Is your index finger long or ring finger long? Some researchers are really trying to "tell fortunes" in this way.

Is your index finger longer or your ring finger longer? I believe that many people have seen unspoken talk related to this on the Internet. For example, the index finger has a higher emotional intelligence than the ring finger, or the ring finger has better fortune than the index finger.

Although these claims may seem as unreliable as looking at palms, surprisingly, some scientists are obsessed with discovering the mystery of finger length. According to the world's top academic journal Science, as of June, more than 20 related research papers have been published this year.

If their findings are correct, the length of the miraculous index finger is longer than the ring finger, which is related to whether a person is more likely to suffer from early-onset heart disease, publish as the first author, become addicted to alcohol, or be blessed with middle age.

As for the scientific logic behind it, scholars who insist on such studies believe that the index finger longer than the ring finger reflects the levels of testosterone and estrogen exposed during the embryonic period, affecting brain and body development.

Decided in the womb?

The index finger ratio ring finger length, also known as the "finger length ratio", is usually recorded as 2d:3d. The finger length of men is smaller than general, i.e. the ring finger of men is usually longer than the index finger. The discovery was first reported by a German anatomist in the 1970s.

But the one who really brought finger ratios into academia was John Manning, an evolutionary biologist at Swansea University in the United Kingdom. In 1998, Manning collaborated with a fertility clinic in Liverpool to study human symmetry. He found that the lower the finger length ratio in the male right hand, the higher the testosterone level tended to be.

In addition, they also found that children under 2 years of age also had sex differences in the length ratio. Manning speculates that this difference is determined before birth, reflecting hormone levels in the womb.

Studying with a simple single variable is inherently tempting, just like the reader reading this article can't help but look into the palm of his hand.

After all, there are marketable arguments about hormone levels in the womb related to acquired aggression, sexual orientation, autism, and addiction, and it is dangerous to sample directly from early embryos. If the finger length ratio can really be used as some kind of variable reflecting the embryonic environment, it is naturally better.

Manning has written more than 60 papers and 2 monographs on finger-length ratios, but did not expect to attract the attention of the world. Today, the field pioneered by Manning has grown beyond imagination. In just 20 years, more than 1,400 papers have been conducted to analyze personalities such as personality, cognitive abilities, sexual orientation, etc., using finger length ratios, or to predict the risk of cardiovascular disease, cancer, and amyotrophic lateral sclerosis. Some researchers have even tried measuring the finger length ratio of the handprints left on cave paintings to speculate whether the artist painting was male or female.

To measure finger length, the scientists used scanners, photocopiers, calipers and X-rays. However, there are also some studies that directly use the self-test data of the participants, which is fast and cheap, and BBC has sponsored an online survey that collected more than 240,000 people's self-test finger ratios.

Is gender difference an illusion?

On the other hand, the study of the length ratio has also attracted a lot of doubtful eyes, and criticisms from different angles such as research methods and research logic have poured in.

For example, some scholars suspect that the difference in the ratio of men to women in the ratio of length is simply an illusion. In the aforementioned BBC online survey, the right finger length ratio for men and women was 0.984 and 0.994, respectively, with little difference. In addition, the geographical distribution and ethnic background of the participants will affect the finger length ratio.

Other skeptics argue that the entire field is based on nihilistic theoretical foundations, and that the conclusions of the study are generally unrepeatable. In their view, there is no difference between studying the finger ratio and studying phrenology.

The fingers of human embryos begin to form around the first trimester of pregnancy, which means that to directly test the correlation between hormone levels and finger length ratios, samples must be taken from early embryos, which is very impractical. As a result, some researchers have shifted their thinking and tried to find circumstantial evidence through animal experiments.

For example, a 2011 paper published in pnas by the University of Florida reported that they injected pregnant mice with androgens or estrogens to measure their hind paws three weeks after they were born. The experimental results show that the fourth toe of the female pups stimulated by androgens is longer and the finger length is lower; in contrast, the fourth toe of the male pups stimulated by estrogen is more segmented and the finger length is higher.

The researchers believe that sex hormones can "hack" into the genetic circuits of mice, controlling bone growth.

However, when a team of researchers at the University of Münster in Germany tried to repeat the above experiment, they got the opposite result: increasing androgen levels in pregnant mice would produce male offspring with higher finger length ratios (more "femaleized"), and lowering androgen levels would produce female offspring with lower finger length ratios (more "maleized").

In addition, two large studies scanned for genetic variants associated with finger length and found no androgens to play a role in it.

Some statisticians caution that studying a "ratio" itself can be misleading because it confuses the relationship between two variables and may be statistically meaningless.

Jaroslav Flegr, an evolutionary biologist at Charles University, tested data from two finger-to-length ratio studies and found that they did not meet the statistical standards. Further mathematical analysis of the data suggests that the sex difference in the finger length ratio may be related to the size of the palm: the palm of the male is usually larger, and as the size of the palm increases, the ring finger becomes longer than the index finger.

In a 2017 study, Fryger found that after controlling for the variable palm size, the difference in finger length ratio between men and women reversed.

Kim Wallen, a neuroendocrologist at Emory University in the United States, believes that the study of forcibly linking the finger ratio to the hormones during pregnancy fundamentally shakes to what is "experimental evidence". When he was editor of the journal Hormones and Behaviors seven years ago, he decided not to accept papers on it.

In a situation where both sides are divided and chickens and ducks are talking, reviewers like Warren can't completely extinguish the enthusiasm for finger-length ratio research. More than 20 papers have been published this year.