerly required scarce hydrogen sulfide to extract energy from sunlight, developed a new molecule capable of using super-abundant water instead. The scale of solar energy unlocked by this event was gargantuan. Every modern plant and alga, and every herbivore, predator, and detritivore that depends on their primary production, owes its existence to this one molecular recipe. That explosion of life didn’t happen all at once. In the short term, the gradual accumulation of oxygen in the atmosphere devastated the ecosystems that had previously dominated the planet. Eventually, though, the atmosphere was richly supplied with oxygen, which was in turn made a resource by other new recipes, enabling further growth.
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| Early oxygen-releasing cyanobactera accreted globular rocks called stromatolites. |
Every major period in the geologic record is marked by such a development. The endosymbiosis of algae in fungi brought photosynthesis to land and created the first organic soil, establishing the entire terrestrial biosphere. The symbiosis of plants with nitrogen-fixing root bacteria allowed terrestrial life to expand far beyond what nitrogen from lightning and ocean drift could support. These innovations have permanently increased the scale and diversity of life on earth. But to focus narrowly on these epochal novelties would be a mistake. This process is endemic to evolution at every level. Every generation of every species is constantly straining to access more resources and fine-tuning its efficiency in using those it already has. These subsequent iterations are just as important as the novelties that enable them.
The current period, the Anthropocene, is no different. It is defined by the environmental changes made by our species, all of which were made possible by an evolutionary search process that yielded recipes for new technologies that allowed us to cross oceans, live in inhospitable climates, and transform wood, coal, and sunlight into consumable forms. That process has caused the extinction of hundreds of other species, transformed most of the Earth’s surface, and involved the consumption of massive quantities of nonrenewable energy. But those are all mere byproducts; the driving process is the same now as it has been for the entire history of life: evolution.
However, the fundamental continuity of human history with the prior evolution of life has become apparent only recently. At the height of the overpopulation debate in the 1970’s, neither side was capable of realizing it.
The ’60s and ’70s were the peak of overpopulation panic. The global human birthrate and life expectancy were both far higher than they had ever been, and the public was becoming concerned about dozens of environmental problems that had previously been ignored. Overpopulation, and especially resource concerns like Peak Oil, were taken for granted by wide swaths of Americans. President Carter openly discussed the threats posed by “increases in world population,” while Nixon called population growth “a world problem which no country can ignore.” The high profile of overpopulation was due in no small part to Paul Ehrlich.
Ehrlich’s 1968 book The Population Bomb was a bestseller and made overpopulation a top priority for environmentalists and development economists. In it, he warned that continued population growth would guarantee a massive famine would kill at least a billion people in the next two decades. While Ehrlich was an ecologist, unlike William Catton, his argument invokes “carrying capacity” only informally and obliquely. His argument is structured in a more general, back-of-the-envelope way, comparing present food production and anticipated demand and pointing out all the environmental problems he could think of — pollution, global warming, biodiversity loss, smog, etc — as if one of them, or all of them put together, simply had to affect population growth eventually.
The thing that made Ehrlich truly influential, though, was the appearances he made on Johnny Carson to push his book. His first interview was so popular that he was invited back again and again. His overall view of humanity and its place in the world — that we were voracious, destructive consumers on a path to disaster which no scientific breakthrough could avert — resonated with people. While Thanos was originally created in 1973, he had very different motivations then. If Infinity War had come out in the ’70s, most of the audience would have found his logic familiar and uncontroversial, and many would have seen him as a hero.
His big epiphany was simply that individual human lives are valuable, and it’s good to have more of them. Humans are clever and creative, and in the past century, they had solved seemingly insurmountable problems of resource scarcity through technological innovations that had been unimaginable to people just decades before. He saw no reason why that same process wouldn’t continue indefinitely into the future, with more people producing more resources to support an ever-growing standard of living. Simon honed this argument in more precise economic terms and embarked on a campaign to share the good news about population growth with the world, attacking Ehrlich in particular.
At the time, this was widely understood as a conflict over values. While Simon identified as a conservationist, and argued that population growth caused innovation that benefited both humans and nature, the fact that the outlets most eager to publish and promote his ideas were conservative thinktanks and politicians opposed to environmental regulations discredited him with the mainstream and radical environmental movements. Ehrlich, conversely, was openly misanthropic. Where Garrett Hardin’s not-so-hidden motivation for embracing overpopulation was racism, Ehrlich’s was conservation.[2] He clearly hoped that the threat of famine and collapse would persuade the public to embrace political and lifestyle changes to protect wild nature. But while his argument relied on an appeal to human self-interest, he frequently rejected the idea that human lives had an inherent value that could be multiplied through population growth. In 1972, when the US population was almost 210 million, he told a reporter that he couldn’t “think of any reason for having more than 150 million people.” To Ehrlich and his followers, Simon’s frank admission that he valued human lives more than animal lives was taken as proof that he was on the wrong side.
For us, the most important part of the Simon-Ehrlich debate is not their values but their intellectual background. Ecology and economics both study how living organisms respond to scarcity. But in Simon and Ehrlich’s time, they used very different conceptual tools to study that question, and came to very different answers. Ecologists understood evolution to be a slow process capable of explaining innovations in other species but not on the timescale of human lives. Hence, population was best understood as the conformation of a species to the fixed limits of its environment. Ehrlich understood human industrial innovation as a fundamentally short-sighted and destructive process, a conversion of nonrenewable resources and self-sustaining natural capital into crumbling physical infrastructure.
Economists were extremely familiar with the power of human ingenuity to overcome scarcity, but framed this in terms of a unique, ineffable creative spark that no other animal possessed. While Simon knew that market forces had not always been able to prevent famine in the past, he was confident that in a global economy of billions of people full of powerful scientific and technical institutions, food production could always outpace rising demand. As he warmed to polemical writing, Simon took this potential to sometimes extravagant places. He fantasized about space colonization and speculated that continuing innovation would eventually achieve literal alchemy, ending the scarcity of metals like copper by allowing us to make them from other metals.
