It makes sense that boys might require a higher fat, protein and calorie 
content in milk than girls. We already know that boys generally take a 
greater volume of milk. That's a cost to the mother as well.

The evolutionary explanation that sons can have more babies and are more 
worth the investment is speculation. Remember the scientist who 
postulated that menstruation existed to regularly wash out the dirty 
male secretions from the vagina? That was based on a non-breastfeeding 
menstrual pattern, not the biological norm for most of human history. It 
can be so hard for us human beings to seperate out our culture from our 
science.

Alternatively, sons provide assistance to the family, and in many 
cultures stay with the birth family and import their wives. In my n of 1 
(family), my 24 y o son is very happy to carry heavy bags, get items 
down from up high, and kill insects that have wandered into the family 
abode. He'll also cook and shop, so maybe he's just a helpful sort (good 
advertisement for bf and attachment parenting). His sister is similarly 
wonderful, but much less enthusiastic about strength and reach requiring 
tasks.

Catherine Watson Genna, BS, IBCLC  NYC   cwgenna.com

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