Twinning and the Inheritance of High Fecundity

George Alter, University of Michigan
Gilles Pison, Institut National d'Études Démographiques (INED)

This paper will examine the importance of a genetic component of fertility by analyzing the birth intervals of daughters and granddaughters of women who gave birth to twins. Women who have twins get pregnant more quickly, and twinning is an inherited trait. We ask whether a family history of twinning reduces the length of birth intervals in subsequent generations. Data are drawn from more than 30,000 family reconstitutions in France from 1670 to 1829 collected by Louis Henry. Event history models suggest that a woman's relative risk of a birth was about five to ten percent higher if her mother gave birth to a twin, and this advantage was passed from daughters to granddaughters. Evidence of inheritance of fecundity disappears after 1780, however, and we suggest that the spread of family limitation makes it more difficult to detect small genetic differences.