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Japan’s Population in the Next Thousand Years: Fertility, Assortative Mating, and the Shape of the Future
Two years ago, I wrote about the likely dynamics of population size in advanced capitalist countries, using Japan as my example. People note that in advanced countries the TFR is well below replacement, and then conclude that the populations in these countries are on the road to extinction. 'More migration,' they cry, showing that these discussions are essentially ideological rather than scientific.
The misfortune of the human race, if we can call it that, is that we evolved - mostly - to like having sex rather than babies. For most of our evolutionary history one thing led to another, however, so this didn't matter in Darwinian terms. However, modern capitalism has provided both effective contraception and many economic and personal incentives to do more interesting things than change nappies and deal with troublesome toddlers. This disconnection between sex and reproduction has led to the collapse in TFR.
However, a minority of men and women actually do want to have families - their emotional drives prompt them to have children despite the many distractions. Evolutionary theory tells us, of course, that future populations are constructed from their ancestors who reproduced, indicating that the prevalence of alleles which promote child-bearing directly will tend to increase in the population, generation by generation.
This is selective advantage in operation: over time the population will be replaced by individuals who actually want to raise families. Let's see how this would work out in Japan.
Japan currently stands near 123 million people, yet its total fertility rate (TFR) is ~1.3, far below the ~2.1 births per woman needed for replacement. What follows sketches thousand-year outcomes using simple demographic arithmetic and population-genetic reasoning.
1. Straight-line decline
With TFR ≈ 1.3 and no offsetting forces, each generation replaces ~60% of itself. Headcount falls steeply: a remnant population of only tens of millions by 2200 and potentially a group in the hundreds of thousands by 3000. Nevertheless, this is a cultural and institutional contraction rather than a biological extinction risk although it's hard to see an advanced civilisation surviving in anything like its current form.
2. Genetics and slow selection
Completed fertility has modest heritability (≈0.2). Directional selection for a stronger “baby-drive” nudges fertility up by roughly +0.01 to +0.03 births per woman per generation. Timescale to reach replacement by genetics under the assumption of random partner selection alone is centuries; the population would likely bottom at a few percent of today’s size before any rebound.
3. Immigration as a bridge
Immigration offsets natural decrease for 1–2 generations because empirically, migrants arrive young and initially have slightly higher fertility. Likely convergence to host norms and global low fertility limit this as a century-scale fix. It buys time; it does not determine the endpoint.
4. Assortative mating as the hidden accelerator
Key dynamic: if a minority of strongly pronatal men and women mostly partner with each other, their subpopulation grows multiplicatively while the mainstream population shrinks towards zero.
Japan’s future is not locked to straight-line decline. Assortative mating among pronatal families can bend the curve upward in roughly a century, especially with supportive culture and policy.
Immigration buys time; automation and biotech buffer the costs of small populations. The decisive variables are partner choice, norm transmission, and targeted pronatal policy. These matter more than slow genetic drift alone.
In fact with the pronatal population exhibiting a TFR of 3.0 (three child families on average) it would take less than 250 years for Japan's population to bounce back to its current 123 million people.
