Gay males' sibling link: men's homosexuality tied to having older brothers.

Birth order may steer some men toward homosexuality in a process that perhaps begins before birth. A new study finds that homosexuality grows more likely with the greater number of biological older brothers--those sharing both father and mother--that a male has.

Men display this tendency toward homosexuality even if they weren't raised with biological older brothers, finds psychologist Anthony F. Bogaert of Brock University in St. Catharines, Ontario. No gay connection appears in men raised with half-brothers, stepbrothers, or adoptive brothers, all deemed non-biological by Bogaert.

"The mechanism underlying this fraternal birth-order effect remains unknown" Bogaert says. It's possible that succeeding pregnancies with male fetuses trigger a maternal immune response. A mother's immune system may treat male fetuses as foreign bodies, attacking them with antibodies that alter sex-related brain development, the Canadian psychologist suggests.

Scientists haven't yet looked for any specific immune reaction during pregnancy that targets later-born boys who become homosexual.

Bogaert's analysis of men's family histories appears in an upcoming Proceedings of the National Academy of Sciences. It confirms an analysis of sexual orientation in 604 men reported in 1996 by Bogaert and a colleague. That report didn't include men raised with non-biological older brothers, leaving open the possibility that some psychological reaction to older brothers fostered homosexuality.

The new investigation consists of 944 Canadian men for whom Bogaert verified background information, including sexual orientation and age, number of biological and non-biological siblings, whether siblings occupied the same house as children, and the biological mother's age at the participant's birth.

Critically, 521 of the men had grown up with one or more non-biological siblings.

The number of biological older brothers correlated with the likelihood of a man being homosexual, regardless of the amount of time spent with those siblings during childhood, Bogaert says. No other sibling characteristic, such as number of older sisters, displayed a link to male sexual orientation.

By accounting for potential psychological effects of having older brothers, Bogaert's data "strengthen the notion that the common denominator between biological brothers, the mother, provides a prenatal environment that fosters homosexuality in her younger sons," say neuroscientist S. Marc Breedlove of Michigan State University in East Lansing and his coworkers in a comment to be published with the new report.

The release of maternal antibodies that boost a boy's probability of becoming gay is a provocative but untested hypothesis, Breedlove and his coworkers note.

It makes sense, though, in light of previous failures to find any older-sibling influences on female homosexuality, they say.

Breedlove's group suspects that some boys are "born to become gay" as a result of genetic and prenatal factors. However, psychologist Daryl J. Bern of Cornell University argues that the new findings don't necessarily support that view.

Bem has proposed that genes and biology orchestrate temperaments that gear kids toward sex-typical or sex-atypical activities. Boys who don't like rough-and-tumble play perceive males as different from themselves, a feeling that may tuna erotic during adolescence, Bern says (SN: 8/10/96, p. 88).

Bogaert's work indicates that for homosexuality to develop, it doesn't matter whether boys feel different from sex-typical older brothers, only that they have older brothers, Bern acknowledges. Still, a maternal immune response could promote homosexuality by lowering a boy's aggression, rather than by stamping a same-sex orientation into the brain, Bem says.