Because physical traits differ with geography, Dr. Snow limited his study to caves in northern Europe. He also gathered hand measurements from over a hundred students of northern-European ethnicity, to create a modern benchmark. He found that hands fall along a continuum, from very dainty, feminine hands to very masculine hands, with a big grey area in between. He eventually developed a two-step process to predict sex just based on hand prints.
First, measure the lengths of the fingers and palm, and put the numbers into a formula Snow developed. That sorted modern hands into male and female with about 80 percent accuracy. But adolescent males, whose hands haven't reached full size, too often got lumped in with women, so Snow added a second step: look at the ratios between the fingers.
For women, the index and ring finger are often the same length, though occasionally the pointer is longer. For men, the ring finger is often noticeably longer. Then compare the pinkie to the others. In most males, young or old, the pinkie is nearly as long as the ring finger, extending up past the ring finger's highest knuckle. In women, the pinkie can be a good inch or so shorter than the ring finger.
The second step alone could distinguish young men from women with 60 percent accuracy, which Snow acknowledges is more ambiguous than he'd like. But then he found something even more surprising: the overlap between male and female hands is a modern problem.
Back in the Upper Paleolithic, physical differences between males and females – what scientists call "sexual dimorphism" – was more pronounced than it is today.
"If you scale all hands in a modern sample on a single scale, they scale from very, very feminine to very, very, masculine, and there's a great deal of overlap in the middle," Snow explains, "but when I plotted the cases from the caves against the modern distribution of several hundred individuals, the cave specimens all came out at the very ends – and in some cases beyond the ends – of the modern range of differences between males and females."