Fitness of twinning.

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.