You must turn off your ad blocker to use Psych Web; however, we are taking pains to keep advertising minimal and unobtrusive (one ad at the top of each page) so interference to your reading should be minimal.

If you need instructions for turning off common ad-blocking programs, click here.

If you already know how to turn off your ad blocker, just hit the refresh icon or F5 after you do it, to see the page.

Psi man mascot

What Causes Homosexuality?

Are homosexuals "born that way" or is homosexuality a product of early experiences? Evidence points to a mix of factors but a predomint influence of genetics and biology.

Not every individual reports the same influences. Sexual exper­iences early in childhood are correlated with same-sex orientation in adulthood, but there is ample evidence for biological influences as well, such as the following:

Children who become homo­sexual are different from an early age.

Homosexual behavior can be produced in non-human animals by manipulating sex hormones.

Male homosexuals have brain areas and biological responses to sexual stimuli that resemble those of heterosexual females more than those of heterosexual males.

What is evidence for biological origins of homosexuality?

One study showed located 56 sets of identical twins in which one member was gay. The other twin was also gay in 52 percent of the cases (Adler, 1992).

Twenty years later, Ngun, Ghahramani, Sanchez, Bocklandt, and Vilain (2011) found 47 identical twin pairs in which one member was homosexual. Of these, there were 10 pairs in which both were homosexual. That would be a 21% concordance rate.

52% vs 21% is a big difference in concordance (the proportion of twins in whom the same trait was expressed). Other studies tilt toward the higher estimate.

Bailey, Pillard, Neale and Aqyei (1993) focused on monozygotic twins in which one was lesbian. The other was also lesbian about half the time (34 of 71).

Whitam, Diamond, and Martin (1993) located 34 male pairs and 4 female pairs of twins where at least one was gay. The concordance rate was 65% for the men and 30% for the women. (Don't ask me how you get a 30% concord­ance rate from four pairs of female twins...the possibilities would seem to be 0, 25, 50, 75 or 100%).

In any event, Ngun (2012) found clear evidence of DNA methylation effects (epigenetic effects) on gonadal hormones. These influence develop­ment of both brain and behavior. He pointed out this could influence sexual orientation.

Epigenetic effects were little known just a few years ago, although the earliest reports of them go back several decades. They occur after the fetus starts developing, so epigenetic effects could create differences between identical twins, despite their identical DNA.

In other words, epigenetic causes of homosexuality might occur even if there was little tendency for identical twins to have the same sexual orientation. As it is, identical twins do have the same orientation about half the time.

This implies that epigenetic effects on homosexuality, if they occur, are probably mixed in with genetic influences. That makes the whole issue more compli­cated to analyze.

Rice, Friberg, and Gavrilets (2013) sug­gested that epigenetic effects influence normal sexual differentiation in all ani­mals, not just humans. Epigenetic effects boost sensitivity to testosterone in XY fetuses and lower sensitivity in XX fetuses.

In a minority of organisms, epigenetic influences carry over across genera­tions and lead to mosaicism for sexual development. Mosaicism occurs when different cells in the same individual respond to different sets of DNA instructions.

This results in phenomena such as a cardinal who is half bright red (the male coloration) and half gray (the female coloration) divided right down the middle. A similar process if mixed genetic activation could result in sexual attraction for the same sex.

Rice, Friberg, and Gavrilets (2013) suggested that if epigenetic effects are widely responsible for sexual different­iation, and random variations in the process cause homosexuality, then homosexuality should be about as common in non-human animals as it is in humans. And it is.

If homosexuality is due to an epigenetic effect, what should be observable in animals?

The incidence of homosexuality in ani­mals was greatly underestimated until recent years because research­ers were too embarrassed or inhibited to mention how often it occurred. Rice, Friberg, and Gavrilets (2013) gave this example:

The early 20th century naturalist George Murray Levick recorded the following observation in his field notes while observing Adelie penguins in Antarctica: "Here on one occasion I saw what I took to be a cock copulating with a hen.

When he had finished, however, and got off, the apparent hen turned out to be a cock. The act was again performed with their positions reversed, the original 'hen' climbing on to the back of the original cock, whereupon the nature of their pro­ceeding was disclosed."

Levick was so taken aback by these "socially inappropriate" behaviors that he hid them in his notebook by recording them in code with Greek letters. He also decided against publishing them except in the relatively obscure expedition's reports—where they were rejected for publication. (Rice, Friberg, and Gavrilets, 2013)

Once Rice, Friberg and Gavrilets noticed this pattern of observations being sup­pressed or mentioned only in asides in an unrelated context, they started looking for similar reports. They found "many hun­dreds" of such examples. "For instance, homosexual behavior has been recorded in 93 species of birds."

