Evolutionary Models of Fertility

Fertility rates have been falling all around the world for decades. Any forecast of human populations without the pollyannish assumption that fertility rates will stabilize automatically projects that global fertility rates will soon fall below replacement.

This is a worrying trend because people, especially people in the developed world where fertility rates are lowest, are the most important input into economic growth and technological progress. If we start to lose population, economic growth rates go to zero in the best case and probably turn negative due to the social and political consequences of falling population and stagnant economies.

The persistence and pervasiveness of falling fertility makes it feel inevitable but there is an intuitive model in which fertility will always rise back up in the long run: natural selection. Natural selection spreads traits that are associated with reproductive fitness and few traits are better associated with reproductive fitness than fertility.

This suggests that the last few centuries of declining fertility may not be a move towards a new stable equilibrium of low fertility but rather a disequilibrium transition before some group with high reproductive fitness wins out.

Kolk Model and Mass Media Extension

There are several papers studying evolutionary models of human fertility but I am going to focus on Martin Kolk, Daniel Cownden and Magnus Enquist’s 2014 paper because it is the simplest while also explicitly modeling both cultural and genetic inheritance.

The paper begins with an empirical observation: in pre-demographic transition societies, fertility outcomes were not heritable, but for modern, industrialized societies the heritability of fertility is high and increasing.

Human societies had tens of thousands of years to optimize for reproductive fitness in subsistence-agricultural civilization. When an outcome like high fertility is strictly optimized by evolution, you don’t see much variance in that outcome. Everyone, regardless of genetic predisposition, cultural ideals, or personal preference displays the high fertility phenotype. Thus, you don’t get much extra information about a person’s fertility by learning about the genetics, culture, or personality of their parents; heritability is low.

The industrial revolution changed the evolutionary playing field so that the new measure of reproductive fitness in industrial civilization was no longer strictly optimized for. A number of heritable traits, including genetics, culture, and religion, modified people’s response to the environmental change of the industrial revolution, introducing heritable variance into fertility outcomes.

This introduction of heritable fertility outcomes suggests that the demographic transition might be self-balancing and temporary. Here’s Kolk et al’s simple model of this change:

In each generation, individuals inherit the lifestyle preferences of their parents. For simplicity, we assume that there are only two possible lifestyles, one with low fertility (L) and the other with high fertility (H).

Individuals then acquire their preferred lifestyle if it is displayed by at least one cultural role model, including parents, otherwise they acquire the other lifestyle. Individuals then produce a variable number of children based on their acquired fertility lifestyle. This process is then repeated every generation with new individuals acquiring the lifestyle preferences of their parents.

So each person inherits a fertility preference from their parents, and then they are randomly assigned some additional cultural role models from the population. If they have a role model whose lifestyle matches their preference, they’ll copy them, otherwise they’ll copy the only lifestyle they know.

Kolk et al somewhat artificially imagine the pre-demographic transition world as one where everyone has the high fertility lifestyle, while most of the population actually has low fertility preferences. But since everyone’s role models are high fertility, everyone continues on with that lifestyle regardless.

Then, and again quite artificially, the authors flip 20% of the population into the low fertility lifestyle, aiming to reflect the social and economic changes that precipitate the demographic transition. Now, all the latent low-fertility preference people have role models to follow and they switch over.

But that switch lowers their fertility, so the population of low-fertility-preference people starts to shrink relative to the high-fertility group. Thus, global average fertility falls for several generations but eventually picks back up.

One extension I made to this model which makes it slightly less contrived is to model the demographic transition as the result of the introduction of various forms of mass media that expands the number of available role models for each person, e.g like soap operas collapsing the fertility of rural Brazilians as they are rolled out.

That way, instead of just exogenously flipping 20% of the population to the low fertility lifestyle, you can have stably high fertility coexisting with a small population of low fertility people for a long time because the number of role models is small, so few of the low-fertility-preference masses ever encounter their preferred lifestyle. Then, once mass media exposes the low fertility lifestyle to the world, it quickly grows to take over. The rebound of fertility as the high fertility group outcompetes the rest works the same.

Responses

The motivating logic behind a high fertility group eventually growing to outcompete less fit low fertility groups is strong, and Kolk’s model elegantly produces this behavior. But I don’t think these or other outcomes of evolutionary fertility models are enough to rely on.

For Kolk’s model to work, you need a tinder-box population with low fertility preferences that are pushed into high fertility lifestyle nonetheless. It's not clear to me that this accurately describes humanity before the demographic transition.

If the demographic transition is not caused by a one-time flip of repressed low-fertility preferences and is instead caused by a more uniform environmental force that lowers the fertility of all groups, then natural selection isn't guaranteed to turn things around. Mormons were a high fertility subgroup with many inheritable traits related to fertility, but they seem to be on the same slope of fertility decline as the rest of the country, just at a higher level.

The fact that heritability has increased in recent decades tells us that the average decline can't be completely uniform across groups, but if genetic or cultural variance determines where your fertility will fall in a range of 0.5 - 2.0 then the population still collapses.

There is also the story of cultural evolution just outpacing natural selection and maintaining the low fertility meme in a shrinking population. The Kolk model assumes perfect transmissibility of lifestyle preferences from parents to children, but if children’s preferences are sometimes changed by cultural memes then the effective replacement level fertility for the high-fertility phenotype rises above replacement. E.g if the Mormons have three kids but on average only one of their kids inherits that preference and the other two have one kid, then the high fertility phenotype would soon die out.

So there are evolutionary models that predict fertility rebound and there are models that predict continued decline. I don't think purely theoretical considerations can decide between the two so this is ultimately an empirical question.

The fertility rebound may always be just around the corner in the data, but the evidence doesn't look great so far. France, who had a particularly early demographic transition, is hovering below replacement with no clear upwards trend. Things don't look better in East Asia or in Sweden. Although completed cohort fertility looks more stable, there’s still no evidence of turn-around.

Some subgroups like the Amish or Haredim have had more success maintaining heritable, high fertility. So they are growing as a share of the world's population and, so far, imperceptibly raising global fertility. They will have to continue this success for many more generations to have a chance at turning the aggregate trends around.