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from "Born that way" theory
Is There a Link Between
Left-Handedness and Homosexuality?
N. E. Whitehead, Ph.D.
whiteh@paradise.net.nz
This year, three Canadian scientists published a study which found some
connection between left-handedness and homosexuality (1).
Analyzing a number of studies, their paper concludes that male
homosexuals are about one third (31%) more likely than heterosexuals to
be left-handed (2), while lesbians are almost twice as likely (91%) to
be left-handed as heterosexual women.
The authors say that their findings support the notion that sexual
orientation in some men and women has an early neurodevelopmental basis.
They trace this to "disruptive events causing developmental
instability." These events may modify sexual differentiation of the
brain, they say, "perhaps through hormonal or immunological mechanisms."
The authors point out that left-handedness has been associated with a
wide range of indicators of reduced fitness, from the standpoint of
natural (Darwinian) selection. Left-handed people, the authors say, have
a smaller number of offspring, higher number of spontaneous abortions,
lower birth weight, higher number of serious accidents, higher rates of
serious disorders, and a shorter life span. Left-handedness has
similarly been linked to neural tube defects, autism, stuttering, and
schizophrenia. The authors conclude by discussing possible biological
reasons for the link between left-handedness and homosexuality. Their
preferred explanation is that both left-handedness and homosexuality
result from "biological developmental errors."
In a related development, two recent studies reported in Archives of
General Psychiatry found significantly higher levels of pathology in the
gay population than among heterosexuals. One of several possible
explanations, said researcher J.M. Bailey in a published commentary that
accompanied the article, is that since evolution naturally selects for
heterosexuality, "homosexuality may represent developmental error" (3).
How Significant is the Latest Study?
I would conclude that there is possibly some link between
left-handedness and homosexuality, but not a highly significant one. The
fact remains that most left-handed persons are not homosexual, and most
homosexual people are not left-handed.
The Canadian scientists' paper is a meta-analysis, which has become an
increasingly popular way of combining data from multiple studies to
overcome the problems of slightly different approaches, and to pinpoint
small effects which individual studies have not enough power to detect.
There are many potential traps in meta-analyses. The paper in question
has avoided most of them--as one would expect, given the prominence and
expertise of its best-known authors, Ray Blanchard and Kenneth Zucker.
One concern, however, is that the paper was published in a
social-science rather than a medical journal. Psychological Bulletin
was certainly a correct choice for reaching the authors' target audience
(psychotherapists working with gay and lesbian clients), but it is quite
doubtful the paper got a rigorous refereeing, since the number of
specialists required to properly analyze the paper would likely exceed
the number of informed referees usually assigned to a paper by the
editorial staff of a journal. The study is severely technical; it uses
odds ratios (statistics from the field of epidemiology), and refers to
fetal masculinization (endocrinology), the Major Histocompatibility
Complex (immunology), and fluctuating asymmetry (developmental
biology).
The main caution, however, must be about the significance of
the findings. Neither the authors nor I can quantify its error, because
it was done using a meta-analysis. A known epidemiological rule of thumb
for individual surveys states that, in a test-population, a prevalence
of twice that in the control group is intriguing but inconclusive, and a
prevalence of three is probably significant. This means that if this
were an individual survey, the result of 31% above normal would be
insignificant, and that of 91% above normal (about twice the control
group) would be considered intriguing, but not decisive. This
rule-of-thumb applies to individual surveys, but it is unknown if the
rule should apply in the same manner to a meta-analysis.
Using their results, it is possible to derive a number, which shows the
extent of any link there may be between homosexuality and
left-handedness. Here's how it's done. Given that 2.7% of adult Western
males are homosexuals and 1.7% of adult Western women female homosexuals
(both figures including bisexuals, and defined as activity in the last
12 months [4]) we can calculate by standard methods that only 3.9% of
left-handed males are homosexual, and only 3.3% of left-handed females
are lesbian (5).
In other words, the overwhelming majority of those who are left-handed,
do not become homosexual. The strength of any underlying factor
producing both is weak. Something unusual appears to happen to a small
number of left-handed people, and their sexual-orientation development
is atypical. Conversely, however, and very importantly, most homosexual
people are not left-handed.
The latest study is intriguing. Nevertheless, the main routes to
homosexuality would not likely be through the route that has caused some
people to be left-handed.
References
- Lalumi¸re, M.L.; Blanchard, R.; Zucker, K.L. (2000): "Sexual
orientation and handedness in Men and Women: a meta-analysis."
Psychological Bulletin 126, 575-592.
- Although for conciseness the term "left-handed" is used in this
article, the original definition in the paper is "all those who are not
exclusively right-handed."
- Bailey, J.M., "Commentary: Homosexuality and Mental Illness,"
Archives of General Psychiatry, October 1999, vol. 56, no. 10, 876-880.
- Whitehead, N.E.; Whitehead, B.K. (1999): My Genes Made Me Do It!
Huntington House, Layfayette, Louisiana. 233 pages.
- The calculation is quite easy and useful to put in perspective other
alleged links. F1 = 0.0277 * R1 (for men) and F2 = 0.0173 * R2, where F1
and F2, the results, give the fraction of left-handers who are
homosexual (men and women respectively), and R1 is the ratio of the
percentage of left-handed homosexual men to the percentage of
left-handed heterosexual men. R2 is similar but for women. The numerical
factors are the ratio of homosexuals to non-homosexuals in a population,
thus it is 2.7/97.3 for males and 1.7/98.3 for females. Other figures
could be substituted if you think them more accurate. The formulae apply
similarly to other biological factors that are alleged to link with
homosexuality; only R1 and R2 change.
Updated: 3 September 2008
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