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.