Simon and Ehrlich never overcame the gulf between these theories. Instead of trying to figure out how the known facts in ecology and economics could be reconciled, they held a bet to determine, in essence, which discipline was right. The specifics of the bet are somewhat misleading. Simon wagered that a set of ten metals would decrease in price over a decade, and they did. It’s not obvious that this truly captures the nature of their competing claims. You could probably imagine a scenario in which Ehrlich had been right about famine and climate change but the price of chromium still went down.
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| Price history of the metal commodities in Ehrlich and Simon’s wager, with the period of the bet highlighted. |
Still, Simon’s victory has been symbolic of a larger sense that Ehrlich’s most dramatic predictions simply haven’t come true. There was no global famine in the 1970s, and the rate of hunger has decreased despite continuing population growth. Ehrlich, who has been prophesying doom for 60 years, is still alive and still arguing that the crash is just around the corner. And of course there are plenty of people who agree with him. But so far, the facts overwhelmingly support Simon. Taking drastic action to curb population growth was not necessary, and doing something evil to achieve it would have been an unjustified atrocity.
As long as carrying capacity was the best tool ecologists had to understand human population growth, it was almost inevitable that an economist like Simon would make better predictions than an ecologist like Ehrlich. The carrying capacity model, with its permanent, external limits, has no place for a species whose growth and consumption creates new resources. For Ehrlich, humans were just another species of animal. They had to conform to the scarce resources available in their environment. Simon believed that humans were unlike other animals in ways that only economics was equipped to understand. His definitive statement on the population debate, The Ultimate Resource, argued that the human mind is something unprecedented in the history of life: a resource-generating engine capable of permanently ending scarcity.
It’s not hard to see why Ehrlich would find this unsatisfying, even ludicrous. The “power of imagination” is not a particularly reassuring answer to the threat of imminent famine and global ecosystem collapse. And yet, for all its human exceptionalism and anthropocentrism, Simon’s “Ultimate Resource” seems to capture the ecological history of our species far better than Leopold and Ehrlich’s narrow fixation on carrying capacity. To truly understand human population growth, ecologists first needed to abandon the constraints of the carrying capacity model. Only then could they open their minds to what Simon knew decades ago and find a way to restate his insight as an ecological process. And now, with the theory of cultural evolution, they have.
Like biological evolution, cultural evolution is an idea that’s difficult to come up with but obvious in hindsight.[3] The idea is simple: if an animal can learn behaviors from other individuals, they can acquire adaptations that can’t be transmitted through genes. And if those behaviors are retained within a population over generations, they will be honed by natural selection and accumulate complexity. Compared to change in genes, evolution through culture is blazingly fast. Its chief advantage is modularity. It acts simultaneously to improve everything from the skills involved in tool use, the design of tools themselves, the skills involved in tool manufacture, the ideas used in tool design, and the social structures needed to support and preserve all of these processes. It doesn’t mean humans are special geniuses, but it is a genuine inflection point in the biosphere’s capacity to evolve.
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| A phylogeny of cornets. From Eldredge 2011. |
The adaptive power of cultural evolution is limited in two key ways. Because it is transmitted through behavior, it can only create and improve adaptations relevant to things people are already doing. And while every organism inherits a complete copy of the genetic code of its species by default, this isn’t the case for culture. Human communities contain more cultural adaptations than any individual can learn in their lifetime. Conversely, while the size of an organism’s genome is relatively fixed, cultural capacity scales with the size of a population. Large populations can develop and maintain more cultural adaptations than small ones. Moreover, because culture propagates through teaching and imitation, it’s the number of connections between people that matters, not just the number of people. When a population grows, its cultural capacity grows even faster.
This is where the differences between carrying capacity and equilibrium abundance become crucial. For many species, population growth changes the environment in ways that decrease their equilibrium abundance, as the carrying capacity model predicts. But for us, population growth changes the environment (and the ways we can interact with it) in ways that enable further population growth. The “rule” of carrying capacity quite literally does not apply to us. A more powerful ecological force has overtaken it. Over the last 12,000 years, humans have driven key prey species to extinction and exhausted rich stores of non-renewable resources like oyster beds, old-growth forests, and soils. Often these events have even been accompanied by local declines or crashes in human population. In the long run, though, on every inhabited continent, these setbacks have always been followed by cultural innovations that allowed even greater growth.
In 1972, Paul Ehrlich could honestly say that as an ecologist, he saw “no reason for having more than 150 million people.” Today, ecology has vindicated Julian Simon: human minds are evolution’s Ultimate Resource, not as vessels of an ineffable creative spark, but as links in a vast network of adaptive knowledge and innovation, capable of constantly raising the equilibrium population of our species in a way unprecedented in the history of life.
Of course, the 20th century overpopulationists were well aware of the massive power cultural evolution had recently bestowed on humanity. Catton even credits cultural evolution by name! They were all, to a lesser or greater degree, conscious that what they were writing about was less population growth per se than the Industrial Revolution that made it possible. They were concerned about the consequences of industrial pollution, from climate change to toxin load to algal blooms. They were troubled by the destruction of habitat and loss of biodiversity enabled by industrial equipment. But more than anything, they were certain that the carrying capacity gains brought by the Industrial Revolution were necessarily temporary because they were enabled by non-renewable fossil fuels.
In their view, humanity is just smart enough to get in over our heads. A bit of cleverness and a lot of luck (good or bad depends on who and when you ask) was enough to get us to the St Matthew Island of fossil fuels, but that wasn’t all that hard, and now that we’re there, we’ve reverted to gluttonous livestock, wastefully consuming the glut of cheap energy, destroying the planet, and embarking on a spree of dead-end growth. Because if there’s one thing they all agree on, it’s that finding fossil fuels is a feat that can never be repeated.
The simple fact of cultural evolution can’t, on its own, settle whether they’re right about that or not. Still, it’s instructive to think about exactly what it took for humanity to access fossil fuels, how those conditions arose, and what might have happened if they had never existed in the first place.
