Handedness and sexual orientation

A relationship between handedness and sexual orientation has been suggested but not verified by a number of researchers, who report that heterosexual individuals are somewhat more likely to be right-handed than homosexual individuals. The relationship between handedness and sexual orientation has been reported within both sexes and may reflect the biological etiology of homosexuality; recent work by Ray Blanchard has linked the relationship to the fraternal birth order effect, which suggests that a man with several older brothers is more likely to be homosexual.

Mustanski et al., 2002 study
Mustanski et al. examined sexual orientation and hand preference in a sample of 382 men (205 heterosexual; 177 homosexual) and 354 women (149 heterosexual; 205 homosexual). Although homosexual women were found to be significantly more left-handed than heterosexual women (18% vs 10%), no significant differences were found between heterosexual and homosexual men with respect to hand preference.

Lippa, 2003 study
Lippa examined sexual orientation and handedness in a sample of 812 men (351 heterosexual; 461 homosexual) and 1189 women (707 heterosexual; 472 homosexual). Homosexual men were 100% more likely to be left-handed than heterosexual men, but no significant differences were found between heterosexual and homosexual women in terms of handedness. When combining men and women into one large sample, homosexual individuals were 100% more likely to be left-handed than heterosexual individuals.

Blanchard et al., 2006 study
Blanchard et al. argued that the fraternal birth order effect (the probability that a boy will be homosexual increases with the number of older brothers who have the same biological mother) appears to be limited to right-handed men. Moreover, the same study indicates that left-handed men without older brothers are more likely to be homosexual than non-right-handed men who have older brothers. As Blanchard et al. said in their report, "the odds of homosexuality is higher for men who have a non-right hand preference or who have older brothers, relative to men with neither of these features, but the odds for men with both features are similar to the odds for men with neither".

Blanchard, 2008 Archives of Sexual Behavior study
A subsequent study by Blanchard found that both right-handed homosexual men and left-handed heterosexual men had a statistically significant number of older male siblings, but that there was no significant observable effect for right-handed heterosexual men or for left-handed homosexual men.

Blanchard, 2008 Laterality study
Blanchard discussed ways in which the fraternal birth order effect and handedness could be explained in terms of the maternal immune hypothesis. In this, the mother is assumed to grow more immune to male antigens with each pregnancy, and thus produce a greater number of "anti-male" antibodies. He suggests two possibilities; that non-right-handed fetuses are less sensitive to the antibodies, or that the mothers of left-handed fetuses do not, for some reason, produce them.