I'm at that age where my buddies from college are starting to sign their kids up for Little League. They'll differ in degree and in approach to helicopter coaching and buying up extravagant, miniaturized baseball equipment, but there's one thing they'll all agree on: They hope their kid's a lefty.


There's a good reason for the preference. As you know, there's a significant over-representation of lefties in Major League Baseball. Somewhere around ten percent of our society is left-handed but southpaws make up close to a quarter of big league rosters, which has led to the common belief that it's easier to make the pros if you're a lefty.1 But I have some bad news: Once the kid is born, it might be too late. In utero might not even be soon enough for you to have any say on the matter. Before the child is conceived, however, there might be something you can do. And as an ex-jock who just had a kid, this is something I increasingly find myself thinking about.

Research has shown that the percentage of lefties in the general population has been stable over the past ten thousand years. This consistency suggests some advantage to being left-handed, otherwise evolution would have done away with it. But other than giving a competitive edge in a sport that came into existence in the 1800's, what's the point in preserving left-handedness? Maybe it's a genetic flaw, one so insignificant that we've just let it hang around, occasionally popping up in Presidents and batting champs. We know there's some subtle geographic variation in human handedness, which means there could be a variable in the environment – something we might not be aware of – that could affect the likelihood of an embryo developing into a southpaw. But if so, what is it?


An unlikely group of scientists—a cadre of academics who had been studying schizophrenia—recently took on the issue of handedness in baseball and arrived at a surprising conclusion involving maternal exposure to sunlight. They previously found that in many non-equatorial parts of the world, people who develop schizophrenia tend to be born in February or March (which means they were conceived in early summer) and are less likely to be conceived in winter.2 This means, among other things, that mothers of schizophrenics tend to be exposed to more sunlight when they're pregnant. Remember that.

The investigators also observed that patients with schizophrenia have less lateralized brains, meaning their cognitive functions like language and creativity aren't dominated by one side of the brain or the other. Put simply, schizophrenics aren't usually ones to be identified as "left brain" or "right brain." And, it turns out, neither are lefties. Because left-handed people tend to have less lateralized brains than right-handers, researchers wanted to see if lefties, like schizophrenics, also had mothers who were exposed to more sunlight while they were pregnant.

After reviewing the records of nearly seven thousand professional baseball players born between 1835 and 1964, the researchers indeed discovered that left-handedness, like schizophrenia, was highly associated with being conceived in May or June (about a month before the summer solstice) when there was maximal maternal exposure to sunlight during a crucial part of fetal development.3 And like schizophrenia, they found that there were far fewer lefties conceived just before the winter solstice, when sun exposure was minimal. But why?

What do these two conditions—schizophrenia and left-handedness—have in common? And why does it matter how much sunlight a pregnant woman is exposed to? Does it somehow affect a developmental process in the fetal brain? And does this predispose someone to left-handedness or schizophrenia?


The answer maybe have something to do with free radicals—molecules that are produced by ultraviolet light, inhibited by antioxidants, and most importantly, can damage DNA. Knowing this, investigators devised something called the 'solistitial' hypothesis. The idea is that sunlight increases free radicals and suppresses antioxidants in maternal blood. The radical-laden maternal blood travels to the placenta and, during the fourth week of gestation, causes the embryo's central nervous system to form improperly (the brain doesn't lateralize and the neural tube doesn't appropriately close). All of this ultimately leads the fetal brain to rewire in such a way that it predisposes a child to becoming a lefty. It may help explain why schizophrenics tend to be left-handed.

This hypothesis dovetails with the finding that certain types of brain damage during infancy and even bacterial meningitis can cause a child to become left-handed, which means our species isn't necessarily preserving left-handedness because it provides a selective advantage; rather, we're seeing it because something went wrong.


It's an intriguing idea—one that I instinctively want to disagree with because I'm left-handed and I don't like to think of it as a defect—but there is evidence to support it. A large Italian study looking at the general population (not just baseball players) also found that left-handers were more frequently conceived in the summer. But it's important to remember the role of sunlight is just a hypothesis and there are certainly other factors—things like genetic inheritance and maternal tobacco consumption—that are involved. And it's possible that light-dependent hormones like melatonin might play a role, too.

On a molecular level, there are many unknowns. We don't yet know how the free radicals lead to genetic alterations in brain tissue and we don't know if antioxidants could prevent someone from becoming a lefty. We don't know precisely how the brain gets rewired and we haven't seen a clustering of lefties around the equator, where sun exposure is theoretically greater. And, of course, there are millions of people who were conceived during the winter who still turned out to be left-handed, though that obviously doesn't disprove a shift in probabilities.


The riddle of left-handedness remains unsolved but there does seem to be a clear association between a summer conception and an increased chance of becoming an elite left-handed baseball player. So short of tying your kid's right arm behind his back, the best way to produce a lefty might be to grab your spouse and head to the bedroom. Summer is almost over.

Matt McCarthy is an assistant professor of medicine at Weill Cornell Medical College in Manhattan. You can follow him on Twitter here.


1 There are several advantages to being left-handed in baseball. For pitchers, it's partly supply and demand. Nearly a quarter of Major League pitchers are lefties, which is more than double the national average, and for a variety of reasons many batters are less comfortable against southpaws. Just ask John Kruk . Meanwhile, left-handed batters are closer to first base, so they've got a couple steps advantage trying to beat out a grounder, Perhaps most importantly, left-handed batters get more off-handed match-ups (lefty facing a righty) which allows for a better view of the ball and a more comfortable positioning of the head.


2 There are a host of other factors that influence one's risk for developing schizophrenia, including family history and obstetric complications