Genetic Family Size Preference And The Re-Arming Of The Population Bomb
Some broadly speaking good news: The Population Bomb Has Been Defused.
The United Nations World Population Prospects 2017 show the median projection as stabilizing over the next century:
These projections take a host of demographic factors into account (see the methodology). Underlying them is the three phase model of fertility transition:
“ Once projected fertility reached Phase III (see figure II.1), the second component of the projection procedure implemented a time series model to project further fertility change, on the assumption that fertility in the long run would approach and fluctuate around country-specific ultimate levels based on a Bayesian hierarchical model (Raftery et al., 2014). The time series model used the empirical evidence from low-fertility countries that have experienced fertility increases from a sub-replacement level following a historic fertility decline. Thus, future long-run fertility levels in the 2017 Revision are country-specific, accounting for the country’s own historical experience and also informed by statistical distributions that incorporate the empirical experience of all low-fertility countries that have already experienced a recovery. The world mean parameter for the country-specific asymptotes was restricted to be no greater than a fertility level of 2.1 births per woman.” (emphasis mine)
So in essence they’re saying that once a country reaches phase III, fertility will bounce back a little but then stay low-ish, and they’re restricting the aggregate below the replacement rate of 2.1. Doesn’t that sound like they’re enforcing a flattening? Well, well.
But.
Historically, humans haven’t really needed a genetic preference for family size — as long as sex drive and fertility were above some thresholds, we’d get large family sizes on average regardless.
In a world with contraceptives, education and low child mortality however, family size is a much more conscious choice.
While there are certainly several nurture-side causes behind any particular individual’s preference (religion, culture, family size growing up, etc), it is quite likely that there will be a genetic component as well and that we’re now selecting very strongly for that trait. If 80% of people want 1.5 children on average, and 20% want 3, then the latter group will grow significantly each generation, eventually causing the now flattening population trajectory to start curving up again.
If this hypothesis is true, then we should eventually see continued growth in birthrates after the bounce-back in Phase III described above, and a re-armed Population Bomb.
We will hopefully be able to in aggregate contain people’s reproductive preferences with only mild incentives, though that will then only more strongly select for ever more aggressive family size preferences.
Queue the dystopian sci-fi thriller What Happened To Monday.
Footnotes:
- It will take generations for any genetic effect to start being seen in the aggregate statistics, so any immediate surges in fertility rates are likely to be from other causes. It might still be possible to tease out genetic effects by longitudinal studies of culturally homogeneous groups under selection pressure, i.e. where the cultural norm is small family sizes.
- Yes, this hypothesis is related to the religious / cultural preference for (large) family size, but culture is much more malleable, and religion seems to be in secular decline (though maybe it’ll rebound?), so culture / religion may not have the same lasting force as a genetic preference difference.