"Even species as familiar as barnyard sheep have about 8% strictly homosexual males," they continue. "Yet almost no one except sheep breeders is aware of this fact, presumably because it has been socially inappropriate to mention it."

That sort of evidence strongly implies biological causation. However, in hu­mans, researchers also find correlations between early sexual experiences and later same-sex orientation.

In some cases early sexual experiences may "imprint" children on same-sex orientation. It is hard to disentangle cause and effect, because a child's natural interests may have some­thing to do with early experiences, but the correlations are there.

What other possible influence is suggested by correlations found in interviews?

Beard et al. (2015) collected data anony­mously from 3,443 adult participants. The 12 researchers found that early same-sex crushes were the most powerful predictor of later homosexuality. Same-sex crushes were also more likely when the opposite-sex parent "modeled an unsatisfactory romantic partner."

Furthermore, people with same-sex crushes were more likely to have early same-sex partners or masturbation experiences. So the crushes might be the result of existing homosexual tendencies, or the crushes might lead to early sexual activity that leaves an imprint on a person, or some combination of those factors.

A person might have suspicions, based on (for example) cross-dressing in early childhood, that it all "came naturally." Sometimes it is obvious. Sometimes it is difficult to know. And people differ.

Looking at averages, male and female homosexuals differ from each other in their perception of free choice. Most male homo­sexuals trace their feelings to childhood and believe they have no choice in the matter of sexual orientation.

By contrast, in a study by Lisa Diamond (2003) over half of lesbians said their sexual orientation was a "free choice." Over a quarter "relinquished their lesbian/bisexual identities" at some point, during a period of five years.

Of the quarter of lesbians who "relin­quished their lesbian/bisexual identities" in Diamond's sample, half reclaimed heterosexual identities, while the other half stopped using sexual identity labels. Diamond reports that these women did not differ from those who remained lesbians in any other way she could find, including early experience or recollection of childhood 'indicators' of same-sex attraction.

Only 20% of lesbians in an older survey viewed lesbianism as an innately determined orientation (Knox, 1984). Recall that Laumann, Gagnon, Michael, and Michaels (1994) found .3% (1 in 333) women claimed exclusively homosexual orientation, while the number was 2.4% (1 in 41) among men.

What accounts for this difference? It may be due to different biological mechanisms at work. Breedlove (2017) pointed out that markers of androgen (testosterone) exposure before birth are higher in lesbians than non-lesbians.

Androgen differences in the womb are not found with males who become gay. Breedlove suggests it may be their response to androgens that is different, not the levels produced in the womb. That is consistent with research on epigenetic effects, because they can alter responsiveness to androgen.

Perhaps there is a completely different sequence of events leading to male versus female homosexuality. This might be reflected in later feelings about inevitability or choice.

Another possibility was raised by Knox (1984). Lesbians may be more likely to feel they have a free choice because they are capable of having sex with a man.

Many lesbians discover their sexual orientation in the context of a failed marriage or troubled heterosexual relationship. The decision to concentrate on lesbian relationships may be a conscious one, made at a certain time, as a free choice.

By contrast, male homosexuals may be physically unable to have sex with a woman, unless they fantasize about a man. In such a case, a man is likely to feel he is naturally homosexual.

What are some possible explanations for the difference between male and female homosexuals in perceived freedom of choice?

Another important point to raise is that the sex act is not necessarily the most important thing that defines the gay and lesbian experience. Indeed, many gays and lesbians object to the "genitalizing" of homosexuality.

Homosexuality is not only about sex, any more than heterosexual relationships are only about sex. Intensity of emotional response may be the most important thing.

Gay men often remember "crushes" on other men, early in adolescence before sexual activity. Lesbian women report similar attachments.

What is "the most important thing" in defining homosexual experience? How do many gays and lesbians prefer to define homosexuality?

For many homosexuals, the best definition of homosexuality is one that does not involve sex. They suggest that homosexuality is simply a strong ten­dency to fall in love with members of the same sex.

The APA Takes a Stand

In 1973, the American Psychiatric Association decided to drop homosex­uality from the list of sexual deviations in the Diagnostic and Statistical Manual Revision Version III (DSM-III), making the decision official in 1974. The DSM manual, described in Chapter 12 (Abnormal Psychology), is used by clinicians to describe mental and behavior problems.

