The New York Times has a popular story by Katy Butler on a new study of sibling violence. All siblings push and shove sometimes, but in 14% of the cases studied they found repeated beatings, usually by a big brother. David Finkelhor and John Caffaro, co-authors of "Sibling Abuse Trauma," found that sibling abuse was most common when disengaged parents headed large families composed entirely of closely spaced boys.
Siblings compete for parental attention, and first-borns often feel entitled to more attention than their little brothers and sisters. When the parents are not paying much attention anyway, the competition would be even fiercer.
This study put me in mind of another fascinating finding about families composed entirely of boys: the youngest boys are much more likely to be gay. No one really knows why this is so, but speculation centers on a conflict between the mother's female hormones and her sons' male hormones, with the mother's hormones getting stronger with each boy baby in her womb. In a series of studies, Ray Blanchard and colleagues found that each additional older brother increases the odds of homosexuality by approximately 33%.
The new study on sibling abuse did not address the siblings' sexual orientation. Putting the two pieces of research together, though, suggests a bad scenario: in a big family of boys, with no parents paying attention, the eldest beats on the little brother, especially if the young one is more effeminate. Scary and sad.