Fertility Links

Inquisitive Bird on age and fertility; Nicholas Eberstadt on depopulation; David Friedman on declining marriage; Tove K on parent shaming

Inquisitive Bird writes,

It’s widely believed that fecundity begins to rapidly decline in the mid-30s, while any decline in the 20s is comparatively minor (e.g., Menken et al., 1986). This is roughly what I believed until I read an excellent paper by Geruso et al. (2023).

…According to their analysis, women’s fecundity peaks when they are aged 19-20 years old.

More importantly, fecundity declines roughly linearly between 20 and 40. Contrary to common belief, there is no sudden accelerating decline in the mid-30s. The drop in fecundity between age 20 and 25 appears no less meaningful than the one between age 35 and 40.

…Though the effect is smaller, paternal age also affects the likelihood of a successful pregnancy.

Fecundity means the likelihood of a full-term baby, conditional on pregnancy.

Suppose that the issue is that as people age, their gametes age, and/or the fetus undergoes more mutations in utero. That would explain why the odds of an unsuccessful pregnancy rise with the mother’s age (and to some extent with the father’s age).

It might also explain why there has been such a dramatic rise in neurological disorders in recent decades. Maybe young mothers and fathers are likely to have children without autism, ADHD, and other afflictions, but fewer couples are having children in their early twenties. I wonder what the trend for autism or ADHD looks like if you control for age of parents when the child was conceived.

Nicholas Eberstadt and others write,

Sub-replacement fertility has even come to North Africa and the greater Middle East. (Contemporary Islam has proved scarcely more successful in warding off low birth rates than Christianity, Buddhism, or Hinduism, much less Confucianism: in fact, seven of the very fastest postwar fertility declines occurred in Muslim-majority societies.) Despite pro-natal mullahs, Iran has been a sub-replacement society for about a quarter century. In Turkey (now officially Türkiye) current fertility levels, like Iran’s, are 20 percent below replacement—slightly lower than in the USA. Lebanon and Tunisia are also sub-replacement locales. In the Gulf, Qatar and the UAE have tilted into to sub-replacement fertility. The latest available birth numbers for Saudi Arabia (2022) suggest the Kingdom may also have submitted to sub-replacement.

The fertility decline is broader and deeper than would have been predicted based solely on the relationship between fertility and economic development.

Countries can veer into sub-replacement with lower incomes, less education, less urbanization, and more extreme poverty than ever before. Today in fact three sub-replacement societies are on the UN’s list of “least developed countries” (LDCs), the grouping for the world’s most impoverished and fragile states: Bangladesh, Myanmar and Nepal. Other LDCs may soon be joining the sub-replacement club.

So what explains the worldwide fertility decline?

Given what is unfolding all around us, old “sociobiology” nostrums about the “hard-wiring” of human beings for procreation may need serious re-examination. By the same token, “mimetic” theories of “social learning” may merit a much closer look. On the face of it, we would seem to learn more about humanity’s current demographic moment from Rene Girard than E.O. Wilson.

Turning to consequences:

If we leave out the LDCs, our global 15-39 contingent—the most educated and tech savvy pool of future planetary workers—starts falling in 2027, just three years from now. That means fewer 15-39 year olds not just in the developed countries, but the majority of developing countries as well. That 15-39 group will be getting smaller for about two thirds of the planet. Little surmise about this decline.

… By 2050, there will be hundreds of millions of fewer people under age 60 outside sub-Saharan Africa than today—up to 13 percent fewer, according to some UNPD projections. … Outside Africa, the 65-plus group will double (to 1.5 billion) between today and 2050. That means a steady 2.7 percent growth per annum—faster than Third World population growth at the very peak of the 1960s “population explosion”. The upsurge in the 80-plus population—call them the “super-old” — will be even more radically rapid. Between now and 2050 that total will triple in the non-African world, leaping to roughly 450 million.

David Friedman writes,

Why has marriage become less common and why has the effective term of the contract become so much shorter?

The simple answer is that the amount of time spent in household production has declined drastically

In the industrial era, men tended to specialize in market labor and women tended to specialize in household labor. As this changed, marriage changed. Justin Wolfers and Betsey Stevenson pointed out that marriage became less about production complementarity and more about joint consumption. In the 1970s, many marriages that had been formed under the old rules broke up. By the 1980s, marriages among the affluent became more stable, as they were formed under the new rules.

The new-rules marriages tend to take place much later, when both spouses have completed higher education. This means that they get a late start on having children.

Tove K writes,

I dare to suggest that instinctual parent shaming is the cause with a big C for the demographic crisis. No society has completely eliminated parent shaming and no society should do so. But creating a culture that handles parent shaming properly is essential. It is the measure that needs to be taken.

…People are naturally critical of other people's parenting. People are naturally anxious that other people will be critical of them.

Her point is that we should be less hasty to criticize parental behavior that seems wrong to us. Let the parent allow the kid to walk to school by himself. Let the parent drag a screaming kid by the arm to get someplace.

Putting demands on parents is the same thing as raising the cost of parenting.

substacks referenced above: @

Parent shaming is natural

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Marriage

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Age and infertility