The number of older brothers a man has may influence his sexual orientation, researchers say.
A first-born son has a 3 percent chance of being homosexual, which is standard for the population. However, the fourth son's chance of being homosexual doubles to about 6 percent.
Sexual orientation researcher and study author Anthony Bogaert, at Brock University in Canada, studied 944 heterosexual and homosexual men in Canada, with either biological or non-biological (adopted or step) brothers.
Previous theories suggested that the older brothers' psychosocial interactions with their younger brothers influenced their sexual orientation. If this were true, then the leading factor would be that the younger brother was raised together with older brothers -- biological or non-biological.
In Bogaert's study, only the number of biological older brothers, regardless if they were raised together, increased the chances that the younger brother would be homosexual.
"In fact, [the gay men] had more biological older brothers who they were never reared with, which means there's probably some biological prenatal factor to account for this older brother effect," Bogaert said.
His research suggests that, in at least some cases, homosexuality is biological and may account for about one of every 7 gay men in North America.
"This is an important contribution," said Dean Hamer, a researcher at the National Cancer Institute, who discovered genetic links to sexual orientation. "It's possible to really show this is a biological rather than social or psychosocial effect."
This older brother effect "does not explain everyone, but this is definitely a part of it," said Sven Bocklandt, a researcher at the David Geffen School of Medicine at UCLA.
The order in which the sons are born to the same mother, called the fraternal birth order, seems to be the source of the biological factor.
"Most studies indicate there is probably a biological basis to men's sexual orientation," Bogaert said. "My study adds to that -- it adds another fairly strong piece to the biological puzzle that underlies, at least in part, men's sexual orientation."
The leading biological theory is the maternal immunization hypothesis, which suggests that something changes with each son a mother conceives.
When a mother gives birth to her first son, she may create antibodies in response to the foreign male proteins of her baby. These antibodies can increase in the mother with each successive male baby, which may affect her son's brain, hormones, and sexual orientation.
Some had speculated that there is not a fraternal birth effect; instead, that mothers are simply older when they give birth to their third or fourth sons and that somehow relates to an increased likelihood of being gay.
Bogaert's previous studies found no difference between gay and straight men, regardless of their mother's age when she gave birth to them.
Other factors, such as genetics and prenatal hormones, especially testosterone, probably play important roles in sexual orientation for both genders.
The maternal immunization theory does not apply to gay women.
"The mechanism behind the fraternal birth effect in males is probably gender-specific just to males," said Bogaert. "There are perhaps different biological routes to sexual orientation in women, relative to men."
Bogaert said there is "quite a bit of evidence" suggesting that lesbians tend to have certain characteristics indicating exposure to higher levels of testosterone in the womb, compared to heterosexual women.
Studies have not been able to show an effect of older or younger siblings on female sexual orientation.