Finger length ratios offer clues to how the womb shapes sexual orientation

A recent analysis of over two hundred thousand people reveals that the relative lengths of a person’s index and ring fingers are linked to their sexual orientation. The research suggests that the hormones a fetus is exposed to in the womb shape both physical development and whom that person is attracted to later in life. These results were recently published in the journal Frontiers in Psychology.

Before birth, developing fetuses are exposed to different levels of sex hormones. These hormones, such as testosterone and estrogen, play a central role in shaping physical differences in the body. They also help regulate how genes are expressed in the developing brain.

Testing exactly how these early chemicals affect human development is difficult. It is not ethical to alter hormone levels in pregnant women to see what happens to their children. Instead, researchers look for physical traits that act as biological markers of the environment inside the womb.

One widely studied marker is the ratio between the length of the index finger and the length of the ring finger. Men generally have a shorter index finger compared to their ring finger. Women typically have index and ring fingers that are closer to the same length.

This physical difference is tied to the hormone environment during fetal development. Fetuses exposed to higher levels of testosterone tend to develop a more male-typical finger length ratio. Those exposed to lower levels of testosterone, or higher levels of estrogen, develop a more female-typical ratio.

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Researchers know this because they have studied people with specific medical conditions. For example, women with congenital adrenal hyperplasia are exposed to elevated levels of testosterone before birth. These women tend to have more male-typical finger proportions.

Conversely, some individuals are born with a condition called androgen insensitivity syndrome. Their bodies do not respond to testosterone. These individuals tend to have more female-typical finger proportions.

Because hormones shape both the brain and the body, researchers use finger measurements to guess how the brain might have been influenced before birth. Sexual orientation is one of the most strongly sexually differentiated psychological traits in humans. Researchers suspect that the same hormones that shape the fingers also shape sexual attraction.

Ashlyn Swift-Gallant, a psychology researcher at Memorial University of Newfoundland and Labrador, led a team to investigate this connection. A previous review of the topic from 2010 found a link between finger proportions and sexual orientation in women. That older review found no such link in men.

That older review had a few limitations. It only looked at published data, which can skew the overall picture. Studies with results that are not statistically significant often go unpublished, sitting forgotten in a filing cabinet.

This is known in science as the file drawer problem. The 2010 review also grouped bisexual individuals together with exclusively homosexual individuals. Swift-Gallant and her team wanted to update the science by including unpublished data while treating bisexual people as a distinct group.

To gather their data, the research team conducted a meta-analysis. A meta-analysis is a statistical method that combines the results of many different independent studies to find broader patterns. This allows researchers to see the big picture across thousands of participants.

The researchers scoured academic databases to find every paper matching their criteria. They did not stop at the published literature. To avoid the bias of only looking at successful experiments, the team contacted nearly three hundred researchers to ask for unpublished data.

In the end, they compiled data from fifty-one studies. This included forty-four datasets focused on men and thirty-four datasets focused on women. In total, the analysis included measurements from 227,648 participants.

The researchers broke down the data by sex and sexual orientation. They compared exclusively heterosexual individuals against exclusively homosexual individuals. They also ran separate comparisons for the bisexual participants.

The results showed a clear pattern for women. Exclusively homosexual women tended to have a lower, more male-typical finger length ratio than exclusively heterosexual women. This pattern held true for measurements taken from both the right and the left hands.

For men, the updated data painted a different picture than the older 2010 review. The team found that exclusively homosexual men tended to have a higher, more female-typical finger ratio than exclusively heterosexual men. This suggests that gay men may be exposed to less testosterone or more estrogen before birth compared to straight men.

The researchers applied statistical tests to check if their results were warped by publication bias. One method they used removes extreme outliers and fills in hypothetical missing studies to balance the data. Another method tests the strength of the data by removing one study at a time to see if the overall pattern collapses.

They found no evidence of publication bias for the data on women. For the men, published studies were slightly more likely to show positive results than unpublished studies. Even so, the mathematical corrections confirmed that the overall pattern for men remained robust.

The data for bisexual participants offered another layer of detail. The researchers found that the finger proportions of bisexual men and women were more similar to those of heterosexual people. They were less similar to the finger proportions of exclusively homosexual people.

This detail might explain why previous reviews missed the pattern in men. By grouping bisexual men together with homosexual men, older analyses may have watered down the mathematical differences. Treating everyone with any level of same-sex attraction as a single group can hide important variations.

The team also performed moderator analyses to see if the way a study was conducted changed its results. A moderator analysis looks at external variables, such as the geographic location of the participants or the tools used to measure the fingers. They found that the method of measurement mattered.

Studies that used photocopies or digital scans of hands produced more reliable data. In contrast, having participants measure their own hands often resulted in weaker data. The researchers suggested that self-reporting is less accurate than using an independent rater to measure a digital scan.

The researchers noted a few caveats to their work. First, the size of the physical difference between the sexual orientation groups is relatively small. The difference in finger proportions between a straight man and a gay man is smaller than the average difference between a man and a woman.

This small effect size makes sense in a biological context. Sexual orientation is shaped by many different biological and environmental factors. Prenatal hormones are just one piece of a much larger puzzle.

Second, the finger length ratio is an imperfect proxy for the environment of the womb. It is influenced by genetic factors alongside testosterone and estrogen. Because of this, the fingers only offer a rough estimate of the chemicals a developing baby was exposed to.

There might also be a ceiling effect for men. Once a male fetus is exposed to enough testosterone to masculinize his body, extra testosterone might not change his finger proportions any further. This could make finger measurements less sensitive to hormonal changes in males than in females.

The team also suggested directions for future research. They pointed out that grouping all homosexual individuals into one category might hide important biological variations. Past research has shown that subgroups, such as lesbians who identify as more masculine, show even more pronounced differences in finger proportions.

Future studies should look at these subgroups more closely to uncover more detailed biological pathways. Researchers might also benefit from measuring attraction to men and attraction to women as separate scales. Placing them on opposite ends of a single continuum might obscure how different hormones affect different types of attraction.

Finally, the team recommended that future studies pay closer attention to demographic factors like ethnicity. Different populations might have different baseline finger proportions. Tracking these demographic details could help clarify the relationship between physical development and human behavior.

The study, “Sexual orientation is associated with 2D:4D finger length ratios in both sexes: an updated and expanded meta-analysis,” was authored by Ashlyn Swift-Gallant, Toe Aung, Stephanie Salia, S. Marc Breedlove and David Puts.