Left-handed people earn 10 percent less than righties

What he found

In the data, around 11 to 13 percent of the population was left-handed. And when broken down by gender — that is, comparing women to women and men to men — those lefties have annual earnings around 10 to 12 percent lower than those of righties, Goodman writes, which is equal to around a year of schooling. (That gap varied by survey and by gender, however.) Most of this gap can be attributed to "observed differences in cognitive skills and emotional or behavioral problems," he writes, adding that since lefties tend to do more manual work than right-handers, the gap appears to be due to differences in cognitive abilities, not physical.

Indeed, lefties in the UK and US are 3 to 4 percentage points more likely than righties to be in the bottom decile of scores on math and reading tests. In some of the surveys, they are also shown to have a greater likelihood of speech problems and learning disabilities. Those problems may translate into educational problems — in the US, lefties were slightly less likely than right-handers to graduate from college.

Interestingly, the left-handed children of left-handed mothers didn't exhibit the cognitive shortcomings that the lefty children of right-handed mothers did. Even more intriguing, the right-handed children of left-handed mothers did show cognitive deficits.

What it means

This study is significant in being the first ever to show that lefties consistently under-earn right-handers, Goodman writes. However, it doesn't mean that left-handedness causes these gaps. It might be that left-handedness is a proxy for other issues. Lower birth weight and complications at birth, for example, are associated with left-handedness, Goodman finds, so those infant health issues may have lasting effects.

As for the issue of the mother's handedness, meanwhile, Goodman presents a fascinating theory: it may be that "left-handed children benefit from being raised by left-handed mothers, perhaps because those mothers model the physical act of writing or perform other cognitive tasks in styles that match their children's capacities more closely." It may just be too hard to learn from mom when you have to mirror her instead of matching her.

Of course, this is just one study. But it sheds some intriguing light on prior studies. One 2007 study, for example, found a pronounced gap among highly educated men but no such gap for women. However, Goodman says that study erred in leaving out the lowest earners, so it underestimated whatever gap there might be. Not only that, but it controlled for cognitive abilities and education, both of which are factors that help explain Goodman's results.

Likewise, as Goodman, that 2007 study, and Orszag point out, there have been mixed study results on the difference between lefties' and righties' cognitive abilities. Earlier papers have tended to show that left-handers have more cognitive and behavioral problems than righties, as both Goodman and that 2007 paper note, but some have shown the opposite. Data has also shown that lefties, for example, are highly represented among high SAT-scorers and people with high IQs. What it may mean, Orszag notes, is that lefties are overrepresented in the intellectual stratosphere, but that for the population as a whole, it's better to be a righty.

Danielle Kurtzleben