It has been claimed by some experts that there is a great similarity between the traits of sexual orientation as compared to the traits of handedness (i.e. right-handed or left-handed), and that this similarity shows that same-sex orientation and handedness may both be the result of genetics.
The following chart is from the book A Separate Creation: The Search for the Biological Origins of Sexual Orientation, by Chandler Burr (Hyperion Publishing, March 1996), who wrote:
"...the trait profiles of the two [handedness and sexuality] are extraordinarily alike, and virtually everything we know about the one, we know about the other. Neither left and right-handedness nor hetero and homo-sexual orientation can be identified simply by looking at a person. Since both are internal orientations, the only way to identify them is by the respective behaviors that express them, motor reflex and sexual response."
Little to none. Handedness of adopted (i.e. non-biological) children shows no relationship to that of adoptive parents, indicating a genetic influence.
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 in families with other left-handed children.
Yes. Elevated rate of homosexuality in families with other homosexual children.
Are monozygotic (identical)twins more likely to shareminority orientation?
MZ concordance for minority orientation  (vs. background rate):
(vs. 8%, so MZ rate is 1.5 times higher)
(vs. 5%, so MZ rate is 10 times higher)
From: A Separate Creation: The Search for the Biological Origins of Sexual Orientation, by Chandler Burr (Hyperion Publishing, March 1996)
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 PsychiatryScience 261 (July 16, 1993): 321-327 [return to top]
1Both traits show a very small number of humans are ambioriented. 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. [return to footnote 1]
3There 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. [return to footnote 3]
5"Segregation" is a genetic term meaning the way the trait shows up in individuals down through generations.[return to footnote 5]
6Indicates that genetics play a significantly greater role in sexual orientation than in handedness.[return to footnote 6]48 (Dec. 1991): 1089-1096; Dean Hamer et al., "A Linkage Between DNA Markers on the X Chromosome and Male Sexual Orientation,"