blogs:
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.
February 15th, 2013 | 9:00 am
From the blog:
I’m an evolutionary partisan, and even I don’t grant that assumption.
February 15th, 2013 | 12:10 pm
When the authors discuss “fertility,” they are discussing the propensity to have kids. In short, they’re arguing that people who come from big families tend to produce big families. All else being equal, as each generation tends to have ever more people with a bias toward big families, we would expect the population to grow.
Note that the tendency to have a big family depends on 1) biological capacity, 2) capacity to attract a mate, and 3) social/psychological factors, such as a desire to have a big family. I suspect this last variable – the desire to have a big family – is the dominant factor for determining the “heritability of fertility.” That is, I suspect that the propensity to have a big family is more a social than a biological variable.
But imagine it weren’t. That is, imagine that family size were mostly a function of biology.
Attributes that tend to result in genes being passed to the next generation get reinforced in future generations; attributes that tend to impede genes being passed to the next generation tend to get suppressed. All else being equal, high fertility would tend to help genes get passed to the next generation.
But all else may not be equal. The authors cite studies suggesting that households with fewer kids are able to expend more resources on each kid, and that these kids acquire social advantages that make them more likely to reproduce than would otherwise be the case. Hypothetically – and this is the big assumption here – the reproductive advantage of having fewer, but more advantaged, kids could offset the reproductive disadvantage of having fewer kids.
If this hypothesis were accurate, then in a world without effective contraception – or among the strict Catholics in our own world — evolution might come to favor less fertile people. That would be the ultimate in Natural Family Planning!
February 18th, 2013 | 9:29 am
Note that, in this model, evolution would gradually work to make NFP less reliable…
February 19th, 2013 | 11:46 am
I’m not sure what numbers the author references, but fertility in almost all developed nations (save Isreal, Austrailia and New Zealand) continue to scrape well below replacement levels (2.1 births per female). Yes, in France the TFR rose from 1.67 to 1.78 for a few years. But, it is still well below replacement levels, and just as important, this increase has only lasted a few years. The sad fact remains that Europe will begin losing population within 3 decades (In Russia and much of Greece and Italy population decreases are already a fact). Japan is losing population (has been for a number of years now), and China will have over 360 million eldery by 2050.
End genetics have little to do with it. It’s all behavior.
February 19th, 2013 | 11:56 am
Additionally, using the UN demographic projections as source material is probably not a good idea. Their population growth models have no basis in current global trends. I’m not exactly sure how they arrive at such fertility increases, as trends all over the world through 2011 surely don’t back their forecasts. As a matter of fact, the birthrate for the US in 2011 hit a historic low of 63 live births per female.
And from a purely statistical point of view, their projected growth rates are almost impossible to arrive at, for the number of fertile women will be decreasing, not increasing. In China, a plunge in fertility and hence population is almost assured; the ration males to females there is 123 to 100. In China, Europe, Russia, and Japan as the populations age and the number of women in their prime fertility years decrease, future women would need to bear between 2 and 3 children just to stop the hemorraging.
February 19th, 2013 | 12:55 pm
WOW. And that’s low?
I’m reminded of the standard line regarding population control: “Somewhere in the world, a woman gives birth four times every second. This woman must be found and stopped.”
February 24th, 2013 | 6:07 pm
JP:
“Yes, in France the TFR rose from 1.67 to 1.78 for a few years.”
?
Fertility rates rose from 1.66 in 1993/4 to 1.78 in 1999, yes. They then continued to keep rising, to 1.87 in 2000, and 2.00 in 2010. They hardly fell.
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