Masters Thesis

"an Evolutionary Hypothesis That Is At Once So Logically Coherent and Impossible to Prove": Testing the Trivers-willard Hypothesis in a Modernized Population

The Trivers-Willard Hypothesis states that parental investment is a function of parental condition and that parents invest in the sex of offspring that offers the greatest reproductive returns. Parental investment is a cost/benefit strategy: parents in worse condition should invest in the sex with the least risk of not reproducing at the cost of high reproductive returns (females) and parents in better condition should risk investing in the sex with the greatest potential returns and highest reproductive variance (males). Tests of this hypothesis in humans have yielded mixed results, with greater support coming from small, economically homogeneous societies. We address two methodological issues that have arisen in tests of the Hypothesis: the lack of a standardized metric by which condition is operationalized and the tendency to treat a modernized population as though it does not have social, cultural, and ethnic differences. This projects seeks to redefine the metrics that are used to operationalize condition, to analyze sex-biased investment in a heterogeneous, modernized population by applying the concept of ecologically-valid reproductive subpopulations, and to investigate whether parents in this population bias their investment in ways that conform to the Trivers-Willard Hypothesis. We tested a sample of 105 mothers across four ZIP Codes. Results suggest that there is no difference in maternal investment based on child sex, nor is there an association between maternal condition and parental investment. However, these conclusions are limited by a small sample size; further research will be required.


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