Coal, gas, and oil contain an enormous amount of energy. But unlike reindeer munching lichens, humans can’t just eat them. Relative to the evolved capacities of our bodies, they’re not a resource at all. The Industrial Revolution, then, was defined not by the discovery of coal, which had been mined for thousands of years, but by the discovery of the alchemical process that turned fossil fuels into food.
Figuring out how to use fossil fuels is a familiar problem in evolution: overcoming apparent irreducible complexity. In order to turn the dense and abundant energy stored in fossil fuels into food, you need to mine something with phosphorus and nitrogen in it (coprolites, guano, or bones), combine it with sulfuric acid, and add it to crops in fairly precise quantities. You also need engines and canals and factories that allow you to use coal to power all of that. In other words, you need a dozen industries staffed by dedicated professionals using specialized techniques. But many of those industries are only worthwhile in the presence of the others, so they all need to spring into existence at the same time. Evolution, stuck pursuing incremental improvements to long-established routines, can’t make that happen.
The necessary precondition of the Industrial Revolution, then, was that all of those dozen industries, or something close, did already exist in the absence of fossil fuels. That could only be the case if there was already an economy that was diverse, specialized, scientific, interconnected, and carrying a relatively large volume of goods. The set of industries that emerged to turn English coal and Pacific Island guano into higher yields of wheat may have seemed like an irreducibly complex novelty, but every part of it descended directly from something that people were already making a living doing. Every piece of scientific and technical knowledge was only a few modifications and fortuitous accidents away from skills and ideas cultural evolution had already honed to serve people in trades that were viable without fossil fuels. And each of those trades in turn descended from others, and still others, for generations. The Industrial Revolution was not a break with the past, but a seamless continuation of the same processes that brought it about in the first place.
Julian Simon’s insight was that those processes weren’t just the accumulation of singular intellectual breakthroughs or the inevitable byproduct of the cumulative functioning of market incentives. Simon realized that population growth itself was a necessary third ingredient. Large populations increase the volume of human minds — our Ultimate Resource — and thus our capacity to develop and retain ideas. But the more important implication, obvious to an economist but easily overlooked by an ecologist, is that large populations mean more trading partners. An economy with more trading partners makes it worthwhile to build the infrastructure necessary to move larger quantities of goods and provides enough customers to support industries that aren’t viable in a smaller world.
There is a self-perpetuating cycle here. Population growth causes economic growth, which creates new trades, where cultural evolution can create new innovations that support further growth in population. To focus on population misses the true scope of this cycle. Every technology in agriculture, sanitation, and medicine that allows an increase in human bodies is valuable. But because cultural evolution acts on connections between people, innovations that increase connectivity are far more potent. Things like travel, postal systems, and printing presses amplify the effect of each thousand births exponentially.
At the time of the Industrial Revolution, the population of the world had grown big enough, and connected enough, that the right set of conditions could kick off a spiral of runaway growth.[4] Increased economic and social connectivity could double, triple, or quadruple the functional population size — and thus the capacity of cultural evolution to produce adaptations — almost overnight.
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| A rough estimate of the history of population growth in Europe. From Atlas of World Population History, McEvedy and Jones 1978. |
The fact that these conditions arose first in Europe was an accident of history. It could have happened anywhere, to any human population, and in another run of Earth’s history it very well might have. There was nothing special about Europeans. This much is obvious to anyone who isn’t a white supremacist. What’s less obvious is that the glut of cheap fossil energy wasn’t crucial either.
Without coal, the Industrial Revolution would not have achieved as much, as quickly as it did. And yet, the conditions that created the Industrial Revolution had nothing to do with fossil fuels. Coal and oil dramatically accelerated growth in some parts of the economy, but their very cheapness suppressed progress in others. This was, ironically, a point commonly made in environmentalist circles at the heyday of overpopulation fears. Modern organic pest control, for instance, is built on a tradition of biological control research abandoned after synthetic pesticides were discovered. Early internal combustion engines could operate on biofuels. Both agricultural revolutions, which directly enabled the massive increase in human population, used fossil fuels only to make their work cheaper and easier. In each case it was the ideas and techniques, not the energy source, which revolutionized the food supply.
With or without fossil fuels, the world would have changed more between Malthus and Simon than it had between Malthus and Christ. Industries would have used draft animals, charcoal, biofuels, wind, water and whale oil for power, with lower energy return on investment and different ecological impacts than coal and oil. Not much else would have been, necessarily, that much different. Miners would still have dug up bones and guano and sold them as fertilizer. Hybrid crop breeding would still have massively increased yields. Communication networks would still have connected the globe. Sanitation, medicine, nutrition, education, and social welfare policies would still have decreased child mortality and increased longevity. As soon as the triple engine of economic growth, cultural evolution, and population growth hit its inflection point, exponential growth was inevitable.
It’s hard to write about the inexorable, exponential growth of human knowledge without coming off as a naively optimistic booster. Reframing human history in this way, focusing on innovation and problem-solving rather than gluttonous consumption, is certainly, as Julian Simon saw it, an effective antidote to Ehrlich, Hardin, and Catton’s overbearing pessimism. But while there are plenty of obvious reasons to be pleased by human progress, the adaptations bestowed on humanity by cultural evolution, and the population growth they have enabled, aren’t necessarily anything to celebrate. They’re neutral ecological facts. We don’t assume that individual squirrels, blowflies, and wildebeest are happier or healthier than pangolins, whales, and rhino lice simply because their species is more abundant. The best news we can take from this shift in perspective is that a future collapse in human abundance can be safely downgraded from “inevitable fact of ecology” to “possible, contingent ecological event.” Human cultural evolution is a seamless extension and amplification of evolutionary adaptation, and evolution is an indifferent natural process that runs on mass death.
In that light, it’s not surprising that the accelerating growth of knowledge and economic activity in the last 200 years has been mind-bogglingly cruel and destructive. The pain and devastation caused by our growth are unprecedented in pace and scope but unremarkable in kind. The anomaly in need of explanation is not poverty and suffering, but the comfort and security a large and growing proportion of humans enjoy. Simon’s optimism aside, this was by no means an inevitable outcome of exponential population growth.
