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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., (© 2022. The Author(s).)

Based on historical data pertaining to the Krummhörn population (eighteenth and nineteenth centuries, Germany), we compared reproductive histories of mothers according to whether the maternal grandmother (MGM) or the paternal grandmother (PGM) or neither of them was resident in the parents' parish at the time of the mother's first birth. In contrast to effects of PGMs, we discovered conditional differences in the MGM's effects between landless people and wealthier, commercial farmers. Our data indicate that the presence of the MGM only lowers the woman's age at marriage (AAM) and her age at the birth of her first child (AFB) in the case of landless families. However, among commercial farmers, who can generally be characterized by a lower AAM and AFB, we found opposite tendencies for the MGM's effect leading to a relatively small delay in AAM and AFB. Moreover, we also analyzed differences in the completed fertility (i.e., children ever born: CEB). Results indicate that landless families in general do have fewer CEB compared with commercial farmers except for those families in which the MGM has been present. Emphasizing that the adaptiveness of investment decisions should depend on the interaction of genetic, lineage-specific (intrinsic) and ecologically imposed (extrinsic) constraints, we conclude that kin strategies consequently address different fitness components under different conditions.

On the basis of a family reconstitution of the rural Krummhörn population (Ostfriesland, Germany) of the eighteenth and nineteenth centuries, we pursued the question of to what extent the birth of twins contributed to the reproductive fitness of their mothers. The twinning rate was 16.2/1000; the secondary sex ratio among twins was 0.93, and it was 1.16 among their singleton siblings. Mothers of twins were older, had a longer generative life phase, and achieved higher age-specific fertility rates with shorter birth intervals. Parity effects on twinning tendency could not be detected. Twin maternities caused reproductive costs, namely, increased maternal, infant, and child mortality and obviously higher intrafamilial competition, because adult twins had fewer local marriage chances and to a higher degree were forced to emigrate. These reproductive disadvantages mean that the productivity of a male pair of twins, as measured by the number of live-born grandchildren, is clearly less than the productivity of a single boy. On the other hand, the birth of a female pair of twins led to more live-born grandchildren than the birth of a single girl. In sum, mothers of twins achieved greater reproductive success, with 13.5% more live-born grandchildren, than mothers of singletons only. The results are discussed against the background of Anderson's (1990) error hypothesis of twinning.