An analysis of the breast milk of 83 less fortunate moms shows that they produced richer, more nutritious milk for their little girls.
PROBLEM: In 1973, evolutionary biologists Robert Trivers and Dan Willard famously posited that parents prefer their sons when times are good and their daughters when times are tough. Few studies, however, have put forth biological proof to support this hypothesis.
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METHODOLOGY: Researchers led by Michigan State University's Masako Fujita assessed the breast milk and feeding frequencies of 83 Kenyan mothers who raise around six children, on average, and live in villages where men can have multiple wives. They controlled for several factors, including age and dietary fat intake.
RESULTS: The moms with less land and fewer livestock produced fattier milk for their daughters than for their sons, and also fed them more frequently. Wealthier mothers, on the contrary, appeared to favor their sons over their daughters.
CONCLUSION: Poor mothers place a larger biological investment in their daughters than their sons.
IMPLICATION: As Trivers and Willard previously suggested, less privileged moms may provide more resources to their daughters, since they stand a greater chance of increasing their status through their child's marriage this way.
SOURCE: The full study, "In Poor Families, Mothers' Milk is Richer for Daughters than Sons: A Test of Trivers-Willard Hypothesis in Agropastoral Settlements in Northern Kenya," is published in the American Journal of Physical Anthropology. It also appears in the journal Nature as "Rich Milk for Poor Girls."