According to a study published in the American Journal of Physical Anthropology, human breast milk produced for girls and boys may differ. Researchers found that of the 72 mothers in rural Kenya involved in the study, mother's milk for male babies was generally richer with 2.8% fat compared with 1.74% for girls. Financially less privileged women, seemed to produce 2.6% creamier milk when they had girl babies versus 2.3% for boys. This study follows up findings that well-to-do, well-nourished mothers in Massachusetts, USA produced more energy-dense milk for male infants.

Apart from fat and protein, other milk components may also differ for boy & girl babies, says Katie Hinde, an assistant professor in human evolutionary biology at Harvard University. Her research demonstrates that differences in breast milk could change infant behaviour and might affect growth and development. She acknowledges, "Only half the story is what the mother's producing. The other is how the infant metabolises the milk."

These findings could help improve formula for bottled milk to optimize development for both boys and girls.