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  • Author or Editor: Thomas v. Pollet x
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Abstract

In a series of recent papers, Kanazawa has extended the Trivers-Willard hypothesis by suggesting that possession of any heritable trait that improves male reproductive success to a greater extent than it does female reproductive success will lead to a male-biased offspring sex ratio (at the individual level). He produces supporting evidence that big and tall parents have more sons than daughters. Here we test this hypothesis using two large datasets from very different populations, one British and one from rural Guatemala. There was no support for Kanazawa's extension of the Trivers-Willard hypothesis in either sample. Maternal marital status was the only predictor of offspring sex ratio but this effect was very small and limited to the British sample. Results are discussed with reference to recent studies of sex-ratio variation in humans.

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Evolutionary theory suggests that maternal grandparents will invest more in their grandchildren than paternal grandparents, due to the difference between the certainty of maternity and the uncertainty of paternity. Tests of this prediction have tended to use retrospective ratings by grandchildren rather than self-reported behaviour by grandparents. Using a large-scale dataset from the Netherlands, we show significant differences between maternal and paternal grandparents in terms of frequencies of contact with their grandchildren, while controlling for a wide range of other variables. Our results show biases consistent with the paternity uncertainty hypothesis.

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As suggested by previous research, childlessness can thoroughly affect the likelihood of giving and receiving help to kin, even in modern societies. In this paper we show that childless women over thirty-five have had more recent contact with their nephews/nieces than mothers. Yet, both groups showed no significant differences in contact with their uncles/aunts. This suggests heightened social investment in kin with high reproductive value by childless women compared to mothers. Results are discussed with reference to kin selection theory.

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