Fertility and longevity in twins, sibs, and parents of twins

Abstract

Genealogical records containing birth and death dates for completed families were analyzed to compare the longevity of twins, sibs and parents. The data are restricted to twins and sibs who survived to adulthood and married. The findings, similar to those found with respect to fertility (Wyshak and White, 1969), show that twins, especially male twins, are disadvantaged in comparison with their male sibs. Sib-sib and parent-offspring correlation analysis confirmed that there is a genetic component in the determination of life span, but environmental factors contribute more to the total variation. No evidence of a stronger maternal than paternal effect was found. Twin bearers also lived longer than non-twin bearers. Even among persons who survived to age 50 or more, parents of twins had more children and lived slightly longer than their twin and non-twin offspring. Regression analysis for persons who survived to 50 or longer indicated that, in addition to life span of parents, secularity (year of death) and fertility (number of children borne) were the best predictors of longevity, though only a small proportion of the variation could be accounted for by these and other demographic variables. Life span has shown a consistent increase over time from the 17th century through the 19th century, while fertility has tended to decline. Among persons surviving to age 50, when the relation between secularity and fertility and secularity and longevity is controlled, a significant correlation between fertility and longevity remains. This relation, observed in populations that did not practice voluntary family size limitation, would not be found in contemporary data. Maternal mortality accounted for the shorter life span of women than for men; eliminating its effect gives women a slight advantage. The fertility and longevity experience of migrants who survived to age 50 is more favorable than that of persons who did not migrate.