The Malthusian Dream
Overpopulation ecologists in the 20th century framed the stakes of population growth in vivid terms of human misery. And they understood the relationship between population and misery in terms of limits. They assumed there must be a limit, and warped their terminology and their models to conform to that assumption. As I’ve argued, framing human population growth relative to an inferred carrying capacity is inconsistent with ecological theory. But perhaps more importantly, it is also profoundly misleading about the relationship between population growth and individual well-being. And on that score, they really don’t have an excuse, because Thomas Malthus got it right the first time, over a century before any of them.
Malthus is almost universally portrayed as the progenitor of the “overpopulation apocalypse prophecy” genre, both by those who see themselves as his successors and by their opponents. His name so dominates the conversation that men like Hardin, Ehrlich, and Catton are generally termed “Neo-Malthusians.” Every single video purporting to “explain” Thanos takes it as a given that his argument might as well be a direct copy of the position first argued by Malthus.
But if Malthus were reanimated as a head in a jar and forced to watch Infinity War, he would have found the overpopulation-induced apocalypse on Thanos’ home planet entirely unexpected. The truth is that Malthus never conceived of the possibility of global famine or human extinction. In fact, he didn’t even make the prediction he’s most famous for: that Britain was heading for a famine.
Writing in 1798, Malthus had no concept of carrying capacity or sustainability. His notion of overpopulation had nothing at all to with the absolute bounds of any ecological system — Britain, Europe, or the Earth. For him, population pressure was defined relative to the current capacity of society to produce and distribute food. The concern driving his work was not the speculative possibility of a future crash, but the daily reality of past and present misery caused by starvation in every human society he was aware of.
Compared to its catastrophizing, pessimistic reputation, the content of Malthus’ argument is shockingly uncontroversial (which is good, because his model was and remains core to the theory of evolution). He put forth a model of population dynamics that is, in essence, an informal precursor of the equilibrium logistic population model mentioned above. Ben Franklin and Leonhard Euler had both recently calculated the exponential trajectory of unchecked human population growth. Malthus observed that this potential clearly didn’t manifest in reality, and inferred the existence of checks, which must necessarily fall into one of two categories: deaths and averted births. The former, which he termed ‘positive checks,’ expressed itself as infant and child mortality, starvation, disease, and war. As a priest, Malthus limited his imagination of the latter, which he called ‘preventive checks,’ to delayed marriage and abstinence outside of wedlock.
In stark contrast with his reputation as a bitter, misanthropic elite looking for an excuse to end charity to the poor, his motivations were also shockingly uncontroversial, especially for a man of his time.[5] Malthus was not just a priest and a Christian, but a Christian Utilitarian.[6] The aspiration of his work was not to limit the size of the poor population, but to follow Jeremy Bentham’s utilitarian principle and reduce their suffering. As Malthus put it, humans were placed on the earth by God to bring about the “greatest sum of human happiness.” Like other utilitarians, Malthus understood this principle to imply that population growth was an inherent and substantial moral good. If Malthus were alive in the 20th century, in other words, he would have been a staunch anti-Malthusian. His anthropocentrism was far more extreme than Julian Simon’s, and Paul Ehrlich’s misanthropy would have appalled him.
Malthus differed from other utilitarians of his day in insisting that high birthrates did little to add to the sum of human happiness as long as death rates, especially among children and infants, remained high. And while he attributed most of these deaths, directly or indirectly, to the inadequacy of the food supply, the core of his argument was that no scheme to increase agricultural production, no matter how wildly successful, could ever change that fact.
Today, the name Malthus is synonymous with pessimism. He is famous for failing to predict the agricultural consequences of the Industrial Revolution, which are generally understood to have “proven him wrong.” It’s true that he massively underestimated the rate at which farm yields could be increased. But the role the relevant passages play in his argument is almost universally misunderstood.
His argument is generally presented in terms of a race between the linear growth of the food supply and the geometric (exponential) growth of population — a race population would inevitably win. But Malthus was well aware that population growth in England was not exponential, because it was already constrained by his positive and preventive checks. And he certainly wasn’t attempting to predict the specific future values of English agricultural production.[7] Rather, his argument is simply intended as a demonstration of the “principle of population”: as long as the positive check, through starvation, was the principle force limiting growth, even under the best imaginable circumstances, food production would always be matched or outstripped by population growth. No amount of agricultural innovation would save the poorest people from the edge of starvation.
The only way to escape this cycle of rising productivity, population growth, and famine was to intentionally and voluntarily embrace the preventive check until population was below the threshold that incurred positive checks. As a Christian, Malthus felt there was only one virtuous way to achieve that: abstinence and voluntarily delayed marriage. He rejected contraception on religious grounds and government regulation of marriage on philosophical grounds. The entirety of his hopes for the future of human happiness rested on voluntary abstinence. He was optimistic that people would embrace the strategy as soon as the potential benefits had been properly explained to them.
The strange thing is that most people living today genuinely don’t know how this story ended. So-called “Anti-Malthusians” believe that the exponential growth in productivity that began immediately after Malthus died has made his obsession with birth rates obsolete. So-called “Neo-Malthusians” believe that by relying on productivity growth and eschewing population control, we’ve put off the problem and made it far more severe. Neither of these positions are consistent with the state of the world today, and unlike his self-proclaimed followers, Malthus himself would have recognized that immediately.
If humans really were like the reindeer of St Matthew Island, every gain in productivity made in the Industrial Revolution would have been swallowed up by accelerating population growth. The rates of poverty, starvation, and probably even infectious disease would all be just as high as they were in Malthus’ day. There would just be a lot more people around to suffer them.
Instead, precisely the opposite has happened. The societies that benefited first and the most from the abundance created by the Industrial Revolution saw their birth rates fall, first gradually, then dramatically — a process called the Demographic Transition. In a few countries, like Japan and Spain, this process has advanced so far that their populations are actually falling.
