One Child

The anthropic principle says that we shouldn’t be surprised by necessary conditions for our existence, because of selection bias: if the conditions weren’t met, we wouldn’t be around to be surprised. The most famous example comes from cosmology, couertesy of Fred Hoyle. Most life forms on Earth are Carbon-12 based, and the only known way get elements heavier than Lithium was through stellar nucleosynthesis. Therefore, it must be possible to form Carbon-12 in stars (even though the mechanism wasn’t understood at the time of the hypothesis).

What works for cosmology also works for demography. Pre-industrial societies had very high mortality. Any society that survived despite this high mortality must have also had high TFR (children born per woman). Therefore, if you’re alive today, your ancestors were almost certainly part of a high-TFR society. As a rough rule of thumb, these societies had to have TFRs of at least 5 in order to survive to modernity.

This observation isn’t vacuous — some past societies did dramatically shrink their populations through low TFRs. One interesting example comes from 1800s Hungary. Vasary’s “The Sin of Transdanubia”:

From the late nineteenth century there was a rigorous system of fertility control in certain regains of rural Hungary for which there have been few parallels. The Hungarian ’egy gyermek rendszer’ translates literally as ‘one-child system’. ‘One-child’ because it refers to the practice of couples aiming towards a completed family of one child, whether male or female. ‘System’ because it was not a question of a random scattering of families that, for a variety of reasons, had few children, but a community-wide practicy of rigorously controlled fertility with a comprehensive system of practices and ethos backing it up.

Ansley Coale commenting on the same situation:

The instances of pre-industrial parity-related limitation of marital fertility cited earlier are examples of how such limitations can reduce fertility below the level consistent with survival. In the villages of Baranya, known by tgheir nineteenth-century contemporaries as addicted to the one-child family, in which marital fertility was very low and strongly restrained by parity-related measures, mortality was still high. In Besence and Vajszlo more than 20 percednt of the newborns died before reaching age one until late in the nineteenth century; many of the villages of southern Transdanubia had a negative rate of natural increase [i.e. a population drop] as a result of such low fertility while mortality remained high. In the French départements where fertility was already low in the early nineteenth century, the rate of childbearing was not adequate to replace the population. In both Lot-et-Garonne and Calvados, the population declined steadily after 1836, by a total of 20% in one instancce and 25% in the otherk, with no substantial out-migration.

In contrast, Benjamin Franklin estimated that Purtin New England had a TFR of about 8. (Franklin himself was the descendent of Puritans.) Others came up with lower estimates, e.g. K.A. Lockridge’s estimate was ≈5, but either way, a one-child policy it was not. The population of the Bay Colony grew from about 20,000 in 1660 to 200,000 by 1760, a growth rate of 2.3% per annum. (Hard to substantiate, but the consensus seems to be that after the Great Migration, the growth was primarily TFR driven rather than immigration driven.)

It’s not surprising that Puritan New England played a bigger role in American History than rural Baranya did in European history, or that more people today are descended from the former than the latter. One is halving every 30 years, and the other is doubling. If you started with 10k people in each society, after 100 years you’d have ≈1k in Baranya and ≈100k people in New England. Simplistically, someone alive 100 years later is 100x more likely to come from New England over Baranya.

So, again, if you’re alive today, then it’s overwhelmingly likely that your ancestors were part of a high-TFR society in the past. And you should expect all sorts of high-TFR beliefs to be prevalent. Hence things like Catholic prohibitions on contraception.

What’s interesting is that TFRs have fallen off a cliff, all across the developed world. Even the virtual elimination of child mortality hasn’t been enough to offset this. So your ancestors all had high-TFR beliefs, but you probably don’t. And this happened slowly, over time: