What can your palm reveal about your sexual preferences? Fingers and the sexual brain

About 30 years ago, it was discovered that there is a difference between men and women in relative finger length (which is not due to men being taller than women) (1). Researchers proposed that by the end of the first trimester of pregnancy, the level of testosterone in the fetus (which is about five times higher in males than in females) determines the relative length of the fingers. Why is this important? Because the level of testosterone in the fetus affects the sexual differentiation of the brain, but we don't know what our testosterone level was when we were fetuses. Therefore, the relative finger length in adults can be used to retroactively infer this. If we know the level of fetal testosterone, we can learn about the sexual development of the brain and, consequently, about later sexual behavior: sexual attraction, gender identity, a child's preferred type of play, and more.

Feeling lost? Let's start now in an organized manner.

We are not born as a "blank slate" regarding sexual behavior. The brain is already shaped in the fetus as either a male or female brain. For example, if a pregnant rat is injected with the male hormone testosterone (so-called because the testes produce it in large quantities), the female offspring are born with a male appearance (an enlarged clitoris resembling a penis). In adulthood, these females try to mount males and mate with them in a male-like manner! It's as if the exposure to very high levels of testosterone during fetal development turned them into lesbians!

Likewise, if a pregnant rat is injected with a substance that prevents testosterone from working, her male offspring will display more female-like behavior in adulthood. Stress during pregnancy creates a similar effect by lowering testosterone levels. This happens because the body produces cortisol (the stress hormone) from the same source-material (cholesterol) that is also used to synthesize testosterone. Therefore, during stress, more cortisol is produced and less testosterone. The result is that when a pregnant rat is exposed to high stress, there is an increased production of cortisol and, more importantly for our discussion, a decreased production of testosterone. Consequently, the male offspring will exhibit more female-like behavior in adulthood (2).

As a side note, in addition to testosterone and cortisol competing for cholesterol for their synthesis, during stress, the adrenal gland produces a substance similar to testosterone that doesn't act like testosterone but tricks the testes into reacting as if testosterone levels are too high. The result is that the testes reduce their testosterone production (this leads to testicular shrinkage, as happens with bodybuilders who use testosterone to increase muscle mass—their testes sense that there is already testosterone and therefore produce less and shrink).

In conclusion, in rats, female fetuses exposed to high levels of testosterone exhibit male sexual behavior in adulthood, and male fetuses exposed to low levels of testosterone exhibit female sexual behavior in adulthood!

Okay, this is about rats, maybe in humans, sexual orientation is determined differently? In the 1960s, it was indeed thought that this was the case and that upbringing was what mattered: in other words, if you raise a baby boy as a girl, they will develop typical female behavior. However, the tragic case of David Reimer (which I wrote about here) showed that this is not the case, and from there, it became clear that in humans as well, the brain is shaped during the fetal stage as either male or female. Upbringing does not affect sexual orientation or gender identity. Since the case of David Reimer, there have been systematic studies that support the same conclusion: we are born with a brain shaped as male or female, and the theory that we are born as a 'blank slate' sexually (The theory of sexual neutrality) is incorrect (3).

I wrote that the brain is shaped as male or female, but most of the brain looks the same between men and women, except that the male brain is larger. (For all the men who may now feel superior to women due to brain size—well, Einstein had a smaller-than-average brain, and an elephant has a larger brain than ours. In other words, a bigger brain doesn’t mean more intelligence.)

Although most of the brain is similar between men and women, there are very specific areas of the brain where differences are noticeable between men and women (and also in rats). Moreover, one area of the brain that is smaller in women than in men appears, on average, like the female brain in gay men (4). Similarly, another area of the brain correlates with gender identity: in trans women (men who feel they are women and have undergone surgery to match their bodies to their feelings), the size of this area is like that of women, not like that of heterosexual men or gay men (5). In other words, the size of this area is related to gender identity, not sexual orientation, whereas the former area is linked to sexual orientation. Turns out, that in rats, the size of these areas is determined by testosterone. In conclusion, we are not born as a "blank slate" in terms of sexual orientation and gender identity, because testosterone shapes the brain as a female or as a male brain.

This field is highly politically charged, and the researchers who published these findings have had to contend with insults and threats to their lives! This is because conservative groups argue that sexual orientation and gender identity (as created by God) must have been heterosexual, and certainly not transgender. These groups claim that flawed education, cultural trends, and even confusion lead people to adopt same-sex orientation and/or transgender identities. Therefore, these groups promote conversion therapy to return to what they consider the true natural state, which, in their view, cannot possibly be anything but heterosexual, let alone a transgender sexual identity. (It is important to note that, based on the available scientific knowledge, the World Health Organization opposes conversion therapy due to its ineffectiveness compared to the psychological risks involved).

