SNPWatch: A tale of two fingers

By David Tran

Look at your hands. Is your ring finger longer than your index finger, or vice versa?

The length of a person’s index finger relative to his or her ring finger may tell us something about that individual’s exposure to sex hormones when he or she was still in the womb. But genetics also plays a big role here and recent studies are shedding light on the interplay between all these factors.

digit ratio histogram flat(1)

This graph shows the estimated distribution of people with different index to ring finger length ratios (X-axis). There’s a small but consistent pattern where women tend to have larger ratios.

While this may seem trivial, understanding how genetics and environmental factors influence finger length may give us insight into important biological functions. It may help us understand not just such things as bone development, but other seemingly unrelated traits and conditions. Recent studies show that people with autism tend to show a smaller index to ring finger ratio, while other studies have suggested associations between finger length ratio and prostate cancer, obesity, ADHD and sexual orientation. These findings are still preliminary, however, and more research is needed.

Finger lengths have fascinated scientists for a long time. As early as 1875, researchers knew there was a difference between the sexes. For many women, the index finger is nearly the same length as the ring finger, while in men the ring finger tends to be longer than the index finger. On average, a woman’s index finger is about 97-98 percent the length of her ring finger, while a man’s index finger is about 96-97 percent the length of his ring finger. A ratio of 100% means your index finger is the same length as your ring finger. One thing to be aware of, ratios can vary significantly between the left and right hands, so researchers often take the average of both hands.

These are not ironclad rules, so if your index to ring finger length ratio seems different from the average, don’t worry, a lot of people have a larger or smaller ratio. (See the accompanying graph for an estimation of the distribution of people with different finger ratios.) You can see there’s a lot of overlap between the sexes and changes in the ratios. Ratios change from person to person and may also be influenced by ancestry.

finger photo

Two volunteers from 23andMe allowed us to measure their fingers and you can see in the picture that the man has a 95% ratio while the woman has a 98% ratio.

In 2013, a team led by John Mollon at Cambridge University in England reported that the single nucleotide polymorphism (SNP) was associated with finger length ratio in people of European ancestry. The AA genotype at was associated with a 0.8 percent increase in finger length ratio, while the GG genotype was associated with a 0.8 percent decrease in finger length ratio. The SNP lies near the SMOC1 gene, which is known to regulate bone growth and has been hypothesized to control finger length growth. Independent studies have suggested that the activity of the SMOC1 gene is controlled by testosterone and estrogen levels.

In 2010, a study found that each A at was associated with a 0.6 percent increase in finger length ratio. This SNP lies near the LIN28B gene, which is important for the development of the embryo. The LIN28B gene is a key player in stem cell development. But how sex hormones affect LIN28B is less clear.

Finger length ratio is of interest to many researchers because it may shed light on exposure to sex hormones in the womb. Some studies suggest that exposure to more estrogen than testosterone is linked to larger finger length ratios, while more testosterone has the opposite effect, leading to smaller finger length ratios.


  • Sam

    Curious to know if there is any relation with those of us with brachydactly?

    • V_Pasquale

      Very interesting, I had to look it up. I can’t wait to see what they say :o)

    • David Tran

      The SNP, rs2332175, so far has not been linked to brachydactyly. Interestingly though, one of genes mentioned above, SMOC1, is also associated with some instances of brachydactyly. But typically these are due to mutations in the gene itself. So at the moment there isn’t any relation of variants at rs2332175 to brachydactyly.

  • Dustin

    In the text of the article, it says that for rs2332175, “the GG genotype was associated with a 0.8 percent increase in finger length ratio”, yet in the box on the right side of the page, it says that GG represents a “Slightly smaller finger length ratio”. Aren’t these two statements in direct conflict with one another? (An equal/opposite conflict also applies to the statements about the AA genotype)

    • ScottH

      Dustin, Thanks for your comment. This was a mistake in the original post. We’ve corrected it, thanks to you, and noted the change.

    • ScottH

      Dustin, You are right. We’ve corrected the typo. Thanks for pointing it out.

  • Deborah

    On my left hand my ring finger is longer and on my right my index is longer, what does that mean?

    • David Tran

      There’s seems to be some variation between the hands too. Often your ratios will vary between the left and right hands; that’s normal.

  • V_Pasquale

    I saw a study many years that stated people with longer ring fingers were fast runners. Since then, I’ve been interested in hearing more.
    I am a female and my ring finger is halfway between my index finger and my middle finger. I also am of European decent… and I **am** a fast runner, or **was** in my youth :o)
    … just thought I’d share.

  • Cate

    What if there’s a big difference in the ratio on the left hand compared to the right?

  • Lidia

    Does it matter what hand you look at? My ring vs index lengths are different on my two hands. The right hand has more even lengths. The left hand has a longer ring finger. (I am right handed.)

  • David Tran

    There have been studies trying to link athleticism and digit ratios. But usually those studies have pretty small sample sizes (~50-200 people) and digit ratios are pretty small to start with. At the moment, a link between digit ratios and athleticism is pretty preliminary.

  • David Tran

    There’s a few comments remarking on differences between the hands. For example, the left hand ratio is bigger than the right or vice versa. That’s normal, there’s a fair amount of variation of ratios between hands, individuals, and ethnicities. Researchers need to look at hundreds of hands to be confident that they are seeing a significant difference.

  • J

    The overlap here is so strong as to render the difference essentially meaningless. I have a “female” ratio (index longer than ring), the genes that correspond with it according to my data on 23andMe, and was assigned “male” at birth (incorrectly, I medically transitioned after high school). These kinds of obsessions over tiny statistical differences reek of pseudoscience.

  • Larka Sojourn

    So, Is it safe to assume that fingers the same length mean equal amount of hormone exposure? I didn’t read any details on that factor.

    • MrWonderful61


    • James Edward Lewis

      A 2D:4D ratio of 1 is much higher than the female mean (about 1.35 standard deviations); the difference is about 1.94 times as great as between the male and female means. It’s about 1.83 standard deviations higher than the male mean (figures from a UAlberta study about a decade ago):

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