You can tell something about a person’s prenatal life by looking at their hands. Specifically, by comparing the length of their index finger to their ring finger.
This measurement, called the 2D:4D ratio, reflects the balance of estrogen and testosterone a fetus was exposed to during the first trimester. High estrogen relative to testosterone means a longer index finger. Low estrogen, more testosterone, and the ring finger pulls ahead. The pattern gets locked in early and stays with you.
It sounds like the kind of thing that shouldn’t matter much. But it does. The ratio correlates with everything from athletic performance to disease risk to cognitive traits. And now, according to research published in Early Human Development,1 it may also offer a window into one of the strangest bargains our species ever made.
