One curious fact about handedness is that men appear to be ever so slightly more prone to left-handedness than women. Over the past few decades, a number of studies have turned up this peculiarity, but there was nothing concrete, nothing consistent. But a recently published meta-analysis in September's Psychological Bulletin (Vol. 134, No. 5) seems to prove the point.

In their analysis of 144 handedness and brain laterality studies—accounting for a total of nearly 1.8 million individuals—University of Oxford psychologists Marietta Papadatou-Pastou, PhD, and Maryanne Martin, PhD, found that males are about 2 percent more likely to be left-handed than females. In other words, they note in the paper, if exactly 10 percent of a population's women were left-handed, then around 12 percent of men would be, too.

Interestingly, they found that in places such as Japan and Mexico, with high levels of "cultural masculinity," which is associated with highly differentiated social roles for men and women, there was an even stronger correlation between males and left-handedness. This suggests a cultural dimension of handedness, the authors point out.

So what else might account for the sex difference? One answer might be basic morphology. Some studies, the researchers note, have found that left-handed and ambidextrous people have larger corpus callosums—brain regions that connect the two hemispheres—than right-handed people, which might be linked to differences in brain development between men and women.

Another possibility Papadatou-Pastou and Martin suggest is testosterone. One theory holds that testosterone accelerates right hemisphere growth in the brain, which could lead to more cases of right hemisphere dominance and, consequently, more left-handedness, although this idea is debated.

There may also be a genetic component. No one has discovered a smoking gene or genes for left-handedness, but if it turns out to be linked to the X chromosome and recessive, like red-green colorblindness, it would make sense for more males to be left-handed.

It's even possible that all of these factors play some role in males' penchant for left-handedness, say Papadatou-Pastou and Martin.

—M. Price