Certain liberals also feel uncomfortable with these findings because they seem to show inherent inequality between men and women. However, differences do not imply inequality. There are differences (duh?) between men and women, and that's it. It doesn't mean one sex deserves more rights because of it, or that one sex is superior. Lastly, LGBT organizations also do not appreciate these results either, as they feel these findings contribute to the pathologization of their identities, meaning they classify them as having a pathology, mutation, or even something close to a disease. Again, however, differences do not mean better or worse, unhealthy, nor do they justify inequality. In any case, the field is sensitive and charged, and many people are angry when science does not support their agenda.

Beyond the political aspect, it should be emphasized that, so far, in humans, only correlations have been found between the size of certain brain regions and sexual orientation and gender identity. However, we do not know what these regions do in this context, nor do we know whether they - or other regions - are responsible for sexual orientation and gender identity. We only know that there are innate differences in the brains of transgender and homosexual individuals, but we do not know what the significance of these differences is. An experiment like the one conducted on rats (injecting testosterone into a pregnant women and examining whether their baby girls develop a sexual orientation and gender identity of a boy) is, of course, shocking, unethical, and would not be conducted. However, it naturally happens in humans that, due to illness during pregnancy with a female fetus, the fetus is exposed to higher levels (relative to other females) of testosterone, and indeed, the result is that a much higher percentage of them develop male sexual orientation and/or gender identity compared to the percentage in other females. Similarly, human males (genetically) who, due to a mutation, do not respond to testosterone during fetal development usually develop with a female sexual orientation and gender identity. The conclusion is that, even in humans, we are not born as a "blank slate," and already at birth, the brain is shaped as male or female, related to sexual orientation and gender identity which is caused by testosterone. However, keep in mind that sexual orientation can change during life, and in addition as of now, we do not know exactly how sexual orientation and gender identity are determined in humans.

In nature, same-sex behavior is very common. However, in nature, there are no social sanctions for such behavior, and therefore, animals that engage in same-sex sexual activity and/or long-term same-sex relationships typically also engage in sexual activity with the opposite sex. However, there are also cases of exclusively same-sex sexual behavior, 8% of rams are like this!

So, what I wanted to say so far is that the brain is already shaped as male or female during the fetal stage, and this occurs under the influence of the sex hormone -testosterone. The different shaping of the brain leads to typical behaviors for women versus men, with the most prominent being sexual orientation: most women (about 95%) are sexually attracted primarily to men, and most men (about 95%) are sexually attracted primarily to women (6). Once the brain has shaped, it is already formed. Although the brain can change to some extent in adulthood ('plasticity') (7), I am not aware of findings supporting plasticity in these regions after birth.

I hope that by now you are convinced that it would be interesting to know the level of testosterone in the embryo during pregnancy, but how can we know what that was? This brings me back to the finger issue: during pregnancy, testosterone not only affects the brain but also impacts the developing body. Testosterone causes the development of the penis, scrotum, prostate, and, importantly for our discussion, it has been found that the level of testosterone in the fetus also influences the relative growth of the fingers (I'm going to explain what this means in a minute), and the relative length of the fingers stays stable throughout life (8). This means that one can measure the length of the fingers in an adult and infer the level of testosterone they were exposed to during fetal development, and from that, deduce whether their brain developed as male or female!

Before continuing with the science, I will now explain how to measure the relative finger lengths. Take either the right or left hand - it doesn't really matter - and look at the palm. The thumb is the first finger, and now measure the length of the second finger (2D) and the fourth finger (4D). Now divide the length of the second finger by the length of the fourth finger. What you get is called the 2D/4D ratio. By the way, for a more accurate measurement, it’s better to take a photo of the hand with a scanner and measure the finger lengths from the image. For example (from reference 9):

Illustration of finger markings on the left hand and the length of each finger

It turns out that this ratio is slightly higher in women compared to men. For example, in this study (10), they found that for women's right hand, the average ratio is 0.979, and for men, it is 0.961. As you can see, the difference between the averages is very, very small, barely a 2 percent difference. While this is a statistically significant difference, it is only less than 2%, and of course, people don't have the exact average; for some people, the ratio is smaller, and for others, it is larger, resulting in a very large overlap between men and women. The main difference is at the extremes. In fact, if we randomly take a man and a woman and measure their 2D/4D ratio, only about 60% of the time will the woman have a larger ratio. (Different studies yield similar but slightly different averages; for example, here (1) they showed a 2D:4D ratio of 1 for women and 0.98 for men. One of the factors for the difference is ethnic origin. However, the small difference between the sexes is always present).

The image below shows the distribution of the 2D:4D ratio for the right hand in women (dashed line) and men (solid line) from another study (11). On the X-axis, you see the size of the 2D:4D ratio, and on the Y-axis, you see how many men and women have this ratio. You can see that for women - the dashed line - the ratio is higher, but there is a lot of overlap with the men (solid line) group. (Here, too, the difference between men and women is close to 2%; the average in this study for the right hand was 0.9770 for women and 0.9594 for men). So, if your ratio is above 1.05, you most likely have a female brain and are a woman, and if the ratio is below 0.9, you most likely have a male brain and are a man. However, different studies give slightly different numbers, and in any case, most people are not at the extremes, and there is a lot of overlap between the groups.

So, if you got excited about the measurement you just did because you thought it would tell you whether your brain is “male” or “female,” it turns out it doesn’t really tell you that. Disappointing? Yes, and more about it at the very end of this post . Anyway, in scientific terms, we say such a connection is statistically significant but with a “small effect size.” In other words, there is a significant correlation between the two variables - the 2D/4D ratio and whether someone is male or female - but you cannot infer an individual’s sex based on this measurement. This effect only becomes visible when you take the average of thousands of measurements in men and women; then you can tell which group is composed of women and which of men.

In fact, there are other bodily markers determined by prenatal testosterone exposure that remain stable after birth and show a much larger difference between men and women than the 2D/4D ratio - for example, the distance from the anus to the base of the genitalia which is much higher in men versus women (12). However, it is much easier to measure finger ratios, which is why this measure has become very popular to infer retroactively embryonic testosterone level .

Many studies have been conducted on the question of whether the 2D/4D ratio differs in homosexuals, lesbians, and transgender individuals, and the results have been quite confusing. However, it now seems that there are indeed differences (13). On average, there are differences, but remember that the difference is very small, so it is only noticeable when averaging a very large number of people. In homosexuals, the 2D/4D ratio is higher than in heterosexual men, and in lesbians, it is lower than in heterosexual women. By the way, not only sexual orientation, but there is also a correlation between the 2D/4D ratio and other traits, such as fertility (1), athletic ability, and social behavior (7), economic behavior and risk-taking (10), autism risk, and finally, yes, there is a correlation with sexual identity: in transgender women (genetically male individuals who transitioned to women), the 2D/4D ratio indeed appears more feminine (higher values) compared to other men (14). However, in transgender men (genetically female individuals who transitioned to men), the 2D/4D ratio does not appear more masculine (14), and it is unclear why. In my opinion, this results from the small effect size that requires large groups to see the effect.

Another interesting preliminary finding: When there is a pregnancy with twins, one male and one female, the female will have a more masculine 2D/4D ratio, likely because she is exposed to some of the testosterone her male-twin produces (15). However, it seems that the behavioral change is not significant, and the play of girls who were in pregnancy with their testosterone-producing brother is not different from the play of other girls. (In animals, the effect of a male fetus's testosterone on its adjacent sister in the womb, as reflected in her adult behavior, is greater).

Approaching the end, and for those who like it deeper (biology wise) I will now write about the mechanism that causes the difference in the relative finger length, and the key word is 'cause'. In research, when you want to show that something - (X) - causes something else - (Y) - you change X and check if Y changes. That is, in the context of this post, you change the level of hormones in the fetus and see if it changes the finger length. We obviously don't do such an experiment on humans, but on animals. The question then is how relevant are the findings from animals to humans. In any case, in mice, the 2D:4D ratio is also larger in females compared to males, and in mice, which allows researchers to experimentally change the testosterone level in the fetus and see if it changes the 2D:4D ratio. Although for simplicity I have so far written only about testosterone, it turns out that the hormone estrogen is also involved, and in fact, it is the testosterone to estrogen ratio that determines it (8). Testosterone causes the growth of the fourth finger, and estrogen inhibits the growth of the fourth finger. The hormones do this by changing the activity-level of genes that control bone growth in the fingers of the fetus. It is difficult to prove that this is also the case in humans, since the testosterone level in the amniotic fluid must be measured in the 13th-14th week of pregnancy, the time when the finger length ratio is determined. The problem is that this is an invasive measurement that can cause a miscarriage, and therefore it is not done for research purposes. So what do we do? Usually, they check the testosterone levels in the umbilical cord at birth and try to find a link to the 2D:4D ratio, but this is long after the time when testosterone affected the fingers (and the brain). In fact, the testosterone level at birth does not indicate what its level was before, and either no link is found or only a weak one between the testosterone level at birth and the finger ratio (16). But on the other hand, there are many correlations between the 2D:4D ratio and situations in which fetal testosterone levels are different from normal. For example, female fetuses exposed to higher levels of testosterone indeed have a lower 2D:4D ratio in adulthood than women who were not exposed to higher levels of testosterone (16).