In the 1973 resolution, the association declared: "A significant proportion of homosexuals are clearly satisfied with their sexual orientation and show no signs of psychopathology (mental illness)." Therefore homosexuality was officially removed as a category of abnormal behavior.

Why was homosexuality removed as a "mental disorder" in DSM-III?

In the late 1970s, a four-year project by 31 psychologists, some gay and some heterosexual, found that homosexuality was not significantly associated with psychological problems. The project's research coordinator, William Paul, summarized some of the conclusions in an interview.

–Most gay people are not exotic or strange; their similarities to straight people are greater than their differences.

–Many homosexuals raise children "and contribute to family cohesive­ness just as straight people do."

–Most gay parents wish to keep their children's sexual orient­ation options open, not necessarily wishing them to be homosexual.

–Most gay parents are discrete with their lovers around the children. Many fear having the children taken away by the courts.

Paul added, "There simply is no evi­dence demonstrating that homosexuality or tolerance of it is either a symptom or a cause of social decline, decadence, or the fall of civilization." ("Report says homosexuals really aren't different...," August 30, 1981)

What findings were uncovered in a four-year project involving 31 psychologists?

Some people have religious or moral objections to homosexuality based on sincere beliefs. They may be offended at accusations of homophobia.

Sometimes they are upset by the "liberal bias" of the national associations such as the two APAs (the American Psychia­tric Association and the American Psychological Association). Letters to national publications such as The APA Monitor (a newsletter for members of the American Psychological Association) periodically expressed this type of senti­ment: "Where did the APA get the idea that homosexuality is normal? This does not represent my beliefs..."

Why does the APA position upset some people?

However, both APAs have taken a strong stand, based on decades of research, that homosexuality is for some people a natural orientation. It is not in itself a sign of psychological disturbance or abnorm­ality, does not necessarily cause distress to a person or harm other people, nor is it correlated with criminal behavior.


Adler, T. (1992, February). Study links genes to sexual orientation. APA Monitor, pp. 12-13.

Bailey, J. M., Pillard, R. C., Neale, M. C., & Aqyei, Y. (1993) Heritable factors influence sexual orientation in women. Archives of General Psychiatry, 50, 217-223.

Beard, K. W. et al. [12 authors] (2015). Childhood and adolescent sexual behaviors predict adult sexual orientations. Cogent Psychology, 2, 1067568. Retrieved from:

Breedlove, S. M. (2017) Prenatal influences on human sexual orientation: Expectations versus data. Archives of Sexual Behavior, 46, 1-10. doi:10.1007/s10508-016-0904-2

Diamond, L. (2003) Was it a phase? Young women's relinquishment of lesbian/bisexual identities over a 5-year period. Journal of Personality and Social Psychology, 84, 352-364.

Holden, C. (1995) More on genes and homosexuality. Science, 268, 1571.

Knox, D. (1984) Human Sexuality: The Search for Understanding St. Paul, MN: West.

Laumann, J. H., Gagnon, J. H., Michael, R. T., & Michaels, S. (1994). The Social Organization of Sexuality: Sexual Practices in the United States. Chicago, IL: University of Chicago Press.

Ngun, T. C. (2012) Molecular mechanisms underlying sexual differentiation of the brain and behavior. PhD Dissertation, UCLA. Retrieved online:

Ngun, T. C., Ghahramani, N., Sanchez, F. J., Bocklandt, S., & Vilain, E. (2011) The genetics of sex differences in brain and behavior. Frontiers of Neuroendocrinology, 32, 227-246. doi:10.1016/j.yfrne.2010.10.001

"Report says homosexuals really aren't different. (1981, August 30). Atlanta Constitution, p.26a.

Rice, W. R., Friberg, U. and Gavrilets, S. (2013). Homosexuality via canalized sexual development: A testing protocol for a new epigenetic model. BioEssays, 35, 764-770. doi:10.1002/bies.201300033

Sanders, A. R., Martin, E. R., Beecham, G. W., & Guo, S. (2014) Genome-wide scan demonstrates significant linkage for male sexual orientation. Psychological Medicine, 45, 1379-1388. doi:

Whitam, F. L., Diamond, M., and Martin, J. (1993) Homosexual orientation in twins: A report on 61 pairs and three triplet sets. Archives of Sexual Behavior, 22, 187-206.

Write to Dr. Dewey at

Don't see what you need? Psych Web has over 1,000 pages, so it may be elsewhere on the site. Do a site-specific Google search using the box below.