Abstract
We present a quantitative model of sex allocation to investigate whether the simple “rules of thumb” suggested by Trivers and Willard (1973) would really maximize numbers of grandchildren in human populations. Using demographic data from the !Kung of southern Africa and the basic assumptions of the Trivers-Willard hypothesis, we calculate expected numbers of grandchildren based on age- and sex-specific reproductive value. Patterns of parental investment that would maximize numbers of expected grandchildren often differ from the Trivers-Willard rules. In particular, the optimum parental behavior is sensitive to population dynamics, type of parental investment, and, most important, relative ages of sons and daughters. It is doubtful whether a parent blindly following the simple Trivers-Willard rules would maximize numbers of expected grandchildren, on average. In addition, we show that sex-specific infanticide will almost never achieve the goal of maximizing expected numbers of grandchildren.
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This research was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. Linda Mealey, Nancy Howell, S. Ryan Johansson, and several anonymous reviewers all kindly read the manuscript and made valuable suggestions for improving it.
Judith Anderson received a B.S. in biochemical sciences from Harvard University and a Ph.D. in ecological genetics from the University of British Columbia. She divides her time between the Department of Psychology at Simon Fraser University, where she is a research associate attempting to integrate evolutionary biology and psychology, and her family and community work. Her current research is focused on reproductive suppression as an explanation for anorexic behavior and on psychological and computer modeling studies of sex allocation in humans.
Charles Crawford received a B.A. from the University of Alberta and a Ph.D. in measurement from McGill University. He discovered sociobiology while on sabbatical leave at the Institute of Behavior Genetics at the University of Colorado, and he is currently working on developing methods of testing evolutionary hypotheses, sex allocation in animals and humans, and the evolutionary significance of human psychopathology.
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Anderson, J.L., Crawford, C.B. Trivers-willard rules for sex allocation. Human Nature 4, 137–174 (1993). https://doi.org/10.1007/BF02734114
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DOI: https://doi.org/10.1007/BF02734114