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

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.

Intergenerational capital flows are central to fitness dynamics and adaptive evolution in humans.

Human fitness dynamics are uniquely and profoundly governed by the flow of capital to subsequent generations. Low socioeconomic status individuals may possess limited capacity to direct capital to descendants and may respond to such constraints adaptively or maladaptively. Mitigation of capital constraints may provide practicable routes to alleviation of the behavioural constellation of deprivation.

Genetic Associations Between Personality Traits and Lifetime Reproductive Success in Humans.

Personality has been associated with reproductive success in humans and other animals, suggesting potential evolutionary selection pressures. However, studies to date have only examined these associations on a phenotypic level, which may be inadequate in estimating evolutionary change. Using a large longitudinal twin dataset of contemporary Finns, we compared the phenotypic (breeder's equation) and genetically informed (the Robertson-Price identity) associations between lifetime reproductive success (LRS) and two personality traits-neuroticism and extraversion.