Editor’s Note: An abridged version of this study appeared at FiveThirtyEight on August 17, 2020 under the title “What Really Gives Left-Handed Pitchers Their Edge?”

Left-handed pitching has long been one of the most prized commodities in professional baseball. Teams strive to obtain lefty pitchers, and those pitchers recognize their competitive edge. Two-sport athlete Tom Glavine explained his career choice this way: “I love both sports, but the deciding factor was, being a left-handed pitcher, I had a huge advantage in baseball because of that, and I didn’t have that type of advantage in hockey.” Even a century ago, Tris Speaker expressed the sport’s reverence for southpaws – if falling short as a trade analyst – when he opined that “taking the best left-handed pitcher in baseball and converting him into a right fielder is one of the dumbest things I ever heard.”

Major league rosters reflect this preference for lefties today. Although just 10% of American men throw with their left hand, fully 28% of innings thrown by major league pitchers come from the left side. And before you blame lefty relief specialists for this disparity, consider that southpaws also make 29% of starts. Any way that you cut the data, lefty pitchers make it to the big leagues about three times as frequently as righties, given their share of the general population. What accounts for this huge surplus of southpaws? Read the rest of this entry »