Canadian researchers have consistently documented a "big-brother effect," finding that the chances of a boy being gay increase with each additional older brother he has. (Birth order does not appear to play a role with lesbians.) So, a male with three older brothers is three times more likely to be gay than one with no older brothers, though there's still a better than 90 percent chance he will be straight.
They argue that this results from a complex interaction involving hormones, antigens, and the mother's immune system.
New York University researcher Lynn S. Hall , who has studied traits determined in the womb, speculates that Patrick was somehow prenatally stressed, probably during the first trimester, when the brain is really developing, particularly the structures like the hypothalamus that influence sexual behavior. This stress might have been based on his position in the womb or the blood flow to him or any of a number of other factors not in his mother's control.
Yet more evidence that identical twins have womb experiences far from identical can be found in their often differing birth weights. Patrick was born a pound lighter than Thomas.
Taken together, the research suggests that early on in the womb, as the fetus's brain develops in either the male or female direction, something fundamental to sexual orientation is happening. Nobody's sure what's causing it. But here's where genes may be involved, perhaps by regulating hormone exposure or by dictating the size of that key clump of neurons in the hypothalamus. Before researchers can sort that out, they'll need to return to the question of whether, in fact, there is a "gay gene."