So, Has Handedness Changed at All?

Sometimes I just like to mess around with data to see if I find something. Today was one of those days. Two years ago, Perri Klass wrote in The New York Times, “The percentage of left-handers in the population seems to be relatively constant, at 10 percent. And this goes back to studies of cave paintings, looking at which hands hunters are using to hold their spears, and to archaeological analyses of ancient artifacts.”

So I wanted to figure out whether handedness had changed at all in baseball. Are there more southpaws or switch-hitters in baseball now than there used to be a decade ago, or half a century ago?

I took a look at all seasons from 1920 to 2012, and took a look at every player active in baseball in each season: just lefties and righties among pitchers, but including switch-hitters among batters. (I wasn’t able to include Pat Venditte.)

Among pitchers, you can see that the proportion of lefties climbed from the 1930s through the 1960s, but in the expansion era, it has remained pretty stable at about 78 percent righty, 22 percent lefty. (The absolute minimum was in 1939, when just 16.3 percent of pitchers were lefties.)

The post-1970 average has been 78.5 percent righty, 21.5 percent lefty; the average of 2011-2012 was 78.6 percent righty, 21.4 percent lefty. Like I said, stable.

So what has it been like for the hitters? A bit more volatile. Switch-hitting was on a steady rise from the 1930s to the 1980s, finally tailing off in recent years. Judging by the fact that both righties and lefties were downward-sloping from about 1930 to 1990, it seems that both natural lefties and natural righties were becoming switch-hitters.

That makes some sense, considering the stability in the proportions of pitchers of each handedness. (If a lot more lefties started coming into baseball at a certain point, you wouldn’t expect as many natural right-handed hitters to start switch-hitting.)

So, what does this tell us? The rough proportion of lefties and righties is still pretty similar to what it was 90 years ago, and the proportion of lefty pitchers in baseball is about twice as high as it is in the population.

It’s a bit harder to tell when it comes to batting, because there are so many people who either switch-hit or hit with a different hand than they throw with, like Rickey Henderson, or Randy Johnson, or youth hockey-playing Canadians like Justin Morneau or Corey Koskie.

Evolutionary selection for lefties is clearly stronger in baseball than it is in nature. Scientists are still trying to figure out the reasons for lefties’ low relative prevalence in nature.

But it’s obvious why they exist in baseball. I mean, just look at Jamie Moyer.