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?
Mental or physical pathology?3
Age of first behavorial appearance of trait
Around age 2
Around age 2
Is either orientation chosen?
Is either orientation pathological?
Can external expression be altered?
Can interior orientation be altered clinically?
Is trait familial/does trait run in families?
Pattern of familiality:
"Maternal effect," implies X-chromosome linkage
"Maternal effect," implies X-chromosome linkage4
Little to none. Handedness of adopted (i.e. non-biological) children
shows no relationship to that of adoptive parents, indicating a genetic
"Maternal effect" implies X-chromosome linkage. 
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
Yes. Elevated rate of homosexuality in families with other homosexual
Are monozygotic (identical twins more likely to share minority
MZ concordance for minority orientation6 (vs. background
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
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.