In An Essay on the Principle of Population, Thomas Malthus in 1798 observed that population tends to outrun food supply, since population increases geometrically while food supply increases arithmetically. He wrote:
The power of population is so superior to the power of the earth to produce subsistence for man, that premature death must in some shape or other visit the human race. The vices of mankind are active and able ministers of depopulation. They are the precursors in the great army of destruction, and often finish the dreadful work themselves. But should they fail in this war of extermination, sickly seasons, epidemics, pestilence, and plague advance in terrific array, and sweep off their thousands and tens of thousands. Should success be still incomplete, gigantic inevitable famine stalks in the rear, and with one mighty blow levels the population with the food of the world.
Imagine that a population of farmers is doing well: They have plenty to eat. It's easy for them to raise more than two children per family—they do so, and the population increases. It continues to increase as long as conditions remain the same. More people need more food, but then there are more workers producing food. As long as per capita production stays the same, the standard of living does not change, even as population increases. However, eventually this expanding population runs out of land, and farmers in the next generation have to farm smaller plots. They may be able to keep per capita production the same by working harder, but in the next generation plots become even smaller. If the methods of food production remain the same, eventually per capita production must decrease as population increases and per capita resources decrease. That decrease will continue until the average farmer produces just enough food to raise two children, at which point population growth stops.
Suppose that farming methods improve, so that productivity per acre goes up by a factor of ten. The population begins to grow—let's say fairly slowly, with each family managing to raise 2.5 children (on average) to adulthood. The population is growing 25 percent per generation. In ten generations—about 250 years—the population has caught up with those improved methods. Living standards are low again, and population growth stops. But 2.5 children per family is by no means an especially high rate of population growth: In colonial America, the average family raised more than 7 children to adulthood. At that rate, population growth could catch up with a tenfold increase in productivity in just two generations.
The point is that even moderate rates of population growth can rapidly catch up with all plausible improvements in food production. Thus, populations should spend most of the time near a Malthusian limit, and there should be no lasting improvement in the standard of living. Malthus himself pointed out that factors other than food shortages can also limit population. Any negative factor that intensifies as population density increases can be the limiting factor—starvation and malnutrition are not the only possibilities. The key is which negative factor shows up at the lowest population density. We believe that the nature of the key limiting factor—which is not necessarily the same in all human populations—can have important effects on human evolution, including the recent changes we have been discussing.
One might imagine that some form of birth control could also effectively limit population, but of course that only works if everyone adopts it. Even smallish groups that do not limit their fertility will rapidly displace (in a few centuries at most) those that do, which brings us back to where we started—a trap where population growth and limitations on population growth keep pace, causing us to remain near the Malthusian limit without achieving lasting improvements in standard of living. In the future, under a disciplined world state, the imposition of birth control could conceivably invalidate the principle, as it could lower the rate of population growth and enable a higher standard of living to take root, but birth control has certainly never worked that way in the past.
War (defined broadly to include all kinds of interpersonal violence) might limit population before starvation occurred if it increased strongly as human density increased. If humans had been unable to form large, well-organized societies, war might have saved us from penury: In fact, war probably has been an important limiting factor in many species other than our own and was probably important for early humans. But humans can cooperate, particularly if there is something worth stealing. In a population with a storable surplus, state formation eventually limited local violence—and peace led to the poorhouse.
Infectious disease is the most serious rival to famine as a population limiter. Certainly the two can work together and often have, since malnutrition can lead to reduced disease resistance, while infectious agents can reduce work output—and thus food production. Furthermore, an infectious disease made worse by population density, or one that killed even well-fed people, could, in principle, be the key population-limiting factor in a society. In such a situation, humans would generally have plenty of food, but other trade-offs would be present. There would, for example, be weaker selection for metabolic efficiency than in a classic famine-driven Malthusian trap. Depending on how most of the people made a living, women might become self-supporting and have a reduced need for paternal investment. There would be strong selection for resistance to the organism or organisms responsible for that strong disease pressure.
Each of those three horsemen—war, pestilence, and famine— has dominated in different populations and time periods.
Primitive warfare was apparently the dominant mechanism limiting population among most foragers before the development of agriculture in the Neolithic period. Infectious disease must have mattered in hunting-gathering societies, but its impact was mitigated by foragers' low population density. Strong climatic swings, such as major droughts or cold waves, must have sometimes rapidly reduced the land's carrying capacity and caused famine—particularly during the climatically unstable glacial periods. But, judging from the abundant evidence of homicide and cannibalism in the archaeological record, our guess is that local violence had a stronger effect. In this sort of system, people were egalitarian, and it shows in the genes: The fraction of men fathering the next generation seems to have been markedly higher than in agricultural societies. Infectious disease, in particular falciparum malaria, may have been the limiting factor in tropical Africa. From what we know, it seems that until very recently population densities stayed lower in Africa than in Europe, the Middle East, or East Asia. The female-dominated farming system seen in much of Bantu Africa, in which women were largely self-supporting, indicates that producing food was fundamentally easier there than in most of Eurasia. In much of Eurasia, hard work from two parents barely allowed break-even reproduction. Disease may have limited the complexity of African state systems—but of course there were many other factors, ranging from Africa's relative isolation from rest of the Old World to elephants attacking the fields of pioneers.13
In many parts of the Old World, particularly among farmers living under strong states, famine and malnutrition were the main factors limiting population. With internal peace, population rapidly bumped up against carrying capacity. In those societies, people living on the bottom rungs of society were regularly short on food, so much so that they often couldn't raise enough children to take their place. However, elites must have had above-replacement fertility, and their less successful offspring would have replaced the missing farmers. Gregory Clark, in A Farewell to Alms, shows that in medieval England the richest members of society had approximately twice the number of surviving offspring as the poorest.14 The bottom of society did not reproduce itself, with the result that, after a millennium or so, nearly everyone was descended from the wealthy classes. There is reason to think that this happened in many places (eastern Asia and much of western Europe, for example), but wealth was not acquired in the same way everywhere, so selection favored different traits in different societies.
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