A recent analysis of more than 200,000 people found that the relative length of the index and ring fingers was associated with sexual orientation. This study suggests that the hormones a fetus is exposed to in the womb determine its physical development and who a person is attracted to later in life. These results were recently published in the journal frontiers of psychology.
Before birth, the developing fetus is exposed to varying levels of sex hormones. These hormones, such as testosterone and estrogen, play a central role in shaping the body’s physical differences. They also help regulate how genes are expressed in the developing brain.
It is difficult to test exactly how these early chemicals affect human development. It is not ethical to alter a pregnant woman’s hormone levels to see what happens to her child. Instead, researchers are looking for physical characteristics that serve as biological markers of the intrauterine environment.
One widely studied indicator is the ratio of index finger length to ring finger length. It is common for men to have a shorter index finger than their ring finger. Women usually have their index and ring fingers close to the same length.
This physical difference is related to the hormonal environment during fetal development. Fetuses exposed to higher levels of testosterone tend to develop more male-typical finger length ratios. Those exposed to lower levels of testosterone, or higher levels of estrogen, develop more female-specific ratios.
Researchers know this because they study people with certain medical conditions. For example, women with congenital adrenal hyperplasia are exposed to high levels of testosterone before birth. These women tend to have finger proportions that are typical of men.
Conversely, some people are born with a condition called androgen insensitivity syndrome. Their bodies do not respond to testosterone. These individuals tend to have finger proportions that are more typical of women.
Hormones shape both the brain and body, so researchers use finger measurements to infer how the brain was affected before birth. Sexual orientation is one of the most strongly sexually differentiated human psychological characteristics. Researchers think that the same hormones that form fingers may also form sexual attractiveness.
Ashlyn Swift-Gallant, a psychology researcher at Memorial University of Newfoundland and Labrador, led a team investigating this link. A previous review on this topic in 2010 found a link between female finger proportions and sexual orientation. That older review found no such association in men.
That old review had some limitations. Because we only looked at publicly available data, the overall picture may be distorted. Studies with results that are not statistically significant often go unpublished and remain forgotten in file cabinets.
This is known in science as the file drawer problem. A 2010 review also grouped bisexual individuals with homosexuals only. Swift Gallant and her team wanted to update the science by including unpublished data while treating bisexual people as a distinct group.
To collect the data, the research team conducted a meta-analysis. Meta-analysis is a statistical method that combines the results of many different independent studies to find broader patterns. This allows researchers to get a complete picture across thousands of participants.
The researchers scoured academic databases to find all papers that matched their criteria. They went beyond published literature. To avoid the bias of looking only at successful experiments, the team contacted about 300 researchers and asked for unpublished data.
Ultimately, they compiled data from 51 studies. This included 44 datasets focused on men and 34 datasets focused on women. In total, the analysis included measurements from 227,648 participants.
The researchers broke down the data by gender and sexual orientation. They compared exclusively heterosexuals with exclusively homosexuals. They also performed separate comparisons for bisexual participants.
The results showed a clear pattern for women. Homosexual-only women had lower finger length ratios that were more male-typical than heterosexual-only women. This pattern holds true for measurements taken from both the right and left hands.
For men, the updated data painted a different picture than the older 2010 review. The researchers found that exclusively homosexual men tended to have a higher proportion of female-specific digits than exclusively heterosexual men. This suggests that homosexual men may receive less testosterone or more estrogen prenatally compared to heterosexual men.
The researchers applied statistical tests to see if the results were skewed by publication bias. One method they used was to remove extreme outliers and fill in missing hypothesis studies to balance the data. Another method is to test the strength of the data by removing one study at a time to see if the overall pattern is broken.
They found no evidence of publication bias for data on women. For men, published studies were slightly more likely to show positive results than unpublished studies. Still, mathematical corrections confirmed that the overall pattern for men remained robust.
Data from bisexual participants revealed further details. Researchers found that bisexual men’s and women’s finger ratios are similar to those of heterosexuals. They did not have much resemblance to the proportions of exclusively homosexual fingers.
This detail may explain why previous reviews missed the male pattern. Older analyzes may have diluted the mathematical differences by grouping bisexual men with gay men. Treating all people with all levels of same-sex attraction as a single group can mask important differences.
The team also conducted a moderator analysis to see if the results changed depending on how the study was conducted. Moderator analyzes examine external variables such as participants’ geographic location and the tools used to measure their fingers. They found that the method of measurement matters.
Studies using hand copies or digital scans have yielded more reliable data. In contrast, asking participants to measure their own hands often results in weaker data. The researchers suggested that self-report was less accurate than using independent raters to measure digital scans.
The researchers noted several caveats to their study. First, the magnitude of physical differences between sexual orientation groups is relatively small. The difference in finger proportions between straight men and gay men is smaller than the average difference between men and women.
This small effect size makes sense in a biological context. Sexual orientation is shaped by a variety of biological and environmental factors. Prenatal hormones are just one piece of a much larger puzzle.
Second, finger length ratio is an imperfect proxy for the uterine environment. Along with testosterone and estrogen, it is influenced by genetic factors. Because of this, fingers only give a rough estimate of the chemicals a developing baby has been exposed to.
There may also be a ceiling effect for men. Once a male fetus is exposed to enough testosterone to masculinize its body, the extra testosterone may no longer change the proportions of its digits. This may make finger measurements less sensitive to hormonal changes for men than for women.
The research team also suggested directions for future research. They pointed out that grouping all homosexuals into one category could mask important biological differences. Previous research has shown that subgroups such as lesbians who identify as more masculine show even more pronounced differences in finger proportions.
Future studies should further examine these subgroups to uncover more detailed biological pathways. Researchers may also benefit from measuring attraction to men and attraction to women as separate scales. Placing them at opposite ends of a single continuum can obscure how different hormones affect different types of attraction.
Finally, the researchers recommended that future studies pay more attention to demographic factors such as ethnicity. Different populations may have different baseline digit proportions. Tracking these demographic details could help reveal the relationship between physical development and human behavior.
The study, “Sexual orientation is associated with 2D:4D finger length ratio 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.