I assume that for most readers, the mechanism is less important and what matters is what finger length says - for example - about your sexual orientation. Well, about you specifically, the 2D/4D ratio does not say not much, but if we add a few thousand more people with the same sexual orientation as you, we will find a difference between you and people of the same sex but with a different sexual orientation. Is it disappointing after reading such a long post? Well, overall, you don't really need someone to tell you what your sexual orientation is. Not based on your finger length, or at all. This shouldn't be disappointing, because I hope the reason you read this was that the topic , of sexual development of the brain and the 2D:4D ratio intrigued you and now you know much more. Your curiosity has been satisfied even if - as I hope is the case - new questions have arisen. I encourage acting out of curiosity and enjoying the never-ending discovery.

This is in science and I think this is also true for love.

I hope you enjoyed the post and I would love to hear your comments.

I spend many hours and work hard

to provide you high-quality, in-depth, and free content on the science of sexuality. If you would like to thanks me, I’d appreciate it if you could buy me a coffee.

Thank you in advance!

Coffee = $4 and it reflects your gratitude which worth for me way more than 4$!

Would you buy me some coffee?

References In most cases I only mention the last author, which is usually the name of the lead researcher from the list of authors.

1. The ratio of 2nd to 4th digit length: A predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen, Human Reproduction, (1998), Lewis-Jones, D. I.

2. Prenatal stress feminizes and demasculinizes the behavior of males. Science, (1972), Ward, I L

3. Discordant sexual identity in some genetic males with cloacal exstrophy assigned to female sex at birth.The New England journal of medicine, (2004), Gearhart, John P.

4. A difference in hypothalamic structure between heterosexual and homosexual men, Science, (1991), LeVay, Simon.

5. Sexual differentiation of the brain and behavior, Best Practice and Research: Clinical Endocrinology and Metabolism, (2007), Swaab, Dick F.

6. In support of 2D:4D: More data exploring its conflicting results on handedness, sexual orientation and sex differences, PLoS ONE, (2023), Turiegano, Enrique.

7. Associative fear learning enhances sparse network coding in primary sensory cortex, Neuron, (2012), Gdalyahu Amos, Trachtenberg Joshua.

8. Developmental basis of sexually dimorphic digit ratios, PNAS - Proceedings of the National Academy of Sciences of the United States of America, (2011), Cohn, Martin J.

9. Investigating the reliability and sex differences of digit lengths, ratios, and hand measures in infants, Scientific Reports, (2021), Schaal, Nora K.

10. Sex, population origin, age and average digit length as predictors of digit ratio in three large world populations, Scientific Reports, (2021), Batsevich, Valery.

11. Experimental And Self‐reported Measures Of Risk Taking And Digit Ratio (2d:4d): Evidence From A Large, Systematic Study, International Economic Review, (2018), Nieboer, Jeroen.

12. Study Of Anogenital Index And 2:4 Digit Ratio In Human Fetuses: An Anthropometric Study From Northern India, Journal of Anatomical Sciences, (2017), Kaushik, Ankit.

13. Sexual orientation is associated with 2D:4D finger length ratios in both sexes: an updated and expanded meta-analysis, Frontiers in Psychology, (2025), Puts, David.

14. Digit ratio (2D:4D) and transgender identity: new original data and a meta-analysis, Scientific Reports, (2020), Kornhuber, Johannes.

15. Finger-length ratios show evidence of prenatal hormone-transfer between opposite-sex twins, Hormones and Behavior, (2006), Wilbur, Christopher J.

16. Relationship between the 2D:4D and prenatal testosterone, adult level testosterone, and testosterone change: Meta-analysis of 54 studies, American Journal of Biological Anthropology, (2024), Kowal, Mart