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When it comes to breast milk, baby boys get the better deal

When she's not writing, Claire Gillespie can most often be found wiping snotty noses, picking up Lego, taking photos of her cat or doing headstands.

Blame evolution: The gender pay gap begins with breast milk

As if the world isn't already tilted in favor of men, we're now being told that women are predisposed to provide their sons with better-quality breast milk than their daughters.

More: How I'm raising a feminist son

In research to be published in the April edition of the Journal of Experimental Child Psychology, psychologists Nancy Segal and Satoshi Kanazawa seek to prove that there is a sex bias in the quality and components of breast milk.

This isn't an entirely new hypothesis; a previous study of U.S. women showed they "produced higher-quality milk, with greater energy, lipids and other constituents, for their sons than for their daughters."

It's known as the Trivers-Willard hypothesis and is based on the idea that parents want to preserve their genes for the future. During good times, parents "invest" more in male offspring to ensure they are strong enough to go out and seek females, mate and produce more children. But when times are not so good, the female children get greater "investment," to help them attract strong males.

It's all very sexist club-them-over-the-head-and-drag-them-back-to-the-cave-type stuff. But can we really argue with evolution?

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According to Segal and Kanazawa, not really.

They analyzed data from the National Longitudinal Study of Adolescent Health, comparing the outcomes of same-sex twins to opposite-sex twins with the belief that mothers with opposite-sex twins wouldn't be able to produce different qualities of breast milk while breastfeeding both babies simultaneously.

"We found that breast-fed same-sex twins were indeed either slightly taller or substantially taller than breast-fed opposite-sex twins," they said. "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."

We can fight for equal rights in education, health care and the work place, but as far as breastfeeding goes, we may have to resign ourselves to the fact that baby boys are always going to get the better deal.

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