Most lesbians are male

The hormones

It seems obvious: if our sexual orientation has biological roots, then hormones should play a role. Because they not only influence many of our moods and reactions, they also shape our gender. In the womb, it is the presence and dosage of sex hormones that ultimately decides whether an embryo will develop into a boy or a girl. And after puberty, it is primarily testosterone and estrogen that control the growth of the beard or bosom.

Does the male sex hormone testosterone play a role in sexual orientation? © HG: Huntstock / thinkstock

What does the hormone level reveal?

But do these hormones also interfere with our sexual orientation? Researchers investigated this question back in the 1980s by analyzing blood samples from homosexual and heterosexual women and men for their hormone content. The result: Contrary to expectations, gay and straight men did not differ in either testosterone or estrogen content. The simple idea that gay men simply produce too little testosterone and are therefore more “feminine” men was largely off the table.

On the other hand, it looks less clear - once again - with the female gender: Here scientists have actually found hormonal differences between homosexual and heterosexual women. In most of the studies, lesbian women had slightly higher testosterone levels. However: Because of the female cycle, the hormone levels in women fluctuate more than in men. And not all studies have taken other influencing factors such as height and weight into account. Researchers now assume that sex hormones hardly play a role in sexual orientation in adulthood.

Two genetic defects and their consequences

The situation is different, however, with the hormone supply in the womb: There are several indications that the amount of prenatal testosterone can influence sexual orientation. Two genetic defects provide an indication: In so-called congenital adrenal hyperplasia (CAH), the hormonal glands of the fetus produce the male sex hormone instead of the stress hormone cortisol. As a result, genetically female unborn babies get way too much testosterone. The result: as adults, significantly more of these women are lesbian than the average.

And there is also the reverse case: In people with complete androgen resistance (CAIS), the receptor for the male sex hormone is deactivated due to a genetic defect. As a result, the cells and tissues of the unborn child do not respond to testosterone - and genetically male fetuses develop as girls. Almost all CAIS patients later feel attracted to men - based on their genetic gender, they are homosexual.

The length ratio of the index finger to the ring finger allows conclusions to be drawn about the prenatal testosterone dose. © Pixels / pixabay

The secret of finger length

Another indication of prenatal hormonal influence is literally obvious: the length of our fingers. Typically, in men, the ring finger is slightly longer than the index finger. In women it is usually the other way around or both fingers are the same length. Researchers have found that this so-called 2D: 4D ratio is determined by the testosterone levels in the womb. The more testosterone the fetus receives, the more pronounced the difference in finger length.

The interesting thing about it: The sexual orientation can often be read from the length of the fingers - at least for women. Studies show that lesbian women tend to have a “male” 2D: 4D ratio on average. As with most men, their ring fingers are therefore longer than their index fingers. In homosexual and heterosexual men, however, the findings are less clear. Some scientists found no differences at all, others even observed "hyper-male" 2D: 4D ratios in gay men.

Not just testosterone?

However, testosterone may not be the only hormone that sets the course in the womb. The corpus luteum hormone progesterone could also play a role, according to a study published in 2017. June Reinisch from Indiana University and her colleagues examined 17 men and 17 women who were born between 1959 and 1961 at Copenhagen University Hospital. Their specialty: All 34 test subjects were exposed to increased progesterone doses in the womb because their mothers had received a hormone preparation against a threatened miscarriage at the time.

In addition to testosterone, progesterone could also have prenatal effects. © janula / thinkstock

The question now was: had the prenatal progesterone glut influenced the sexual orientation of these people? The evaluation showed: “Compared with non-exposed persons, fewer of these men and women identified themselves as heterosexual,” report the researchers. “They also reported more and more about same-sex attraction and corresponding sexual contacts.” According to Reinisch and her colleagues, this suggests that progesterone could also be involved in the complex structure of our sexual orientation.

In summary, this means: There are numerous indications that hormone levels in the womb play an important role in our sexual orientation. However, which mechanisms take effect and which hormones have which effects remain largely unexplained.

Nadja Podbregar
Status: June 29, 2018

June 29, 2018