Do Grandmothers Promote Inclusive Fitness?: Evidence from the Utah Population Database

Ken R. Smith, University of Utah
Edward Christenson, University of Utah
Alan Rogers, University of Utah

The Grandmother Hypothesis suggests that menopause stops a mother from investing in offspring that are less likely to survive to reproductive maturity and allows that investment to be applied to her earlier offspring and their progeny. This should increase a women's own inclusive fitness over what she could have realized by having more offspring of her own. There have been few tests of these hypotheses. We test key aspects of the hypothesis by using the Utah Population Data Base (UPDB) based on data from over 40,000 mothers, their mothers (and fathers), and 185,000 of their offspring, all drawn from a period of time reflecting natural fertility conditions in the Western frontier (mid to late 1800s). We find evidence indicating that grandmothers are associated with shorter birth intervals of their daughters, and favorable survival patterns for their grandchildren.