FITNESS OF TWINNING

On the basis of a family reconstitution of the rural Krummhorn populat ion (Ostfriesland, Germany) of the eighteenth and nineteenth centuries , we pursued the question of to what extent the birth of twins contrib uted to the reproductive fitness of their mothers. The twinning rate w as 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 fert ility rates with shorter birth intervals. Parity effects on twinning t endency could not be detected. Twin maternities caused reproductive co sts, namely, increased maternal, infant, and child mortality and obvio usly higher intrafamilial competition, because adult twins had fewer l ocal 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, i s clearly less than the productivity of a single boy. On the other han d, the birth of a female pair of twins led to more live-born grandchil dren than the birth of a single girl. In sum, mothers of twins achieve d greater reproductive success, with 13.5% more live-born grandchildre n, than mothers of singletons only. The results are discussed against the background of Anderson's (1990) error hypothesis of twinning.