Not Cheaper by the Dozen

Abstract

GENETICS Fitness, defined as an individual's reproductive success, is measured by the number of offspring, and parents with more offspring are generally considered fitter. However, Penn and Smith show that in married couples in preindustrial societies, the survival of both the mother and the father depended on the number of offspring the wife had and families with fewer children had more surviving grandchildren. In an analysis of 21,000 records of survival in late 19th-century Utah, survival rates of the parents and children were significantly lower in the largest families, that is, those with 12 or more children. The negative effect of bearing children on fitness was stronger in females, lasted after menopause, and was evident even in women with only 1 to 3 offspring. However, after the birth of the fourth child, the risk of death became significantly greater in fathers, too. In addition, those children with the largest number of siblings were less likely to survive to the age of 18, with the youngest siblings at greatest risk. The ability of human females to survive long after the loss of reproductive function has previously been explained by the positive effect a mother's survival has on the success of her offspring; without a mother, survivorship of the children is lowered. Nevertheless, this study demonstrates that there may be a selective pressure for females to stop reproducing before they complete child rearing. — LMZ Proc. Natl. Acad. Sci. U.S.A. 104 , 553 (2007).