Genetics of Sexual Orientation
Sexual orientation is a scientifically important and fundamental human
difference.Why some people are attracted to men, others to women, and
others to both men and women is a fascinating and as yet unanswered question.
Most men are primarily attracted to women, and most women are primarily
attracted to men. This is perhaps the largest psychological sex difference.
But there are exceptions–men who are attracted to men, and women
attracted to women. These individuals are sex-atypical, in other respects,
on average. Thus, understanding the origins and development of sexual
orientation can help us understand the origins and development of sex
differences as well.
Even in 2003, sexual orientation remains a controversial topic. I study
it primarily for basic scientific reasons, but I also believe that socially
controversial issues should be illuminated in a scientific light. Social
controversies can often benefit from scientific data. (I agree with the
motto of Faber College, from the movie "Animal House": "Knowledge
My current research on sexual orientation is comprised of three distinct
sub-areas: gender nonconformity, sexual arousal, and genetics.
You can get a pretty good idea about my current research interests and
ideas from reading my book.
Gender nonconformity is another term for sex atypicality. One of the best
established correlates of sexual orientation is childhood gender nonconformity.
(Download a review
of this topic by Ken Zucker and me.) Very feminine boys–those who
cross-dress, play stereotypic girls' games, prefer girls as playmates,
and who may even say they want to be girls–are likely to grow up
to become gay men. Furthermore, the average gay man recalls being more
feminine than the average heterosexual man. This link is probably weaker
for females–most masculine girls probably grow up to be heterosexual
adults–but lesbians tend to remember being particularly masculine.
Although the link between childhood gender nonconformity and adult homosexuality
is well established (especially for men), much less research has been
done on adult gender nonconformity. I have focused a great deal of attention
on this topic.
One of my first publications in this general area showed that with respect
to sexual psychology, gay men tend to be like heterosexual men, and lesbians
like heterosexual women. This is interesting because it shows that the
developmental systems that cause sexual orientation to sexually differentiate
are different from those that cause, say, interest in casual sex to sexually
But there are other ways that gay men and lesbians may differ from same-sex
heterosexuals. Stereotypes about homosexual people include sex-atypical
occupational and recreational interests, superficial aspects of behavior
such as movement and speech, and in the case of lesbians, physical appearance.
My lab is examining the validity of these stereotypes. If they are true,
on average, it will then be important to determine whether they represent
innate or social causation (or both). The idea that people's sexual orientations
can sometimes be accurately judged by listening to them say a few (nonsexual)
words is fascinating and potentially important in illuminating the origins
of sexual orientation and sex differences in articulation patterns. (Here
is a link to an interesting
page about gay articulation patterns.)
In general our lab is confirming the stereotypes, on average.
Gender nonconformity is socially important to gay men (especially) and
lesbians. In one study, we showed that gay men are massively biased against
feminine romantic partners, and lesbians are biased (but less so) against
masculine partners. (Download
a copy of this article.) However, we don't know yet what feminine
traits, precisely, gay men (or lesbians) tend to dislike. My graduate
student, Gerulf Rieger, is pursuing this question.
Gender nonconforming people (particularly gender nonconforming boys) are
often mistreated in our society. It would not be surprising if this mistreatment
had negative consequences with respect to mental health or general adjustment.
(The fact that gender nonconforming homosexual people sometimes experience
prejudice from other homosexual people cannot help.) My graduate student,
Chris Skidmore, is investigating this issue. My colleague, Galen
Bodenhausen, is also collaborating in this research.
Read an article about my
interest and research in gender nonconformity.
Sexual arousal is comprised of the subjective and genital sensation that
we get when exposed to provocative sexual stimuli. For men, sexual arousal
may be the primary way they learn their sexual orientation. That is, heterosexual
men become aroused by women, and homosexual men by men. There has long
been the suspicion that women's sexual arousal is less predictable by
their sexual orientation, and we have recently demonstrated that this
is true. In this line of research, we have studied men's and women's subjective
and physiological (i.e., genital) response to purely male, purely female,
or mixed (male-female) sexual stimuli. As expected based on prior research,
men's responses were category specific; that is, homosexual men were aroused
by male stimuli and heterosexual men were aroused by heterosexual stimuli.
Women had a very different, bisexual, pattern of arousal, on average,
no matter whether they were heterosexual or homosexual. This probably
means that men's and women's brains are organized quite differently, and
furthermore, that sexual arousal does not play as important a role in
women's sexual orientation development as it does in men's. You can download
our article, which was published in Psychological
On December 23, 2002, the Washington Times printed an article
on this research that was highly misleading. Read my initial
response to this article. When I wrote this response, our article
was being reviewed by the scientific journal where it was published,
Science, and thus I was somewhat constrained about what I could say.
I urge anyone who is interested to compare the Times article with our
In our latest work in this area, we have conducted a neuroimaging study
of heterosexual and homsoexual men while they view sexual and nonsexual
photographs. This study was led by undergraduates, Adam Safron and Ben Barch.
of Sexual Orientation
Much of my early work on sexual orientation focused on behavioral genetics.
I did several twin and family studies, which suggested that both male
and female homosexuality run in families, and that male and female sexual
orientation are moderately (but far from completely) heritable. You can
download the most
recent twin study. Here is an article about gay
Because identical (monozygotic, or MZ) twins are often discordant for
homosexuality, environment must matter. It is important to realize, though,
that "environmental" is not equivalent to "social."
There can be biological causes of MZ twin differences. We hope to begin
a study of discordant MZ twins (i.e., twin pairs with one homosexual and
one heterosexual twin).
With collaborators Alan Sanders, Khytam Dawood, Elliot Gershon, and others,
we have begun a genetic linkage study to try to replicate Dean Hamer's
famous finding of linkage on chromosomal site Xq28, and to search for
other linkage sites. This study will take several years, but we expect
a definitive answer to the question whether there is linkage at Xq28.