OTTOWA – Sometimes a difference between the sexes is not based on sex at all. Women have a finer sense of touch than men do, but a new study shows that this is simply because their fingertips tend to be smaller.

Neuroscientist Daniel Goldreich of McMaster University in Hamilton, Canada, and his colleagues first became curious about the sex difference while studying differences between blind and sighted people. They found that blind people are better than those with normal vision at distinguishing fine textures but that, within each group, women are better than men.

The researchers thought that the discrepancy might be the result of brain differences between men and women, but they first wanted to see if something simpler could explain it. So they tested 50 women and 50 men on a simple task: Each person touched a small, grooved surface and tried to identify the orientation of the grooves. As the grooves got closer together, it became more difficult to determine their direction.

As expected, women performed better at this task than men did, but when the scientists looked at the results by finger size, they found that the sex difference disappeared: On average, men and women with the same size fingertips perform at the same level, the team reports in the 16 December issue of The Journal of Neuroscience. (Finger size does not explain all individual variability, however; there are differences between people with the same size fingers, perhaps as a result of differences in the mechanical properties of skin or in how each person’s brain processes the information.)

The researchers also came up with a potential explanation for the size effect. Cells in the finger called Merkel cells appear to transmit this type of touch information to the brain. Goldreich and his co-workers measured the number of Merkel cells in their subjects’ fingertips and found that everyone had about the same number, regardless of finger size. They suspect that this explains the effect: Merkel cells are spaced more densely in smaller hands, giving those hands the ability to distinguish finer textures.

The paper is “very solid and convincing” regarding the effect of fingertip size on this specific task, says neuroscientist François Tremblay of the University of Ottawa in Canada. However, other types of tactile tasks may not work the same way, he adds. For example, passively pressing the skin against a textured object–as the study participants did–involves different neuronal pathways than actively moving the fingers around an object and may be controlled differently.