In light of the emerging evidence of sex-specific variation in breast milk quality, the Trivers-Willard hypothesis implies that breast-feeding may be less beneficial for opposite-sex twins than for same-sex twins. Same-sex twins, after all, can benefit from their mother’s sex-tailored breast milk just as non-twins can. However, the breast milk of mothers of opposite-sex twins cannot be tailored for either sex. Perhaps it is selectively tailored for the wrong sex part of the time, or for neither sex all of the time.

Thus, the Trivers-Willard hypothesis led us to surmise that opposite-sex twins would be disadvantaged for health and growth. This would be reflected in their being shorter and weighing less than same-sex twins.

Using data from the National Longitudinal Study of Adolescent Health — a large, nationally representative, long-term behavioral and medical study of young Americans — we tested this idea. More than 20,000 adolescents were interviewed in their homes several times: once in 1994 or 1995, and again in 1996, 2001 to 2002, and 2007 to 2008. The subjects’ self-reported height and weight were recorded at all four points, and their height and weight were also directly measured by interviewers from 2007 to 2008, yielding five data points. Information about breast-feeding was provided by mothers.

Our study focused on 546 same-sex individual twins (277 females and 269 males) and 233 opposite-sex individual twins (120 females and 113 males). Using all five measures of height and weight, we compared their growth over time.

We found that breast-fed same-sex twins were indeed either slightly taller or substantially taller than breast-fed opposite-sex twins at four of the five time points (the exception was the first, when they were just entering adolescence). Same-sex twins were, on average, nearly one inch taller than their opposite-sex counterparts. Similarly, same-sex twins were substantially heavier than the opposite-sex twins except during the first measurement period. The same-sex twins were, on average, 12 pounds heavier than the opposite-sex twins.

To the best of our knowledge, ours is the first study to demonstrate that there are observable physical consequences of sex bias in human breast milk. Additional research is needed, of course, to confirm the presence of sex bias in human breast milk and the differences in the quality of breast milk in accordance with the conditions of the parents. But for now, the Trivers-Willard hypothesis seems to hold.