Fertility is going to go up

In my latest working paper, co-authored with Oliver Richards, we argue that recent fertility increases in developed countries may only be the beginning. From the abstract:

We propose that the recent rise in the fertility rate in developed countries is the beginning of a broad-based increase in fertility towards above-replacement levels. Environmental shocks that reduced fertility over the past 200 years changed the composition of fertility-related traits in the population and temporarily raised fertility heritability. As those with higher fertility are selected for, the “high-fertility” genotypes are expected to come to dominate the population, causing the fertility rate to return to its pre-shock level. We show that even with relatively low levels of genetically based variation in fertility, there can be a rapid return to a high-fertility state, with recovery to above-replacement levels usually occurring within a few generations. In the longer term, this implies that the proportion of elderly in the population will be lower than projected, reducing the fiscal burden of ageing on developed world governments. However, the rise in the fertility rate increases the population size and proportion of dependent young, presenting other fiscal and policy challenges.

We’re certainly not the first to hint at the idea that selection of high fertility individuals will increase fertility. Fisher noted the power of higher fertility groups in The Genetical Theory of Natural Selection. I’ve seen Razib Kahn, Robin Hanson and John Hawks mention the idea in blog posts. There is one great paper by Murphy and Wang (which I will blog about soon) that has part of this argument buried in the micro-simulation. Many papers on the heritability of fertility hint at it. Rowthorn’s paper on fertility and religiosity also points in this direction. But what we couldn’t find was someone who sought to tie down the idea - particularly in the way we have.

I actually thought a paper of this nature would already be written. We were interested in the economic implication of the argument, but because there was no clear statement of the evolutionary foundations that we could use in the way we wanted, we decided to build our argument from the ground up. We’re hoping that this working paper receives some solid critique that will allow us to decide whether our angle of attack is useful or can be improved. We have constructed three basic genetic models, but are they useful? Are there better alternatives? Once we address those questions, we have some ideas for empirical tests and we hope to use the concept in some more detailed economic and cultural analysis. Ultimately, this paper will need to be tied in with a large and growing literature on the biosocial basis of fertility.

I’m the first to admit we could be wrong in the prediction of a fertility increase. What other shocks are still to come? Will the continually changing environment drown out the underlying evolutionary dynamics? Our instinct is that most of the shocks that can affect fertility have played out in the developed world - increased incomes, effective contraception, female choice and so on. But what further shocks could reduce fertility?

In presenting this paper, we tend to receive  two major classes of response. The first and obvious question is whether these dynamics will play out in time frames that matter. As its been 200 years since some populations underwent the demographic transition, there has  been enough time for selection to have occurred on a trait as important to fitness as fertility (in some populations we have evidence of this). The more interesting question is what will be the magnitude of the effect over the next 50 or 100 years? I’m not sure of the answer to this, but even a small total fertility rate increase of 0.1 children per female can have material effects on population size and structure.

The other response we tend to receive is that fertility is affected by policy, incentives, female opportunities and so on. Any trend we see today is a response to those factors. And that may be true. But to the extent there is variation in the response to the policy, incentives or opportunities and there is a genetic basis to that variation, we can see selection of those with higher fertility.

Having said that, throughout the paper we are deliberately agnostic about the merits of the various theories of what has caused the fertility decline in the developed world to date. For the purposes of our hypothesis, it is sufficient to know that there was a decline in fertility and that variation in fertility is heritable after the decline. As the first law of behaviour genetics is that all human behaviour is heritable, its not a very high bar to clear. It would be difficult if it was otherwise, particularly when you consider the raft of current theories. And even if you believe a certain factor is behind the decline, what is the causative pathway? As an example, consider the spread of the pill and the factors which are relevant to it reducing fertility. First, there is desire of someone with access to the pill to have children. Then there is their desire to take the pill to control pregnancy. Do they take the pill as instructed (possibly related to conscientiousness)? Is the pill physiologically effective? Do they experience side-effects that deter continued use? Variation along any of those dimensions would affect fertility.

The biggest simplification in the way we present our models is that, unlike our models, developed countries did not receive a single fertility shock across the population. Rather, multiple shocks hit different parts of the population at different times. This is why fertility has generally declined for much of the last 200 years, rather than suddenly suffering a single large drop. Of note, fertility tended to decrease among the wealthy first. As our framework would suggest that fertility rates will increase first among groups that experienced the shock earlier, we would predict that groups with a history of higher socioeconomic status will tend to increase their fertility rates earlier.

Immigration also presents some interesting issues. Immigrants tend to have higher fertility when arriving in a country that has undergone the demographic transition. But our framework would suggest that following generations will experience a decline as they undergo the fertility shock. To the extent that the immigrant population has not been exposed to the shock before, their fertility may decline and recover later than native populations.

These issues offer some basis for testing the hypothesis. But first, we’re keen to nail down some good ways of thinking about the problem. The working paper and the models within are part of that process. So, if you have any thoughts or criticisms, we’d be grateful to hear them.