Women's Health Research Institute

Years ago, two scientists named Robert Trivers and Dan Willard came up with what is now known as the Trivers-Willard hypothesis. The hypothesis basically states that during good conditions, male offspring are favored through parental investment, and during poor conditions, female offspring are favored. Conditions could include the quality of the external environment, the health of the parent, and access to resources that support longevity.

Masako Fujita from Michigan State University was interested in this hypothesis and how it translated to breastfeeding, so he led a study to assess the breast milk and feeding frequencies of 83 Kenyan mothers. These women typically raise an average of six children and live in villages where men often have more than one wife. Poorer women included in the study lived in environments with less access to land and livestock compared to their wealthier counterparts, and Fujita was interested in how such environmental differences could influence the quality of milk and frequency of feeding through the lens of the Trivers-Willard hypothesis.

It turns out that the women with less land and fewer livestock produced fattier milk for their daughters compared to sons, and also fed them more often than sons.  The wealthier mothers with land and livestock, on the contrary, appeared to favor their sons over their daughters based on fat content in breast milk. The results were only significant for milk fat concentration, but the hypothesis was supported in that regard.

Although much more research is needed on breastfeeding and biological investment of parents to offspring, the implications of these results suggest that underprivileged moms may biologically allocate more resources to daughters compared to sons since the chances of increasing social status are higher through a girl’s marriage. Wealthier women do not necessarily have a threat to social status, so by this logic the favoring of males would result. The outcomes of this research are not necessarily applicable to other communities (in the United States and beyond) since they were limited to large-household Kenyan women, but it is likely that more research like this will emerge that will include a range of participants from a variety of environments such as low-income urban areas in the US.

For the full study, click here.