Are there advantages to being left-handed?

When our son was born two years ago, my right-handed wife hoped he would be a lefty because she believed lefties were disproportionately talented. It seems that he is left-handed, as am I and my older son, but new research suggests that may not carry the benefit my wife thought it would.

About 12 percent of people globally are southpaws. You’re more likely to be one if you’re male than female and if your mother was left-handed. Lefties’ brain structure and use appear to differ from righties. For example, the neural fibers connecting the left and right sides of the brain (the corpus callosum) are larger in lefties. And lefties differ in how the brain responds to language.

But does belonging to the 12 percent bring some IQ or other benefit? Of the seven presidents since Gerald Ford, at least four have been lefties, reinforcing that view. That count excludes Ronald Reagan, who was reportedly born left-handed but learned to write with his right.

Along comes Joshua Goodman, an economist at Harvard’s Kennedy School, whose new analysis suggests left-handedness may generally be a curse rather than a gift. He found lefties on average score lower on cognitive tests than righties, even after taking into account factors such as their health as infants and family background.

Goodman also shows that lefties earn about 10 to 12 percent less than righties, which is about equal to the earnings gain from an extra year of schooling. That gap comes in large part because “lefties have more emotional and behavioral problems, have more learning disabilities such as dyslexia, complete less schooling, and work in occupations requiring less cognitive skill.”

These findings come from five databases, three from the U.S. and two from Britain. The databases contain not only handedness and family characteristics, but also variables such as earnings and test scores from vocational aptitude, math, and reading tests.

In both the U.S. and Britain, lefties are three or four percentage points more likely to be in the bottom tenth of test scores than righties. That finding—that lefties are disproportionately represented at the bottom—is consistent with several previous analyses. What is most surprising in the new research is the picture at the top end of the skills distribution. Previous studies have found lefties to be more likely among very high scorers on the SAT, and more likely to have an IQ above 131. Goodman, though, finds that lefties in the U.S. are less likely to be in the top 10 percent of cognitive skills. And they are no more likely to be high earners than righties.

It is still possible that lefties are disproportionately represented among the very top of the skills distribution; the databases Goodman uses don’t contain enough detail about these extremes to say either way. The SAT study, for example, examined those in the top 1 out of 10,000, and IQs above 131 are found only in the top 2 percent of the population.

If there is any southpaw gift, it would seem to be restricted at most to a very small share of people. And for most people, being lefty is a disadvantage. But why would that be? The starting point is that which hand you use isn’t entirely genetic. For example, between a fifth and a quarter of identical twins do not share handedness, so it can’t be that genes are destiny. Environmental factors, including stressors during pregnancy or complications at birth, play some role as well, and those factors may help to explain the lefty disadvantage.

One of the most intriguing findings from Goodman’s research is that the lefty disadvantage applies only to those born to right-handed mothers. Lefties born to left-handed mothers have outcomes similar to righties.

What could explain that? Here’s one clue: Right-handed children of left-handed mothers experience adverse effects that are similar to left-handed children of right-handed mothers, according to Goodman’s findings. It’s possible that learning to mimic one’s mother in early learning is more challenging when handedness differs between mother and child.

All of which suggests that expectant mothers shouldn’t wish for their children to be lefties. For the record, Goodman is a righty.