Poor mothers will invest more resources in daughters, who stand a greater chance of increasing their status through marriage than do sons, suggests a study in the American Journal of Physical Anthropology.
Masako Fujita, Michigan State University anthropologist, and her fellow researchers tested the breast milk of mothers in northern Kenya and found that poor mothers produced fattier milk for their daughters than for their sons.
On the contrary, mothers who were better off financially favored sons over daughters.
The results, also featured in the journal Nature, support a 1973 hypothesis that predicts poor mothers will favor their daughters.
The Nature article, titled "Rich milk for poor girls," notes that Fujita and her team assessed the fat content from 83 mothers living in villages in which men can have multiple wives. In these villages, taking multiple wives requires wealth to support a larger family, leaving poor males less competitive for marriage. The researchers found that mothers with less land and fewer livestock provided richer milk to their daughters than to their sons.
On average, a mother in northern Kenya raises six children.
The study is one of the first to explore parents' unequal biological investment in their children, such as the nutritional content of breast milk.
Fujita is an assistant professor of anthropology at MSU. The research team also includes Eric Roth of the University of Victoria in British Columbia, Canada; and Yun-Jia Lo, Carolyn Hurst, Jennifer Vollner and Ashley Kendell from MSU.
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