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When I was a kid back in the 1920s, we used to play a little game in our idle moments at school. It went like this: Find some unsuspecting sod. Tell her or him: “The second sign of lunacy is hair on your palm." Unsuspecting as they were, she/he would start examining their palms, which set them up perfectly for the clincher: “The first sign of lunacy is looking for hair on your palm." Cue plenty of cackling at the sod’s expense.

We couldn’t play the game very long, because it spread through school like wildfire. Soon anyone I buttonholed and sprang it on would, with a knowing grin on her face, pointedly avoid looking at her palms.

In recent years, there has been a reason for people to look at their palms in search of a sign of something, though not lunacy. And, in the arguments about what it signifies, there hangs a tale—about science, statistics, evidence. Perhaps even about gullibility.

What I’m getting at is called the 2D:4D ratio. That is, the ratio of the length of your second (index, or 2D) finger to the length of the fourth (ring, or 4D) finger. If you look at your fingers —I bet you already are—you’ll probably conclude the two are close to the same length. That’s the case on both my hands. Though if I examine them more closely, I’d say my fourth fingers on both hands are slightly longer than the respective second fingers. What’s more, I suspect the difference is a tiny bit greater on my left hand than on my right; that is, the 2D:4D ratio is less for my left hand than for my right.

I doubt there’s any real significance to that difference. Anyway, why should I, or anyone, take an interest in finger lengths and this ratio? I never had. But then I read recently about a whole edifice of scientific endeavour built on the 2D:4D ratio. In particular, there are claims that the ratio is affected while we are in our mothers’ wombs, by exposure to testosterone and estrogen. If your index finger is shorter than the ring finger, you were exposed to higher levels of testosterone as a foetus. If the ring finger is shorter, that means lower levels of testosterone. Conversely, in each case, for estrogen. Put it all together, and there’s a negative correlation between the prenatal testosterone to estrogen ratio and the 2D:4D ratio after birth. Put another way, the 2D:4D ratio is in effect a sex marker of sorts. Just in case you are unable to tell if a given person before you is male or female, you can measure their second and fourth fingers, take the ratio of those lengths, and … well all right, that single calculation itself won’t lead you anywhere in particular in trying to determine the person’s gender. Look for other markers, please.

But as a general rule, 2D:4D ratios are slightly lower for men than for women.

But not by much. One study in 1998 looked at about 400 men and 400 women. The average 2D:4D ratio was 1 among the women (that is, the two fingers were, on average, the same length), and 0.98 among the men (the ring finger was a bit longer). A more recent online survey by the BBC attracted as many as 240,000 people who measured their fingers and then submitted 2D:4D ratios for their right hands. The average was 0.984 among men and 0.994 among women.

Ring finger slightly longer in both genders, and a tiny difference indeed between the genders. Perhaps you’re inclined to dismiss this as insignificant. But since the evolutionary biologist John Manning first proposed this correlation two decades ago, there’s been plenty of research that finds the 2D:4D ratio cropping up in all kinds of places. In fact, it’s such a seductive idea that some scientists have even measured finger lengths on handprints found on cave walls, so they can make an educated guess about the gender of the prehistoric artist who drew on those walls.

Apart from that, scientists have also claimed correlations between the ratio and plenty of behavioural and physical characteristics. For example, a lower ratio suggests, among many others, these: A longer penis, a greater risk of anorexia nervosa in women, more masculine handwriting in women, decreased empathy among men, a better feel for numbers in children, leanings towards polygamy and better musical ability. (No, not all at the same time.) A higher ratio suggests these: Belief in superstition and the paranormal, ineptness in financial matters, a womanly preference for more manly men, greater anxiety and depression among men, a tendency to develop a belly and less chance of addiction to video games. (No, also not all at the same time.) Perhaps predictably, there have also been attempts to link the ratio to sexuality: One study concluded that among lesbians, the finger ratio was lower than among straight women. More masculine, in other words.

At least some of these claims strain all credulity. Addiction to video games? Decreased empathy? (How do you measure that in a definitive way?) More masculine handwriting? (What about handwriting is identifiably gender-related anyway?) Higher numeracy? To a lot of people, this laundry-list of phenomena linked to the 2D:4D ratio brings back uncomfortable memories of “scientific" endeavours of the past that weren’t quite so scientific after all. Like physiognomy, which postulated that facial appearance is an indicator of personality traits and intelligence. Or phrenology, which measured supposed bumps on the skull and linked that to various mental characteristics. Or eugenics, which sought to use measurements like these to decide that some humans are inferior to others and should not be allowed to reproduce, so as to “improve" the human race. What’s more, any suggestion that lesbians are less “feminine" than other women should be met, I believe at a minimum, with serious scepticism.

The reported correlations have also been hard to replicate. Two Australian studies involving several thousand people, for example, concluded that there was “(no) strong evidence of testosterone involvement". Then, since experimenting on human foetuses is impossible, some studies have worked with pregnant mice, giving them shots of either testosterone or estrogen. At best, such research can only be suggestive about effects on humans—but nevertheless, while one Florida team’s results supported the connection to the hormones, a German team concluded the opposite: There was no link.

But apart from that, there are questions about the numbers—the measurements and the ratio itself. For example, some sceptics point out that the simple reality of male hands being larger than female ones could account for the ratio’s gender difference. For, as hands grow, each finger grows differently, and it’s possible the index finger’s growth simply slows down. Inevitably, a team of Czech and British scientists decided to correct for just such growth while they were investigating the ratio. The result? Males, not females, had the higher ratio. In which case, the whole testosterone/estrogen thesis crumbles.

Besides, there’s a more fundamental statistical problem in considering a ratio, too. If we extrapolate observed finger lengths downward, is there a match when both lengths reach zero? If not, any claimed relationship has problems.

To understand this, let’s say Panaji reports 30°C (86°F) on the same day that Pondicherry reports 20°C (68°F). Suppose someone divides these pairs (°F/°C), getting 2.87 for Panaji and 3.4 for Pondicherry, and then claims that the Fahrenheit/Celsius ratio suggests which coast you’re on: Lower meaning the west, higher meaning the east. I hope you’re scoffing, because you should. It’s not just that this hypothesis goes out the window if Panaji has a 20°C day. It’s really that this ratio makes no sense to calculate, because the zeros of each scale are different: 0°C corresponds to 32°F, and 0°F corresponds to -17.8°C. This, and not the coastal location, is why there is a difference in the ratio at all.

The same Czech scientists looked at several hundred finger lengths and did just this extrapolation. The zeros did not match. Thus, any claim made for the ratio of those lengths is meaningless.

Yet the idea of a single ratio predicting so much about us is so seductive, as I said above, that all this scepticism hasn’t stopped the research from marching on. To one 2D:4D sceptic, Kim Wallen of Emory University, this itself “raises some fundamental issues about what we consider evidence."

That’s the question, really. Given all these claims for the 2D:4D ratio—given any claim about anything at all, really—we should ask: What’s the evidence? If it cannot stand, neither can the claim. You can stop looking at your hand now.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun