World

The Fermi paradox is often framed as a discrepancy between the apparent opportunities for technological civilizations to emerge and expand and the absence of unambiguous evidence for them. This paper develops a demographic interpretation of that problem by asking whether demographic transition can operate as a post-technological probabilistic filter on civilizational expansion and detectability. Drawing on demographic transition theory, research on low fertility and population ageing, long-run population projections, and the SETI and futures literatures, the paper treats low fertility as an imperfect empirical proxy for a broader transformation in social reproduction. Mortality decline, reproductive control, ageing, intensive investment in new generations, and changing intergenerational priorities may reduce population-driven expansion pressure and increase the opportunity costs of long-duration settlement. The proposed filter is non-extinctionary and conditional: it applies most plausibly to biological civilizations with costly reproduction and generational institutions. Human demographic trajectories are used as an empirical analogue, not as a universal law for extraterrestrial life. Four stylized pathways are used as stress tests: growth-expansion, demographic homeostasis, low-fertility automation, and post-biological decoupling. The framework separates technological capability from expansionary motivation, institutional continuity, and detectability, and shows why demographic regimes should be treated as explicit variables in long-term models of technological civilizations.

This paper studies the effects of population growth on welfare in an OLG model, in which lower population growth raises output per worker but also reduces the ratio of workers to retirees. Under fairly mild restrictions on the model’s parameters, there exists an optimal population-growth rate—one that maximizes steady-state welfare. This growth rate may well be negative: it may be optimal for the population to shrink over time. The introduction of a pay-as-you-go social security system reduces the optimal population-growth rate.

Many population projections used by international and government agencies assume that, in today's low-fertility societies, future fertility rates will stop declining and roughly stabilize. This paper assesses the evidence for an equilibrium point or rebound from low levels. We show that there is no robust evidence for a systematic fertility rebound, or for a stable equilibrium point at any fertility level conventionally assumed. Instead, where and when fertility has fallen low, it has tended to keep falling. This is true across a range of data sources, time periods, and fertility measures: United Nations data on period total fertility rates for all country-years from 1950 to 2023; Human Fertility Database data on completed fertility for cohorts born between 1935 and 1975 in 31 countries; and fertility rates in subnational geographies in Japan, India, and the United States. We reconcile these results with a prior literature that documented fertility reversals among some low-fertility countries in the early 2000s. Our findings provide an empirical basis for re-examining the equilibrium and rebound assumptions embedded in widely used population projections.

As of 2026, humanity is likely below replacement fertility. That has never happened before, not in wars or pandemics. But the real surprise is that the fall has been concentrated in low- and middle-income countries and among poorer and less educated women. We fit a single-factor model to 236 countries since 1950: the common component peaked in 1978, and what drives fertility down today are country-specific trends, 219 of them negative and not one leveling off. None of the commonly cited mechanisms can account for this pattern, so we offer a conjecture: modernity itself, which makes a third child expensive and childlessness cheap. Children come in integers, so it takes very little to move a cohort’s fertility rate from 1.8 to 1.3. And nothing in an economy pushes fertility back to 2.1. We close with the main economic consequences, in particular slow growth.