More evidence that biological factors play a significant role in the development of sexual orientation

Nov 27 2007

A study of men in Ontario, Canada provides a new twist on the connection between sexual/relational orientation and right or left-handedness.

Whereas earlier studies showed that gay men (and lesbians) were 39 percent more likely than heterosexuals to be left-handed, the new data “provides evidence that gay or bisexual men also have an elevated incidence of extreme right-handedness.”

To complicate matters, another factor is involved – the often-reported finding that having older brothers may be a predictor for men being gay. In the new study, results indicate that the number of “older brothers moderates the relationship between handedness and sexual orientation.” That is, the extreme right-handedness finding is only seen in men with no or few older brothers.

“These new research findings add further weight to the idea that biological factors play a significant role in the development of sexual orientation,” said Robert-Jay Green, Executive Director of the Rockway Institute, a national center for lesbian, gay, bisexual, and transgender research and public policy at Alliant International University.

The study was conducted by Anthony F. Bogaert of Brock University in St. Catharines Ontario and published in the journal Neuropsychology (2007, Vol. 21, No. 1, 141-148). Bogaert asked about the sexual attractions and behavior of 538 gay or bisexual men and 373 heterosexual men. The men were questioned about their right or left hand usage for 10 physical activities. They also were asked if they had biological brothers.

Most of the men were right-handed. However, the gay and bisexual men had a higher likelihood of both left-handedness and extreme right-handedness when compared to the heterosexual men. “The number of older brothers increased the likelihood of being gay or bisexual in moderate right-handers only,” Bogaert wrote. “In both non-right-handers and in extreme right-handers, older brothers either did not increase or lowered the likelihood of being gay or bisexual.”

Bogaert went on to conclude: “If elevated extreme right-handedness is an indication of early neurodevelopmental anomalies, then an elevation of this handedness pattern in gay or bisexual men gives additional evidence that one route to same-sex attraction is through early developmental stressors” (during pregnancy) “or through a factor correlated with such stressors.”

However, Bogaert wrote, “a genetic explanation can also be forwarded.” He noted that genes have been linked to both handedness and sexual orientation. Specific genes have been linked to handedness and immune system functioning, but this relationship has not been sufficiently researched. Immune reactions are suspected in the male birth order findings.

“In conclusion,” he continues, “the main findings—evidence of extreme right-handedness in gay men, along with the moderating effect of older brothers at both ends of the handedness continuum—potentially move forward two important research programs (on handedness and birth order) related to men's sexual-orientation development.”

“The results of this research suggest there is a biological predisposition to homosexuality among a significant number of gay/bisexual men,” said Green. “What we don't know yet is how strong or widespread such biological predisposition is or whether it is a result of genes, maternal hormones during pregnancy, or maternal immune system functioning during conception.”

Green continued: “Although many legal scholars and others argue that lesbian/gay citizens deserve equal treatment regardless of the causes of sexual orientation, previous research shows that people who believe being gay is inborn are more apt to endorse equal rights. Thus research evidence like Bogaert's, which is consistent with a biological explanation, may inform public opinion and policies in favor of lesbian/gay equality in areas such as employment opportunity and marriage rights.”

About Rockway Institute: The nonpartisan Rockway Institute promotes scientific and professional expertise to counter antigay prejudice and improve public policies affecting lesbian, gay, bisexual, and transgender (LGBT) people. The Institute's view is that public opinion, policies, and programs should be shaped by the facts about LGBT lives, not by political ideology. A primary goal is to organize the most knowledgeable social scientists, mental health professionals, and physicians in the United States to provide accurate information about LGBT issues to the media, legislatures, and the courts. The Institute also conducts targeted research projects to address the nation's most pressing LGBT public policy concerns. Website: http://www.rockwayinstitute.org