The Demographic Transition is impossible to understand in the terms of the carrying capacity model. Unlike the peak of the St Matthew Island reindeer population, the peak of Japan’s population was not brought on by a wave of starvation. The resources they rely on have not become scarcer in any meaningful way. If, like Paul Ehrlich, you believe that the current population is well above the Earth’s carrying capacity, you still wouldn’t expect birthrates to fall until the famines and wars and epidemics kick in to “enforce” carrying capacity. The slow and steady decline of births in rich countries makes no sense in this model.
In the Malthusian model, on the other hand, the Demographic Transition is perfectly comprehensible. Which shouldn’t be surprising, because it’s exactly the outcome he hoped to inspire. We are living in the world Malthus dreamed of, and in the broadest strokes, we got here by doing exactly what he prescribed: lowering the birth rate at approximately the same pace as the death rate. This fact has led many people to the reasonable conclusion that the demographic transition was the result of Malthusian activism. As a prudish Christian ignorant of modern birth control, Malthus obviously overstated the role of abstinence in the process, but it’s not hard to imagine that people were persuaded by his key point: having fewer children would make their lives better. Scientists developed contraceptive technologies that made birth control cheaper, safer, and less unpleasant. Governments, particularly in China, enacted policies explicitly designed to lower birthrates. Non-profits spent billions trying to convince and empower people to have fewer children. Doctors performed (genocidal) campaigns of mass sterilization.
And yet, while all of this is true, and all of it undeniably had some effect on population growth in particular times and places, none of it can explain why the Demographic Transition happened when and where it did. When birth rates first began to fall in Britain, both religious and feminist leaders were fanatically opposed to contraception (the latter because they thought consequence-free sex would degrade married women). And in many poor countries, aggressive contraception distribution and family planning campaigning failed to lower birth rates for decades — until they suddenly and mysteriously began to fall of their own accord.
Contemporary overpopulationists interpreted this pattern in two different ways. For people like Paul Ehrlich, these campaigns were at best a good start — often successful, but applied much less aggressively than the scale of the problem demands. For people like Garrett Hardin, they were evidence of a racial difference. Europeans were smart enough to see the benefits of Malthus’ plan and wise enough to make individual sacrifices to achieve it. But because nonwhite people were incapable of restraining themselves, they would, through foreign aid, charity, and immigration, exploit the extra resources white people had intentionally set aside, ruining things for everyone unless they could be prevented by force.
The
truth is more straightforward (and a lot less racist). British people
in the 19th century had small families for the same reason that couples
in Niger have large families today: because they feel it’s the best
choice for them given their circumstances. Sure, plenty of politicians
and academics believed some version of Malthus’ argument and set out to
enact his vision, but there’s no good evidence that any society has
actually lowered its birth rate for altruistic reasons. Hardin wasn’t
wrong about the lack of coordination among nonwhite people — he was
wrong to think white people were any different. Everyone, to the extent
they are free to control their reproductive decisions, chooses their
family size selfishly. If people want smaller families, they’ll make
that happen whether the Church and Feminism approve or not, even if that means passing around underground publications about contraception techniques.
And if they want more kids, they’ll do their best to make that happen,
whether Paul Ehrlich and the government want them to or not. Cheap,
effective contraception could accelerate a decline already underway, and
religious resistance could slow it down, but ultimately this is all
ephemeral to the real question: what determines the number of children
people want to have?
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Kitty Marion distributing Birth Control Review in 1915 |
We know that desired fertility changes in a predictable relationship with income. Regardless of race, culture, religion, or climate, people start wanting and having fewer children when they become richer. In the way the demographic transition is presented, this is often implicitly understood as a consequence of falling death rates. As people get richer, they can buy better medicine, food, sanitation, and childcare, and their kids just don’t die as often, so they need to have fewer new babies to replace them. The assumption is that most change in fertility is just an adjustment for falling child mortality, and that the dramatic increase in population was caused by the lag as falling birth rates failed to keep pace with the precipitous fall of death rates.
This just buries the real question a step deeper. Why, at a moment when resources are becoming more abundant and people are getting richer, should they fastidiously maintain the same family size as ever? Can’t they afford to loosen their condoms at least a little? But no: the richest people, who could best afford to provide for more children, lowered their birth rate even faster than everyone else.
While that question is still debated in evolutionary demography, the hypothesis I find most convincing (and the one favored by Julian Simon) is that family size is, and has always been, an economic decision. As a rule, people decide when to have children, and how many children they should have, in order to maximize their wealth.
That might seem like a strange way to think about having babies. We tend to conceive of “wealth,” and economic behavior in general, in narrowly human, cultural terms, and often as a very recent phenomenon. This is misleading. Just as cultural evolution is a direct extension of evolution in general, human economic exchange is a seamless extension of primate foraging. Gradually, the way our ancestors survived shifted from near-independent hunting and gathering to total reliance on trade. One major implication of that shift was that evolution honed the techniques humans used to earn a living, improving the literal tools they used to collect resources. But this is just a side effect of the more fundamental process: cultural evolution gave our ancestors general strategies that let them succeed in this novel, dynamic economic niche. In short, it favored strategies that maximized wealth.[8]
Before the Industrial Revolution, the factor limiting the success of any given human lineage wasn’t birth rates, but death rates. Natural selection heavily favored any cultural strategies that reduced death rates. And because most of the best ways to protect children from early death cost money, the effective pursuit of wealth was itself the target of cultural selection.
One of the most important parts of that pursuit was to have the right number of kids and have them at the right time. Like any mammal with a long period of parental care, children are a major resource-sink for parents. They have presumably been the biggest lifetime cost incurred by most people in human history. But unlike most such mammals, humans get a return on that investment. Children pay for themselves. They help with chores and farm labor, including helping to raise other children. They can provide for their parents when they get sick or injured or grow old. For most of human history, children were both the best insurance policy and the best retirement investment people had access to. It was just a matter of getting the numbers right for your circumstances.
The Demographic Transition makes blatant and undeniable something that was likely the case for generations: cultural evolution overrides the existing mechanisms that control human reproduction. It provides techniques like abstinence, contraception, abortion, and infanticide to literally override biological reproductive processes, but it also tells people when and why to use those techniques. These culturally evolved cues don’t necessarily feel like economic calculations when people experience them. They express themselves as traditions and norms, representations of the future that activate our hopes and anxieties. It is the selection process of cultural evolution that makes these choices economically optimal, not the way we conceive of them as individuals.
