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Richard Harper links via Twitter to an interesting blog post by Harvard researcher Kate Hinde who suggests that “mothers have the capacity to synthesize milk in relation to infant sex.” Hinde shows that in general and across species, mothers tend to produce milk with higher fat concentration for males compared to females. In humans, socioeconomic factors come into play. Paging Kevin Williamson:
More recently, Fujita and colleagues (2012) revealed sex-biases in the milk fat concentration among 72 women in rural Kenya. On average, mothers of sons produced significantly higher fat concentrations in milk. However there was an interaction between infant sex and maternal socio-economic status. Relatively wealthier women- women whose household owned more land and more dairy animals (such as camels, cattle, goats, and sheep)- produced milk with higher fat concentrations for sons. But relatively poorer women produced higher fat concentrations for daughters. Interestingly, higher socio-economic status was associated with lower mean fat concentrations in milk, possibly because such mothers were producing more milk. However investigating milk volume among Ariaal people in Kenya presents ethical and cultural challenges. The data from this study suggests that sex-biases in milk synthesis among humans may be sensitive to cultural and economic factors. These results provide support for the Trivers-Willard hypothesis (1973) that mothers in the best condition should favor sons because they will likely have high reproductive success, but that women in poor condition should favor the “safer” investment in daughters.
Hinde points out that this topic has been sparsely researched, possibly because of fears that a “milk is sexist” argument won’t sit well with many.