Never more popular, the splitter divides lefties and righties.

Clayton Kershaw delivers a pitch during a game at Fenway Park in Boston, on Oct. 23, 2018. Kershaw wound up throwing only 126 splitters over his final three seasons with the Los Angeles Dodgers, but even so, he joined an exclusive club: left-handers who throw split-finger fastballs. (Chang W. Lee/The New York Times)

By ENO SARRIS and FABIAN ARDAYA / THE ATHLETIC

In his final seasons, Clayton Kershaw, who retired from Major League Baseball at the end of last year, experimented with baseball’s trendiest pitch. For years, he had tried and failed to throw a changeup. His delivery did not allow him to pronate enough to throw the pitch consistently. So he tried a split-finger fastball.

Kershaw wound up throwing only 126 splitters over his final three seasons with the Los Angeles Dodgers, but even so, he joined an exclusive club: left-handers who throw split-finger fastballs.

The split-finger fastball, or splitter, has had its moments, specifically during the 1986 season. But in our current era of pitch tracking, the splitter has never been more popular than it is right now. MLB pitchers are throwing almost three times as many splitters as they did when the league first started tracking pitch types. Eight pitchers are throwing a splitter they did not regularly throw last year, and 11 pitchers added the pitch last season.

While the pitch has grown in popularity, its use has not been evenly distributed. Right-handers are four times more likely than left-handers to throw a splitter. Right-handers threw more than 11 times as many splitters as lefties from 2022-25.

So why don’t lefties throw splitters more often? There are some theories.

The splitter has a reputation for being an injury risk.

The effect the pitch has on a pitcher’s health often comes up.

“The reason that I don’t throw the splitter anymore is I did blow out my elbow when I was throwing it,” Robbie Ray, a left-handed starter for the San Francisco Giants, said. “I don’t think there’s any correlation between the two, but it just happened. I was throwing it a lot in spring training, I was working on it, and never had any issue. I think my elbow was just ready to go.”

The perception has long been that splitters lead to elbow injuries, but there is not much hard evidence for a link between the two at the major league level. According to FanGraphs, pitchers who throw the splitter at least 10% of the time have a 54% chance of hitting the injured list in any given year. Pitchers who don’t throw a splitter? That percentage falls to 52%. This mirrors past research that does not show a direct link between the pitch and injury risk.

Still, it is hard to dismiss the possibility of an increased risk of injury out of hand. If you put the ball between your first two fingers and feel the muscles in your forearm and near your elbow, those muscles are engaged in a way that they are not with a traditional fastball grip. Those muscles redirect stress away from the elbow ligament.

“I always thought that if thrown properly with the fingers really split like a forkball, that’s when you can get hurt because there’s no resistance against the ball being thrown and it really puts a lot of pressure on the elbow,” Joe Maddon told The Associated Press in 2011 when he was managing the Tampa Bay Rays.

“The theory of the splitter being more harmful is due to the split fingers and the alleged pressure on the forearm/elbow,” said Kyle Boddy, a co-founder of Driveline Baseball. “But that’s not possible to model using the systems we use today.”

Roger Craig, the manager and pitching coach who is credited with helping bring about the wave of split-finger fastball adopters in the mid-1980s, often pushed back on this critique. He pointed out that you could hurt your arm on any pitch.

“I’m not an idiot,” Craig, who died in 2023, once said. “I wouldn’t have taught them the splitter if I thought it was hurting them.”

The splitter was always a right-hander’s pitch, so it has remained one.

In 1986, Mike Scott of the Houston Astros won the Cy Young Award using the splitter as his signature pitch. Craig taught it to anyone who would listen, and the grip spread.

“The grip does all the work,” Craig once said. “You just throw it like a fastball.”

Craig’s most famous pupils were right-handers — Scott, Jack Morris, John Burkett and Steve Bedrosian, among others. Even his pupils’ pupils were right-handers. He taught the grip to Ron Perranoski, who taught it to Orel Hershiser, Bob Welch and others (most of whom were right-handed).

Even when the pitch was at its trendiest, it was thrown primarily by right-handers. Interestingly, the rise in splitter use in MLB today is nowhere near as significant as its rise in Japan.

In Japan, according to Yuri Karasawa of Yakyu Cosmopolitan, a website for Japanese baseball, around 14% of all pitches have been splitters or forkballs this season. That’s more than four times as many splitters and forkballs as currently being thrown in MLB.

But guess what? Even though they throw more splitters in Japan, they have a righty-lefty split in that league, too. Since 2021, right-handers in Nippon Professional Baseball have been twice as likely to throw splitters as left-handers.

It’s a much smaller difference than in MLB, but there’s still a platoon split. That seems to imply that it isn’t only MLB tradition that is keeping the pitch right-hander dominant, but something else.

The splitter doesn’t work as well with left-handers’ mechanics.

One working theory as to why lefties do not throw splitters is that their mechanics are not suited to the pitch. It seems that so many left-handers have lower arm slots, à la Chris Sale and Randy Johnson. Could the splitter be best suited for an over-the-top delivery that is just easier for right-handers to learn?

There does not seem to be a lot of evidence for this. The median arm slot for lefties this year is 38.5 degrees, and the median arm slot for right-handers is 38.7, so there is not a huge difference, on average. The average righty arm angle on a splitter is 41 degrees, suggesting the pitch is slightly more likely to be thrown by a pitcher using a higher arm angle, so maybe there’s a little something there. But there are plenty of low-slot splitters, all thrown by right-handers: Paul Skenes, Joe Ryan and Kirby Yates throw split-fingers from a lower than 30-degree arm slot.

Pitchers’ early development may play a role.

Watch any youth baseball game, and you’ll notice it: There are way fewer lefties at the plate and on the mound than in MLB. Young amateur pitchers will generally face multiple teams that do not have a left-handed batter in the lineup over the course of a short season. That puts pressure on left-handers to find a pitch to use against right-handers more than it does the reverse, which puts the focus on left-handers developing changeups rather than breaking balls early in their development

“Left-handed pitchers have a greater need to develop platoon-neutral off-speed pitches earlier,” Connor White of Driveline said.

In the major leagues, lefties throw nonsplitter changeups about 50% more often than right-handers, so there’s something to this idea. Southpaws have to face a lot of right-handers growing up, and they have to find a changeup that works to get them out. Right-handed pitchers can go a lot longer throwing only breaking balls and fastballs because they do not see that many lefties.

Here’s where the role of injury comes in. Fair or not, the splitter is associated with injury, so it’s not taught to young players looking to find a changeup. Lefties who succeed early in their careers usually develop a nonsplitter changeup, while right-handers can ascend without one. Then, once they get to the big leagues, the right-handers who are still looking for a changeup have more need for the splitter.

Consequently, those left-handers throwing a splitter in the majors are just generally looking to improve upon a bad changeup more than anything.

“You can kind of get away with an OK changeup when you’re babying it and you’re not throwing as hard coming up,” Ray said, “but then the second I started throwing harder, the changeup wasn’t as good, and I needed something new.”

This MLB quirk — that the lefty splitter is a relative rarity — is probably because of all of the aforementioned theories working together rather than any single reason. Lefties are just a little less likely to need the splitter once they get to the big leagues because of who they faced on the way up.

As the splitter-thrower Justin Wrobleski, a left-hander for the Dodgers, put it: “The hitters will let you know what you need to throw.”