The Demographic Transition is defined by shifts in nearly every economic variable underlying these calculations. The opportunity cost of raising an additional child increased when women entered the workforce. Shifts from agriculture to cities, followed by child labor laws in industry, decreased the short-term payoff of children as laborers. Education made each child more expensive to raise, but increased their long-term payoff even more. Falling child mortality made it more viable to invest heavily in each child. Institutions like banks, pensions, and insurance companies reduced reliance on children to mitigate risk. Paradoxically, the economics of having large families became less and less viable as people and their societies got richer. And thanks to cultural evolution, people, on some level, knew that and adjusted accordingly.
In that light, it’s no wonder poor people resisted population control campaigns. They couldn’t be convinced that population control was in their own best interest because they knew it just wasn’t true. Conversely, when British women surreptitiously sought out contraceptive techniques all but banned by the church and civil society, they did so because they knew the economic calculus was changing. Women could earn more working at factories than they could by raising children. Because of falling mortality risk and rising wages, paying to educate a few children was becoming more worthwhile than having many uneducated ones.
And that’s pretty much it. Changing social norms and contraception technologies have pushed the cost of having an additional child up or down, but the base rate has been set by underlying economic factors that probably haven’t changed much in generations. For the last 200 years, those factors have shifted farther and farther, and in more parts of the world, to favor smaller families. In a sense, Malthus got exactly what he wanted: people had fewer children because they saw that doing so would be in their best interests, and everyone has benefited from it. It’s just that those decisions were made out of selfishness, not altruism, and they were a consequence, not the cause, of the underlying socio-economic changes that created modern standards of living.
Compared to dramatic transformations wrought by the technological innovations and resources driving the Industrial Revolution, the Demographic Transition is almost universally taken for granted. Julian Simon paid it more attention than most economists, but even he broadly treated it as, in retrospect, easily predictable and probably inevitable. In this, as in discussions of resource limits, economists act condescending toward ecologists who are shocked that humans might violate a core prediction of evolution. That attitude is misplaced.
For one thing, there’s no good way to apportion credit for our wealth between accelerating production and slowing population growth. The two occurred in tandem, and fans of neither can claim the other was dispensable. The Industrial Revolution challenged Malthus’ assumption that “the power of population is indefinitely greater than the power in the earth to produce subsistence for man,” but did not contradict it. Paul Ehrlich’s claim that the Green Revolution would do little to forestall famine as long as population growth continued might very well have been correct. Famine was avoided because “the power of population” was tamed. Simon may have won the bet, but his wildly optimistic prediction that resource growth could always exceed population growth was never truly tested.
But there’s a deeper reason economists fail to grasp why the demographic transition seems so strange to ecologists. Contrary to what many economists assume, self-interest is not a fixed and independent driver of human behavior. It’s a direct product of natural selection. Until the Industrial Revolution, natural selection acted relentlessly to favor anything that reduced mortality rates. As long as becoming wealthy was the best all-purpose way to increase their odds of survival, cultural evolution could simply select for better ways to get wealth. Now that mortality rates have fallen so dramatically, this strategy is no longer the one that maximizes reproductive success. Because this relationship is inverted, evolution will start to select for traits that increase the conversion of wealth into children. The economist’s assumption that individuals do their best to maximize their own perceived self-interest will still be true. It’s just that the humans of the future will be more likely to value large families as a cultural norm to fulfill or simply a pleasure to indulge than a means to increase their wealth.
At the moment, this is a distant, speculative concern.[9] In the next century, more problems will be caused by declining population growth than rising fertility. And if there’s anything to be learned from the intellectual history of overpopulation, it’s that extrapolating from simple premises to a catastrophic future is a good way to be wildly wrong. But it is undeniably the case that, in theoretical terms, the current Malthusian utopia is unstable. High and growing per capita resources offer an enormous opportunity. If traits emerge to exploit that opportunity the human population will resume growing until most people once again have only the bare minimum needed to survive. If that ever happens, we will finally get to test Simon’s faith that population growth will always enable resource growth to outpace demand. Then again, it’s also possible that environmental catastrophes will end the growth of industrial civilization. In that case, it will be a moot point.
This is essentially all there is to say about the history of the human population, from an ecological point of view. Cultural evolution supercharged the inherent tendency of evolution to furnish adaptations that produce more resources and enable population growth. The Demographic Transition slowed population growth while resource growth continued to accelerate, yielding an unprecedented increase in resources per capita. None of those trends are guaranteed to continue, but neither are they certain to end in any foreseeable future. Population ecology simply is not equipped to answer the question overpopulationists put to it: is a crash coming?
If you focus on our capacity for adaptation, it seems unlikely. No matter what we need to do to survive the next challenges to our survival, we’re in a better place to figure them out than we’ve ever been. If the Industrial Revolution could have proceeded without fossil fuels three centuries ago, there's no question we could live without them now. The scale of human cultural capacity is so enormous, with not only the most humans who have ever lived but the greatest degree of education, economic integration, and communication, that we can solve more difficult problems more quickly than ever before. Julian Simon was right. The Ultimate Resource is a universal problem-solving mechanism that can achieve anything it sets its mind to.
On the other hand, it’s not at all difficult to imagine a challenge so deadly that this unprecedented engine of adaptation can’t adapt to it. A big enough rock could do us in without much trouble. An unusually large volcanic eruption. An especially severe solar flare. None of these have been particularly rare in Earth’s history. Climate change, with its long-term, locked-in impacts and potentially catastrophic feedback loops, could easily be such a challenge. Climate change could become so severe, so quickly, that humans can’t adapt to it, causing a population crash. The outcome seems, for the moment, genuinely unclear. It depends very much on how well and how soon we take action.
