Human Handedness Human Sexual Orientation
Distribution1 Stable bimodalism, behaviorally expressed Stable bimodalism, behaviorally expressed
Population distribution Majority and minority orientations Majority and minority orientations
Population distribution of orientations according to sex Male: 9%, Female 7% Male: 6%, Female 3%
Male:Female ratio for minority orientations 1.3:1 Minority orientation, 30% higher in men than women 2:1 Minority orientation, 100% higher in men than women
Does minority orientation correlate with race? No No
...or geography? No No
...or culture2 No No
Mental or physical pathology?3 No No
Age of first behavorial appearance of trait Around age 2 Around age 2
Is either orientation chosen? No No
Is either orientation pathological? No No
Can external expression be altered? Yes Yes
Can interior orientation be altered clinically? No No
Is trait familial/does trait run in families? Yes Yes
Pattern of familiality: "Maternal effect," implies X-chromosome linkage "Maternal effect," implies X-chromosome linkage4
Parent-to-child segregation5 Little to none. Handedness of adopted (i.e. non-biological) children shows no relationship to that of adoptive parents, indicating a genetic influence. "Maternal effect" implies X-chromosome linkage. [4]
Little to none. Sexual orientation of adopted (i.e. non-biological) children shows no relationship to that of adoptive parents, indicating a genetic influence.
Do siblings of those with minority orientation have increased rates of minority orientation? Yes. Elevated rate of left-handedness families with other left-handed children. Yes. Elevated rate of homosexuality in families with other homosexual children.
Are monozygotic (identical twins more likely to share minority orientation? Yes Yes
MZ concordance for minority orientation6 (vs. background rate) 12% (vs. 8%, so MZ rate is 1.5 times higher) 25% (vs. 5%, so MZ rate is 5 t imes higher)
From: A Separate Creation: The Search for the Biological Origins of Sexual Orientation

Sources: I.C. McManus, "The Inheritance of Left-Handedness," Biological Asymmetry and Handedness, Ciba Foundation Symposium 162. (Chichester) John Wiley & Sons: 1991, 251-267; J. Michael Bailey and Richard Pillard, "A Genetic Study of Male Sexual Orientation," Archives of General Psychiatry 48 (December 1991): 1089-1096; Dean Hamer et al., "A Linkage Between DNA Markers on the X Chromosome and Male Sexual Orientation," Science 261 (July 16, 1993): 321-327

1 - Both traits show a very small number of humans are ambi-oriented. Handedness shows almost none for both men and women—McManus: "Measures of handedness usually show a bimodal distribution with few subjects appearing truly ambidextrous." Sexual orientation, likewise, shows almost none for men but a still small though significant number for women.

2 - However, may highly influence expression.

3 - There is currently fierce debate over the existence of a correlation between left-handedness and certain pathologies, most notably schizophrenia. Some researchers assert that handedness, thought to reflect one aspect of brain lateralization, may be a result of a cause--in some manner a concomitant—of schizophrenia's etiology or pathophysiology. A study done by Charles Boklage ("Schizophrenia, brain asymmetry deveopment, and twinning," Biol. Psychiatry 12, 19-35, 1997) powerfully developed the hypothesis, and Nancy Segal ("Origins and implications of handedness and relative birth weight for IQ in monozygotic pairs," Neuropsychology 27, 549-561, 1989) also supports some form of correlation. On the other hand, Luchins et al. (1980) and Lewis et al. (1989), in their respective replication attempts of Boklage's work, found little support, and Gottesman et al. ("Handedness in twins with schizophrenia: was Boklage correct?" Schizophrenia Research 9, 83-85, 1993) conclude that there does not appear to be an association between handedness and schizophrenia. (See Gottesman for a more complete bibliography.) The point, however, is the distinct difference between the trait profile of handedness and that of sexual orientation: while there is clinical debate in scientific and research circles over whether handedness correlates in some way with psychobiological abnormalities, no such debate exists regarding sexual orientation, and neither heterosexuality nor homosexuality are implicated in any mental or physical pathology.

4 - A subset of gay men show the maternal effect. It does not appear in women.

5 - "Segregation" is a genetic term of art meaning the way the trait shows up in individuals down through generations.

6 - Indicates that genetics play a significantly greater role in sexual orientation than in handedness.