This Zenodo/GitHub repository provides the data and R code behind the paper Mothers with higher twinning propensity had lower fertility in pre-industrial Europe by Rickard et al. (Nature Communications, 2022) 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 physiological predisposition to conceive easily – 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 – in fact had fewer births, and when twin mortality was high, fewer offspring reaching adulthood. Twinning rates may thus be driven by within-individual and among-population variation in its mortality costs, rather than variation in intrinsic fertility.