Interesting. My lovely and talented wife pointed out to me that a recent study out of Canada has established a link between homosexuality and “fraternal birth order” — i.e., how many older brothers a male has. While numerous previous studies have shown that a man’s sexual orientation can be influenced by the number of older brothers that he has, it has remained unclear whether or not this is simply an environmental effect of being reared with brothers, or if there is some underlying (but unknown) biological mechanism at work. Well…
It now seems that prenatal factors are responsible. Indeed, whether or not boys were raised together, the number of biological brothers that an individual has influences sexual orientation, whereas an increase in the number of non-biological brothers in the household had no effect. Of course, this is only a tiny part of the story… According to the study, a woman’s first son has a 3% chance of being homosexual, whereas her fourth son has a 6% chance. Thus, while the likelihood doubles, it’s still a rather subtle effect.
The leading explanation for this phenomenon is the ‘maternal immunization hypothesis,’ in which a mother’s body creates antibodies against foreign male proteins. These antibodies can increase in the mother with each successive male baby, and it’s believe that they can affect a baby’s brain development and hormonal levels.
As the third of three brothers, I can honestly say that haven’t seen this effect in action. Then again, I’m the father of four young boys, so only time will tell.