01 · The question Europe has not yet askedOne number has governed demographic thinking for 70 years. It may no longer be enough.
European demographic policy, Eurostat projections and annual ageing reviews all measure the distance between the observed fertility rate and 2.1.
The Institute for Demographic Future opens a question that the European debate has not yet addressed with full seriousness: is 2.1 still what we think it is? Not because the mathematics has changed. The mathematics is the same as it was in 1945. What has changed is the society on which that mathematics operates.
The figure 2.1 implicitly assumes that almost all women — and men — in a population become parents at some point in their lives. That assumption was empirically defensible in mid-20th-century societies. It is no longer empirically defensible in European societies in the second quarter of the 21st century.
If this hypothesis is correct, the implications for European demographic policy are far-reaching. Slovenia’s distance from replacement fertility may not be 0.58, but considerably larger. Italy’s distance may not be 0.9, but considerably larger. The same logic applies across Europe.
02 · Historical origin of 2.1From Pearson and Lotka to Notestein: mathematics for a world that no longer exists.
The number originates in stable population theory, developed by Karl Pearson and Alfred James Lotka and formalised by Frank Notestein within the Princeton demographic school in 1945. A fertility rate of 2.0 would be sufficient in purely mathematical terms; the additional 0.1 compensates for the sex ratio at birth and mortality before reproductive age.
The key issue: the concept assumes that reproductive participation is close to universal. Variation lies mainly in how many children people have, not in whether they become parents at all. This was broadly true for the 1940 cohort. It is no longer true for the 1975 cohort.
In several developed countries, the share of permanently childless women in the 1975 cohort has moved from a marginal phenomenon into a structural one. This changes the meaning of replacement fertility.
03 · BirthgapMothers still have roughly the same number of children. Something else has changed.
Stephen J. Shaw’s Birthgap — Childless World highlights one of the most striking empirical observations in contemporary demography: in several developed low-fertility countries, the average number of children among women who become mothers has remained close to 2.3–2.4.
The decline in the total fertility rate therefore does not primarily come from smaller families. It comes from a growing share of people who never become parents.
This reframes the European fertility debate. The core issue is not only how many children parents have, but how large the reproductive core of society remains.
04 · Proposed conceptual frameworkEffective replacement fertility.
The fertility level that would have to be reached by those members of a population who actually become parents for the population as a whole to reach replacement.
If 20% of women in a cohort never give birth, the remaining 80% must have 2.625 children on average for the population as a whole to reach replacement. At 30% childlessness, the required level is 3.0 children per mother. At 40%, it is 3.5.
Recalibrating the distances
For Italy, where permanent childlessness is close to 25% in relevant cohorts, the effective replacement level is not 2.1 but around 2.8. For Slovenia, with estimated levels closer to 12–15%, the effective replacement level would be roughly 2.4–2.5.
05 · Empirical frameworkChildlessness is not only a women’s issue. In most European countries, the share is higher among men.
An important but often overlooked pattern is that the share of men reaching ages 45–49 without children is higher than the share among women of the same cohort. The difference is often between 5 and 13 percentage points.
| Share childless (p) | ERF | Empirical comparison |
|---|
| 0% | 2.10 | Lotka–Notestein idealised world |
| 15% | 2.47 | Slovenia, France |
| 20% | 2.63 | EU 1975 cohort average |
| 25% | 2.80 | Italy, Austria, Germany |
| 30% | 3.00 | Mediterranean Europe scenario |
| 35% | 3.23 | Hong Kong |
06 · Four scenarios for Europe 2050A structured thought experiment — not a forecast.
The scenarios below are not predictions. They show what different levels of reproductive-core erosion would imply for Europe’s long-term demographic future.
Scenario A · Stabilisation
Childlessness stabilises around 20%. Population decline continues but does not accelerate dramatically.
Estimated probability · 20–30%
Scenario B · Moderate erosion
Childlessness rises toward 25% in cohorts born after 1985. Mediterranean and Central European demographic decline accelerates.
Estimated probability · 35–45%
Scenario C · Italianisation
Childlessness approaches 30% in cohorts born after 1990. Domestic demographic dynamics can no longer stabilise population decline.
Estimated probability · 20–25%
Scenario D · Polarisation
Society splits into a smaller reproductive core and a larger reproductive margin, increasing pressure on welfare and intergenerational systems.
Estimated probability · 5–10%
07 · Institutional consequencesFour systems calibrated for a different population structure.
If Europe’s effective replacement fertility is not 2.1 but 2.5 to 3.0 children per mother, the consequences are direct: pensions, labour markets, welfare systems and intergenerational solidarity must all be recalibrated.
The erosion of the reproductive core is not the same as population ageing. Ageing is a consequence that is already well measured. Reproductive-core erosion is a cause that remains poorly measured.
08 · Proposal for EurostatReproductive Core Erosion Index (RCEI).
The Institute proposes that Eurostat develop a standardised indicator — the Reproductive Core Erosion Index — tracking three dimensions across EU Member States: the share of the population that never becomes parents, the speed of change across cohorts, and the regional distribution of the reproductive core within countries.
Without measuring the cause, public policy can only describe the consequences. For empirical demographic policy, that is not enough.
“If 2.1 was born in the demographic world of the previous century, what is the replacement fertility of the world we live in today?”
The Institute presents this framework for national and European institutions, parliamentary groups, universities, journalists’ associations and research networks.
info@demografskaprihodnost.si