Mothers with higher twinning propensity had lower fertility in pre-industrial Europe

Abstract

Historically, mothers producing twins gave birth, on average, more often than non-twinners. This observation has been interpreted as twinners having higher intrinsic fertility – a tendency to conceive easily irrespective of age and other factors – which has shaped both hypotheses about why twinning persists and varies across populations, and the design of medical studies on female fertility. Here we show in >20k pre-industrial European mothers that this interpretation results from an ecological fallacy: twinners had more births not due to higher intrinsic fertility, but because mothers that gave birth more accumulated more opportunities to produce twins. Controlling for variation in the exposure to the risk of twinning reveals that mothers with higher twinning propensity – a physiological predisposition to producing twins – had fewer births, and when twin mortality was high, fewer offspring reaching adulthood. Twinning rates may thus be driven by variation in its mortality costs, rather than variation in intrinsic fertility.

The Evolutionary Puzzle of Human Twinship

“…Twinning is also likely to affect parents' future reproduction, due to its direct (e.g., in terms of mortality risk) and indirect (e.g., in terms of parental investment costs) effects on mothers. Indeed, an analysis of several pre-industrial European populations concluded that a twinning event decreased the chance of a future birth, ultimately leading women with higher twinning propensity to have lower reproductive output (Rickard et al, 2022).…”

“…Recent evidence, however, has cast doubt on these conclusions, as the analytical strategies used to evaluate the empirical data may have failed to account for differential exposure to the total risk of twinning (see Rickard et al, 2012, for technical details). The re-analysis of historical demographic data from pre-industrial Europe by Rickard et al (2022) shows that twinners in fact experience lower fertility compared to non-twinners, after appropriately accounting for exposure.…”

“…Monozygotic (MZ) twinning is normally thought to result from random biological processes, and it occurs at a low, geographically invariant rate across human populations (Bulmer et al, 1970;Hoekstra et al, 2008b); MZ twinning in humans is not all that puzzling. In contrast, dizygotic (DZ) twinning results from polyovulation (i.e., the release of multiple ova during a single fertility cycle), and shows signs of both genetic heritability (Hoekstra et al, 2008b) and geographic heterogeneity, occurring at variable rates in human populations (e.g., from 0.7% to 2.7% of all births; Rickard et al, 2022). DZ twinning in humans is puzzling, both for its persistence despite apparent costs, and for its relatively large geographic variation.…”