If we fail, evolution’s experiment in cumulative culture might turn out to be a dead end after all. Or, like early oxygen-producing cyanobacteria, we might collapse the biosphere and crash our population a dozen times along the way to a permanently transformed world dominated by culturally evolved ways of living. The future is both uncertain and undetermined; we won’t know what’s to come until we make it happen.
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| The history of severe extinction events in the last 542 million years. The y-axis shows percent of extent animal genera becoming extinct within each interval. |
To say something is “not the end of the world” is to dismiss it as an acceptable loss. It feels like a betrayal to admit that each act of environmental devastation committed in the name of consumption and every avoidable climate disaster inflicted by the rich on the poor is not a building block in the deus ex machina of our comeuppance, but just a senseless tragedy. That doesn’t make it any less true. There is no account to be settled. The atrocities of the past, and the ones yet to come, are what they are, and must be confronted on their own terms. Our urge to mourn them, and our commitment to prevent them, does not and should not depend on their relation to some even greater tragedy yet to come.
Julian Simon believed that the Ultimate Resource would, given the chance, address all of these problems together. Evolution might be indifferent to the comfort of individual humans, but markets are driven by our subjective desires. Economic growth benefits those within the economy almost by definition. Any existential threat to humanity would become our highest priority and the market would focus the Ultimate Resource to overcome it. His optimism wasn’t limited to human welfare, though. He saw in his time the beginning of a pair of processes he imagined would accelerate over the coming century. As people became wealthier, they would spend more money supporting ecological preservation and restoration. And new techniques would be developed that would make resource extraction both cheaper and less destructive. A wealthier, more technologically advanced society would protect us from extinction, make everyone richer and eliminate extreme poverty, and protect wild habitats, all at once.
So far, the results are mixed. The price of solar energy has fallen far faster than anyone predicted. Farmers are getting larger yields from less farmland. Populations of hundreds if not thousands of once-threatened species have recovered to stable levels, and others have recolonized ranges where they had been extirpated. Poverty and hunger have fallen dramatically in absolute terms even as the population has grown. But total carbon emissions are still rising, old-growth habitat is still being destroyed, and nearly a billion people still live in extreme poverty.
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| Sea otters, once nearly extirpated along the Pacific Coast, were reintroduced in the 1960s and have experienced rapid population growth. Photo by Tony Wood. |
Simon was right, in other words, that we’re capable of solving all of these problems. We’re on track and we’ll get there given time. But time is precisely the problem. The longer we take to effectively address conservation problems, the more species will be lost forever and the harder it will be to restore damaged ecosystems. The longer we take to eliminate extreme poverty, or contain infectious diseases, the more people will suffer and die. And the longer we take to end greenhouse gas emissions, the harder we’ll have to work to protect ourselves from severe climate change.
In a sense, the observations naïve overpopulationists take as undeniable proof of their position really are undeniably true. Humanity’s rise to ecological dominance has been enormously destructive in proportion to the increase in its population and the consumption required to supply it. In fact, they’re more right than they know. Population growth has been harmful not just by demanding ever-increasing volumes of consumption, but by accelerating cultural evolution’s capacity to overcome barriers and give us to access more and more resources.
Their obsession with fixed, external limits, however, has led the overpopulationists to completely misunderstand the fundamentally dynamic nature of human population growth. We are a rocket flying toward orbit. The relationship between population and the environment has been utterly transformed. In the early stages of our flight, blows to our population did indeed reduce consumption. Today, population and consumption are decoupled. The Demographic Transition has slowed population growth and is likely to end it completely within a century, but consumption will continue to increase regardless. By focusing on consumption rather than cultural evolution, though, they’ve overlooked the variable that will determine our future trajectory.
The very things that make cultural evolution a uniquely rapid and powerful mode of adaptation also make it uniquely vulnerable to disruption. The evolutionary potential of our species today continues to increase not through our simple abundance but through ever-increasing connectivity and collaboration. By hindering the potential of our “Ultimate Resource,” any political trends or natural disasters that limit or sever those connections (the early impacts of climate change, for instance), even if they lower birthrates or standards of living, can only make things worse. We are in a critical stage of our flight. To slow innovation while continuing to consume the resources we use now, with the techniques we have available now, would guarantee disaster even if a few billion people were snapped out of existence.
There is an optimistic inverse to that point, however. The coming end of population growth poses no limit to the acceleration of our adaptive potential. As long as we put them to their best use through education, internet connections, open borders, and truly free trade, the minds already being born can overcome the problems posed by climate change, reverse the ecological damage we’ve caused, and finish realizing Malthus’ vision of a world without poverty. The specifics of these solutions are, from an evolutionary perspective, immaterial. Whether they end up favoring markets or regulations, solar or nuclear energy, eco-friendly organic or high-density conventional agriculture, and whether we call it degrowth or green growth, capitalism or socialism, they have to solve the same problems and will require the same level of combined, cooperative ingenuity.
[1] ↩
The standard form of the logistic population growth equation is
where t is the unit of time (eg years), r is the growth rate of the population per year, N is the population in the current year, and K is the carrying capacity, and dN/dt is the actual change in the population from this year to the next (transformed to a percent change by dividing by N). The parenthetical term (1-N/K) thus modulates potential growth: when N/K is small, growth is determined entirely by r, but as N approaches K, growth approaches 0. If the population exceeds K, the parenthetical term becomes negative, causing the population to decline.
This version of the equation was modified from this original form
where α is the amount of growth prevented by the presence of other individuals (called the density-dependent crowding factor). Both functions describe a logistic growth curve with a horizontal asymptote at K or r/α. If r/α is substituted for K, the equations are algebraically identical.
The problem is that the r-K equation conceals the fact that K includes r and thus invites us to think of them as independent. Specifically, it invites us to think that r includes all density-independent environmental effects and K only includes density-dependent effects. Ecologists thus often observe that deaths match or exceed births (ie, r is at or below 0) in a given habitat, but still posit a positive K. Because K= r/α, a negative or 0 r means a negative or 0 K; if this adjustment is not made, the equation produces paradoxes: populations over carrying capacity with a negative growth rate grow to infinity because the negatives cancel out; populations with a growth rate of 0 remain constant regardless of where they are relative to K — where in both cases the r- α equation shows the population should decline by αN or more. The confusion is exacerbated by the fact that r is often presented to students as the “intrinsic” rate of growth for a given species, something that represents its maximum possible reproductive rate in ideal conditions which is therefore always positive. Coupled with an always-positive K representing “carrying capacity,” this definition leaves no room at all for density-independent effects on reproduction imposed by the environment.
