Testosterone Levels, Finger Length, and Fidelity

How much can you tell about a man simply by looking at his fingers?

Until quite recently, most people probably presumed that a man’s fingers revealed relatively little beyond his marital status and, perhaps, his alma mater.

A pair of studies, however, concluded that a glance at a man’s fingers can reveal how likely he is to be nice to women and faithful in relationships.

The vital clue, according to the study authors, is the relative length of the second (index) finger and the fourth (ring) finger, which is generally called the 2D:4D ratio. Prior studies indicate that lower ratios indicate greater fetal exposure to testosterone and other androgens.

The authors of the first study, which was published in Personality and Individual Differences, recruited 155 people who spent 20 days listing every social interaction that lasted more than 5 minutes and checking boxes to indicate behaviors they exhibited during each.

After the researchers tallied each man’s agreeable and quarrelsome behaviors with women, they found that men with low 2D:4D ratios engaged in about a third fewer quarrelsome behaviors and a third more agreeable behaviors during their interactions with women.

“When with women, men with smaller ratios were more likely to listen attentively, smile and laugh, compromise or compliment the other person,” said Debbie Moskowitz, lead author and professor of psychology at McGill University.

But women might want to think twice before setting their hearts on a small-ratio man. The second study, published in Biology Letters, suggested that they are the ones who are more likely to cheat.

Researchers questioned 575 North American and British people about their attitudes toward sex and commitment and then measured the 2D:4D ratios of 1,314 British men and women.

Interview analysis revealed that, for both sexes, preferences were not distributed evenly across the “Stay — Stray” scale or in a standard distribution with a single peak. Instead, the distributions had 2 sharp peaks, 1 close to strict monogamy and 1 close to zero commitment.

The researchers — who knew that prior research suggests that men with relatively long ring fingers tend to be more promiscuous than men with relatively long index fingers — then compared these 2 double-peaked distributions (1 for each sex) in relationship preferences to the digit-ratio distributions of the other 1,314 subjects.

The digit-ratio distributions were also double-peaked. The distribution of male digit ratios, moreover, looked very similar (but not identical) to the distribution of male relationship preferences, and the distribution of female digit ratios looked very similar to the distribution of female preferences.

From this correlation, the researchers hypothesized that exposure to testosterone in the womb could be the key, or at least a key, to adult preferences for sexual fidelity.

The hypothesis made news around the world, and nearly every story warned readers to steer clear of those cheating men and their long ring fingers.

It wasn’t the first time. Since the discovery that digit length probably reflects fetal testosterone exposure, researchers have spent a lot of time trying to find correlations between digit ratios and medical outcomes, questionnaire answers, and a host of other things. A Google Scholar search of the terms “womb” and “digit ratio” returns 2,150 results.

Many of those papers generated media coverage suggesting you can tell X about a men or women simply by looking at their fingers. In some cases, the actual studies do suggest this. In most cases, however, studies contain the sort of caveat found in the British study.

“While not predictive of individual behavior,” its authors warn, “the length of the ring finger versus the index finger can help identify the group of people who are more likely to be promiscuous.”