Does the length of your fingers predict sexual orientation?

Imagine my surprise last week when I saw an article in Science that claimed "finger lengths can predict personality and health."* Huh?

The author, science writer Mitch Leslie, gives us the rather startling number that over the past 20 years, more than 1400 papers have been published linking finger lengths to personality, sexual orientation, cardiovascular disease, cancer, and more.

What is this magical finger length ratio? Simple: it's the ratio between the lengths of your index (2nd) and ring (4th) fingers, also called the 2D:4D ratio. Take a look: is your index finger longer than your ring finger?

It turns out that most people have slightly longer ring fingers than index fingers, and in men the difference is a bit larger. If the ringer finger is longer, than the 2D:4D ratio is less than one. One recent study reported that this ratio was 0.947 in men and 0.965 in women. Another study found average values of 0.984 and 0.994 for men and women. Not only is this a tiny difference, but in every study, the 2D:4D ratio among men and women overlapped, meaning the number alone doesn't tell you very much.

Nonetheless, some researchers have taken this tiny physiological difference and run with it. Nearly 20 years ago, Berkeley psychologist Marc Breedlove (now at Michigan State) published a study in Nature where he and his colleagues measured finger-length ratios in 720 adults in San Francisco. Based on this data, they concluded that finger-length ratios show

"evidence that homosexual women are exposed to more prenatal androgen than heterosexual women are; also, men with more than one older brother, who are more likely than first-born males to be homosexual in adulthood, are exposed to more prenatal androgen than eldest sons."

Whoa! They are not only claiming that the 2D:4D ratio is predictive of homosexuality, but also that exposure to prenatal androgen is the root cause of both finger lengths and sexual orientation. (Confusing correlation with causation, perhaps?) Not surprisingly, this claim is not widely accepted.

There are many, many more claims out there. In 2010, the BBC boldly reported that 

“The length of a man's fingers can provide clues to his risk of prostate cancer, according to new research.”

based on this study in the British Journal of Cancer. That study found that men whose index fingers were longer than their ring fingers had a reduced risk of cancer. (I don't believe it for a second, but if it makes you feel better, go ahead.) And a 2016 report found that both men and women with a low 2D:4D ratio (longer ring fingers) had better athletic abilities. 

The Science article goes on to explain, though, that "the results often can't be replicated." Most of these studies are small, the measurement techniques vary widely, and efforts to reproduce them (when others have tried, which isn't that often) usually fail. It didn't take me long to find a few, such as this study from 2012, which swas the 2nd failure to replicate a result claiming a link between sex hormone exposure and the 2D:4D ratio.

The author's left hand

After reading the whole Science article, one comes away with the impression that finger ratio science is almost certainly bogus. The presentation, though, gives far more space to the claims of those who believe in it, and one gets the strong impression that the journalist (Mitch Leslie) is on their side. A hint to that is in his last sentence where, after saying that the two sides are "talking past one another," he writes "more than 20 papers using the digit ratio have already come out last year."

And since the last sentence is often a giveaway for what the writer really thinks, let me conclude by saying that both my ring fingers are longer than my index fingers.

[*The print version of Science contains precisely this claim in the subheading to the article: "Some researchers say a simple ratio of finger lengths can predict personality and health." Interestingly, the online version of the same article does not have this headline. Instead, it reads "Scientists try to debunk idea that finger length can reveal personality and health." It appears as if the online editors were more skeptical than the print editors.]