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 effect, which suggests that a man with several older biological brothers is more likely to be homosexual.
[5] 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.
[8] 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.
[11] Among a sample of university students in Malaysia and the United States, it was found that same-sex attraction was linked with left-handedness among women.
[12] A 2014 Internet study attempted to analyze the relationship between self-identification as asexual, handedness and other biological markers, in comparison to individuals of other sexual orientation groups.