A relationship between handedness and sexual orientation has been suggested by a number of researchers, who report that heterosexual individuals are somewhat more likely to be right-handed than are homosexual individuals. The relationship between handedness and sexual orientation has been suggested within both sexes and may reflect the biological etiology of sexual orientation ; work by Ray Blanchard has linked the relationship to the fraternal birth order effectwhich suggests that a man with several older biological brothers is more likely to be homosexual.
The theory is still tentative and not all research has been able to confirm the disparity between homosexual and heterosexual men. In a study involving men— homosexual and heterosexual—no significant association emerged between handedness and sexual orientation.
Lippa examined sexual orientation and handedness in a sample of men heterosexual; homosexual and women heterosexual; homosexual. 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.