The r- α equation might feel more opaque — density-dependent and density-independent effects are less intuitive than a growth rate and a limit to growth — but the r-K equation contains precisely the same problems. It just obscures them in a way that makes them feel more accessible than they actually are, engendering precisely the confused overconfidence that plagues the overpopulation debate.
It’s also worth noting that no matter how you rearrange this equation, r and α (and thus K) cannot be directly measured. Deaths and births can be measured, but it’s impossible to completely disentangle their density-dependent and density-independent causes. Population models, even quite accurate ones in simple, controlled systems, are thus approximations, not calculations through the logistic equation.
See Mallet 2012 for the original formulation of this argument.↩
[2] ↩
That’s not to say Ehrlich’s thinking on overpopulation was entirely
pure of racism. His motivation to explore conservation came from
personal experiences of habitat destruction in areas he liked to collect
insects as a child, but his introduction to conservation ideology came
from books (mostly forgotten today) that were openly and explicitly
eugenicist. By the time he became a biology professor, however, Ehrlich
believed race had no biological basis, and as a political activist, he
considered himself a liberal and an anti-racist. While the Population
Bomb opens with a fairly troubling paragraph about the profusion of
human flesh in a densely populated city in India, his main concern as an
environmentalist was reducing the number of Americans, because their
consumption habits made them disproportionately harmful to the
biosphere. Ehrlich spent years crafting and advocating for an
anti-racist immigration restriction policy, one that would reduce the
number of future Americans without discriminating based on race. His
organization Zero Population Growth draw criticism from Garrett Hardin
because, by trying to lower birthrates among white Americans, it was
exacerbating the real problem posed by overpopulation: “white genocide.”
Whether that rescues him and his argument from being fundamentally
racist, or an accessory to racism, you’ll have to decide for yourself.
A
full intellectual biography of Ehrlich, and of Julian Simon, can be
found in the main source for this section, Paul Sabin’s book The Bet. ↩
[3] ↩
For a good introduction to what cultural evolution is, how it works, why we know it’s a good theory, and why it emerged in Homo sapiens and no other species, see Joe Henrich’s The Secret of Our Success.↩
[4] ↩
See Joel Mokyr’s A Culture of Growth
for a cultural evolutionary exploration of what, specifically, those
conditions were and why they came about when and where they did.↩
[5] ↩
That said, Malthus was undeniably a racist. In the second edition of his Essay on the Principle of Population,
he credulously passed on wildly racist reports from imperialist
explorers and scholars regarding the state of indigenous communities in
Africa, the Americas, and Australia. He also speculated blithely about
the possibility that colonists would increase the productivity of those
continents by either eradicating or “educating” the natives to impose
European land management techniques.↩
[6] ↩
The claim, made by Giorgos Kallis in his book Limits,
that Malthus saw the suffering of poverty as a divine goad to ceaseless
economic growth, rather than an evil to be eradicated by embracing
abstinence, is entirely unsupported by what Malthus actually wrote, at
least to my reading. He is perhaps the most widely and thoroughly
misrepresented thinker I’m aware of, and if you’re tempted to dismiss my
claim (or accept it and repeat it yourself) you should really read the Essay and make up your mind for yourself. It’s available for free online. Robert Mayhew’s intellectual biography and retrospective Malthus: The Life and Legacies of an Untimely Prophet was also enlightening.↩
[7] ↩
Malthus did engage in this speculation elsewhere, however. He guessed
that Britain’s population would double or triple in the next 200 years.
This might seem low compared to a future in which we know the population
grew 6 times in that period, but it was far from pessimistic at the
time.↩
[8] ↩
The popular understanding of this fairly uncontroversial
observation — that evolution makes animals make good decisions — has
been set back immeasurably by the use of the word “rationality.” The
word conjures an image of a dispassionate accountant calculating the
best choice, allowing no emotions, biases, or superstitions to cloud
their judgment. Economics, anthropology, and biology have actively
perpetuated this framing by studying the behavior of individual animals
or humans in explicit comparison with this ideal. Animals are said to
behave “as if” they were rational, though of course it would be
anthropomorphic to imply they were literally thinking like an
accountant. This is utterly backward. The general notion referred to as
“rationality” is, by default, an unconscious process of optimization and
adaptation defined by uncertainty and failure and which acts through
biases, emotions, superstitions, rules of thumb, and flawed reasoning.
The human ideal of rationality is simply a new expression of the same
process. It has frequently overruled prior adaptations, often creating
meaningful improvements but often, in its hubris, destroying them with
catastrophic effects. ↩
[9] ↩
This dynamic, in which reproductive output is heritable and is strongly
selected for, is a core element of Garrett Hardin’s argument (echoing
one of Darwin’s eugenicist grandsons) in “The Tragedy of the Commons.”
However, Hardin profoundly misunderstood (or maliciously misrepresented)
the relevance of this process for humans. He led his readers to believe
that humans have large families to satisfy an irrational emotional need
despite the economic costs of doing so. To the extent that white people
were smart enough to notice this cycle and intentionally end it, they
became rich. Other races, however, were not willing or able to make this
choice, so they continued to multiply.
The
truth is that while Hardin was right that this process is a fact of
evolution, inevitable given a sufficiently long timescale without other
changes, it is one that has only barely begun in humans, if at all.
Natural selection in the past acted almost entirely on lowering
mortality. By favoring cultural adaptations that maximized wealth, this
caused family size to decrease with wealth, not increase. After
generations of uniformly low mortality, however, natural selection
begins to favor variants that increase fertility. The result is that
selection for fertility is, ironically for Hardin, likely to emerge
first in Scandinavia, not Africa.↩
