Handedness and sexual orientation
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 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 biological brothers is more likely to be homosexual.
Lalumière et al., 2000 meta-analysis
Lalumière et al. conducted a meta-analysis of 20 studies with a total of 6,987 homosexual and 16,423 heterosexual participants. They found that homosexual men had a 34% greater odds of non-right-handedness, and homosexual women had a 91% greater odds (39% overall). 
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 a significantly higher proportion of homosexual women was found to be left-handed compared to 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 82% 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 50% 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.
A 2014 study attempting to analyze the biological markers of asexuality, asserts that non-sexual men and women were 2.4 and 2.5 times, respectively, more likely to be non-right-handed than their heterosexual counterparts.
- Handedness § Sexuality and gender identity
- Biology and sexual orientation
- Prenatal hormones and sexual orientation
- Digit ratio
- Lalumière, M. L., Blanchard, R., & Zucker, K. J. (2000). "Sexual orientation and handedness in men and women: a meta-analysis". Psychological Bulletin 126 (4): 575–592.
- Mustanski, B. S.; Bailey, J. M.; Kaspar, S. (2002). "Dermatoglyphics, handedness, sex, and sexual orientation". Archives of Sexual Behavior 31 (1): 113–122. PMID 11910784. doi:10.1023/A:1014039403752.
- Lippa, R. (2003). "Handedness, sexual orientation, and gender-related personality traits in men and women". Archives of Sexual Behavior 32 (2): 103–114. PMID 12710825. doi:10.1023/A:1022444223812.
- Blanchard, R.; Cantor, J.; Bogaert, A.; Breedlove, S.; Ellis, L. (2006). "Interaction of fraternal birth order and handedness in the development of male homosexuality". Hormones and Behavior 49 (3): 405–14. PMID 16246335. doi:10.1016/j.yhbeh.2005.09.002.
- Blanchard, R. (2006). "Sex Ratio of Older Siblings in Heterosexual and Homosexual, Right-Handed and Non-Right-Handed Men". Archives of Sexual Behavior 37 (6): 977–981. PMID 17186124. doi:10.1007/s10508-006-9119-2.
- Blanchard, R. (2008). "Review and theory of handedness, birth order, and homosexuality in men". Laterality: Asymmetries of Body, Brain and Cognition 13: 51–70. PMID 18050001. doi:10.1080/13576500701710432.
- Yule MA, Brotto LA, Gorzalka BB. "Biological markers of asexuality: Handedness, birth order, and finger length ratios in self-identified asexual men and women.". Arch Sex Behav. Retrieved 2014.