Born Gay: The Brother Factor

If you grew up as a younger brother in a houseful of boys, you have probably lost count of the ways your big brothers shaped your life. Now a new study suggests that if you're also gay, they may have had a role in that too.

Since the 1990s, many social psychologists have concluded that gays represent about 3% of all males—although that's a much debated figure. Others put it as low as 1% or as high as 10%. Those who subscribe to the 3% figure believe they have documented an interesting wrinkle: among boys with one older brother, the figure goes up to about 4%; two older brothers tick it up to 5%; and with three or more, it tops out at about 6%. What nobody could answer is, Why?

Last week psychologist Anthony Bogaert of Brock University in Ontario, Canada, provided some clues from a study of 944 men. Some were raised with biological brothers or stepbrothers; others had biological brothers but were raised apart. In almost no case did homosexuality seem to correlate with stepbrothers living under the same roof. It was only the existence of older biological brothers—whether or not they were raised together—that influenced younger brothers' sexuality.

Bogaert believes the answer may lie in the mother's immune system. Mothers' bodies naturally recognize boy fetuses as slightly more alien than girl fetuses, since all of us carry sex-specific proteins in our bloodstreams. Some mothers may develop antibodies to those male proteins. In subsequent boy pregnancies, Bogaert theorizes, the antibodies may cross the placenta and affect regions of the fetal brain that determine sexual orientation.