Why having older brothers may nudge a boy's sexual orientation
Since the 1990s researchers have found that the more sons a woman has, the likelier each later son is to be attracted to men. One proposed explanation involves the mother's immune system reacting to male proteins during each pregnancy. It is one strand of a wider search pointing to biology, rather than upbringing, as the main influence.
Nobody knows exactly what determines sexual orientation. Scientists suspect a tangled mix of genes, hormones and environment, and they generally favour biological explanations, because evidence that childhood social surroundings shape orientation is weak, particularly for men. The immune theory runs like this: male fetal cells cross into the mother's bloodstream and trigger antibodies. In later pregnancies those antibodies may interfere with proteins tied to masculinising the brain, and the effect appears to build with each additional son.
The most influential idea concerns hormones in the womb. A fetal brain starts in a female-typical state until testosterone, released once a gene on the Y chromosome triggers the testes, masculinises certain regions. One of these, a small cluster in the hypothalamus called INAH-3, is typically larger in men, and dissection studies found it smaller in gay men than straight men. Finger length ratios, a rough marker of prenatal testosterone, also differ on average: a large review found lesbians had more masculine ratios and gay men more feminine ones, while bisexual people resembled heterosexuals.
Animals offer parallels. Long-running work with domestic sheep led by Charles Roselli found that 6 to 8 percent of rams prefer other males throughout life, and those rams have a smaller, female-like version of a brain structure that forms before birth. Brain imaging led by Ivanka Savic found straight men's right hemispheres slightly larger than the left, while gay men's hemispheres were symmetrical, like those of straight women.
One grim chapter tested the opposite view. When twentieth-century theory held that babies were born sexually neutral and shaped by upbringing, some infant boys injured in botched circumcisions were surgically reassigned and raised as girls without being told. In every published case describing their orientation, they grew up attracted to women.
Source: Biology and sexual orientation
