The finding contradicts earlier studies that had shown that gay men (and lesbians) were 39 percent more likely than heterosexuals to be left-handed.
The new study, conducted by Anthony F. Bogaert of Brock University in St. Catharines Ontario, reviewed 538 gay or bisexual men and 373 heterosexual men.
Bogaert asked about the sexual attractions and behaviour of all the men. They were questioned about their right or left hand usage for 10 physical activities. They also were asked if they had biological brothers.
The analysis of the study found that 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 said. '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,' he said.
'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,' he added.
The results of the study also indicated 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 centre for lesbian, gay, bisexual, and transgender research and public policy at Alliant International University.
However, Bogaert said, 'a genetic explanation can also be forwarded.' Bogaert contemplated 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, 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 added.
The study is published in the journal Neuropsychology.