Study Links Gay Men to Birth Order

The more boys a mother has produced, the greater the chance that the next son will be gay, a study says.

The reason might involve the mother’s immune system acting on the developing male fetus, researchers speculated, or it could be a psychological effect on a boy of growing up with older brothers.

Several earlier studies found that gay men tended to have more previously born siblings than heterosexual men did. The new study says this effect comes only from brothers.

Previously born brothers are not necessarily older brothers, because the study also counted brothers who died before study participants were born.

Psychologist Ray Blanchard, head of the clinical sexology program at the Clarke Institute of Psychiatry in Toronto, reported the work in the January issue of the American Journal of Psychiatry with Anthony Bogaert, who has since left the institute.

Questionnaires were filled out by 302 gay men and 302 heterosexual men.

Each gay man was matched for year of birth to a heterosexual man. The average age was 39. While 50% of the overall sample was gay, the rate was 45% for men with no previously born brothers, 53% with one brother, 64% with two brothers. Blanchard said those rates pertain only to the sample and not the general population.

Prior studies have spurred a variety of psychological and social explanations for the sibling effect.

Blanchard and Bogaert proposed the immune-system hypothesis.

The immune system recognizes one or two particular proteins that males but not females have on cell surfaces, Blanchard said. Maybe these proteins progressively alter the mother’s immune system response to male fetuses, and this somehow affects the fetal brain in a way that predisposes to homosexuality.