Below Replacement: How a Falling Birth Rate Reshapes the Politics of Migration

The slowest-moving force in migration politics is also the most decisive: the birth rate. The Factrail causal graph traces a chain from the world's Total fertility rate (births per woman), through the rising Old-age dependency ratio (world), into the Demographic ageing and labour-shortage pressure driver — and from there into the labour-immigration reforms now reshaping destination countries.

What the numbers show. Global fertility fell from 3.31 births per woman in 1990 to 2.20 in 2023, edging toward the ~2.1 replacement threshold the indicator uses as its normal line. Over the same period the old-age dependency ratio — people aged 65 and over per 100 working-age adults — rose from about 9.97% in 1990 to 15.36% in 2023. These are real reported values drawn from the World Bank series sourced to the UN World Population Prospects, with no interpolation.

Why it matters. Replacement-level fertility is the demographic dividing line at which a population renews itself. Sitting just above it globally masks far steeper declines in many high- and middle-income economies, where the working-age base is already shrinking. In the Factrail model, falling fertility feeds the ageing driver (a weakening link on the fertility indicator) while the old-age dependency ratio measures it (a strong strengthening link). Ageing, in turn, exerts upward pressure on the International migrant stock with roughly a one-year policy lag.

From demography to policy. Two facts illustrate the response. Japan created the Specified Skilled Worker visa in 2019 explicitly to fill shortages in one of the world's fastest-ageing populations. But ageing is not destiny: Hungary's 2015 border fence and asylum tightening were paired with pro-natalist family policy, an explicit attempt to answer demographic decline through higher domestic births rather than immigration. The two facts mark the poles of the debate — import workers, or try to raise the birth rate at home.

Relative to the normal line. Fertility is approaching, not breaching, replacement at the world level, so the global indicator has not yet crossed its norm. The old-age dependency ratio, by contrast, has deteriorated well beyond its 1990 baseline — a one-directional structural shift that no near-term policy reverses, because today's elderly cohorts are already born.

Verified vs. uncertain. The fertility and dependency series are high-confidence official data. The causal ordering — fertility feeding ageing, ageing pulling migration — is well established demographically and coded high-confidence in the dossier. What remains genuinely uncertain is the political response: whether ageing societies lean toward migration or toward pro-natalism is a contested policy choice, and the Hungary fact is flagged needs_review given its sensitive framing.

Alternative explanations. Labour shortages can also be met by automation, raising the retirement age, or lifting female and older-worker participation — substitutes for migration not captured in this dossier. Pro-natalist incentives have a mixed empirical record and act only with a generational lag, so they cannot resolve near-term shortages even where pursued.

What may happen next. If fertility stays at or below replacement while the dependency ratio keeps climbing, the Factrail model expects sustained structural pressure for labour migration, with skills-based reforms spreading even where asylum policy tightens. Pro-natalism may slow the trend over decades but is unlikely to reverse the near-term arithmetic. Sources supporting this analysis include the World Bank fertility and old-age-dependency series (UN World Population Prospects), Japan's Immigration Services Agency, and the Migration Policy Institute analysis of Hungary's demographic-strategy turn.