June 26, 2006 -- Men may be more likely to be homosexual if they share their birth mother with older brothers, even if they didn't grow up with those brothers.
"These results strongly suggest a prenatal origin to the fraternal birth-order effect," writes Canada's Anthony Bogaert, PhD, in the early online edition of Proceedings of the National Academy of Sciences.
Bogaert works in the community health science and psychology departments at Brock University in St. Catharines, Ontario, Canada.
He studied 944 Canadian men, some of whom were heterosexual college students. Other men were recruited through gay-oriented publications.
The men were asked about their siblings, birth mother, and sexual orientation. The men noted which siblings shared the same mother, which ones were adopted or step-siblings, and how long (if at all) participants had spent with those siblings while growing up.
The men rated their sexual attraction (thoughts and feelings) to men and women on a scale of one to seven. A one-point score meant "exclusively homosexual/gay." A seven-point score meant "exclusively heterosexual/straight."
The men also used a seven-point scale to rate their sexual behavior (actual sexual experiences) with men and women. Bogaert calculated sexual orientation based on the men's average sexual attraction and sexual behavior ratings.
Of the 944 men who participated in the study, 905 weren't twins and had complete sibling data. They included 329 men with "exclusive or near exclusive heterosexuality," 151 "bisexual" men, and 425 with "exclusive or near exclusive homosexuality," Bogaert notes.
Homosexuality was more common among men who shared their birth mother with older brothers, even if they hadn't been raised with those brothers, the study shows.
To be sure of that, Bogaert checked the data from 234 participants who came from nonbiological or 'blended" families. The results held.
"Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men's sexual orientation, regardless of the amount of time reared with these siblings," writes Bogaert.
Bogaert's findings don't prove that birth order determines sexual orientation. However, Bogaert writes that the study supports the idea that sexual orientation starts before birth.
No data was available on any incomplete pregnancies among the men's birth mothers. So it's not clear if the pattern only applies to births or pregnancies, Bogaert notes.