A Show of Sinister Force? Left-Handed Pitchers Are Dominating MLB Like Never Before ...Middle East

Roughly halfway through the 2025 season, left-handed pitchers have the biggest edge over right-handers in MLB history. We try to gauge just how real this historic split might be.

Pick a left-handed pitcher in Major League Baseball, one active in 2025.

Check the stats.

More than ever before in baseball history, odds are the numbers look good.

Roughly halfway through the 2025 season, left-handed pitchers have the biggest edge over right-handers in MLB history, dating back to 1903. Collectively, left-handed pitchers have a 3.64 ERA, which comes out 11% better than league average via park-adjusted ERA. The much larger pool of right-handed pitchers checks in with a 4.22 ERA – 4% worse than average.

It’s an extreme deviation from the norm, even across three months. By virtue of being a smaller and less-familiar looking group, lefties often perform slightly better than their more common brethren. But sailing this far clear of the league average is unprecedented.

Lefties’ park-adjusted ERA hasn’t been more than 4% better than the league in a full season since 1953, and they have never eclipsed 10%.

Entering July, 13 of the top 25 qualified starters by park-adjusted ERA are left-handed, even though only 22 of the 72 qualifiers are lefties. If you want to temporarily forget how “average” usually works, 63 of the 87 southpaws who have tossed even 30 innings are running better-than-average run prevention numbers.

And four of the top six qualifiers in largest decrease in ERA from 2024 to this season are lefties:

These are numbers that make you look for a mistake, a fluke. They are numbers that recall the origin of the term “southpaw,” which traces its linguistic origins through baseball and boxing, back to the days when Latin was spoken, and the word “sinister” simply meant “left.”

“In the Middle Ages, for instance, the left-hander lived in danger of being accused of practicing witchcraft,” TIME Magazine explained in 1969. “The Devil himself was considered a southpaw, and he and other evil spirits were always conjured up by left-handed gestures.”

In trying to explain MLB’s left-handed advantage in 2025, a show of sinister force might be one of the only theories that can be ruled out immediately.

So let’s dive in and try to gauge just how real this historic split might be.

Theory: An unusual cluster of left-handed aces is skewing the numbers

What if Tarik Skubal is just so good he’s throwing off the sample? Sort of kidding. It’s not really possible for one pitcher to do that by himself, but a cluster of the league’s top aces happening to be left-handed for a snapshot in time?

With Skubal, Garrett Crochet, Max Fried and pre-injury Chris Sale looking dominant, it was at least worth checking.

However, even if you remove outliers from both the right- and left-handed groups of pitchers (the top and bottom five pitchers with at least 50 innings, by ERA+), the 2025 lefty edge persists. The remaining lefties have an average ERA+ of about 122 in 2025, the best mark of any handedness in any full season dating to integration in 1947.

Fans of the Kansas City Royals and Cincinnati Reds, for instance, could attest to the fact that it’s not just established Cy Young contenders throwing up zeros. Long underwhelming, Kris Bubic has emerged as a dominant starter in KC with a sparkling 2.25 ERA and 70 raw value- (RV-) that ranks seventh among pitchers with at least 80 innings.

In Cincinnati, Nick Lodolo is putting up a career-best season while steadily solid Andrew Abbott is suddenly sporting a double-take-inducing 1.79 ERA through 14 starts.

Theory: Lefties are stifling the running game

The pitch clock and accompanying rules unleashed a new fervor around stolen bases and aggressive running.

It’s true that some teams, such as the Detroit Tigers, find real value on the bases. And it’s true that left-handers, who face first base on the mound, have a built-in advantage.

Yet there’s no good evidence that lefties have made notable strides in this arena, nor that it amounts to a large-enough chunk of value to cause the phenomenon we’re seeing in the splits.

Theory: The slider spike

Chris Langin, the former director of pitching at influential performance lab Driveline and now founder of Unfiltered Labs, told Opta Analyst he would expect the overall numbers to regress toward less extreme splits.

He did, however, note one way lefties might be making real gains.

Increasingly advanced pitch design and usage strategies are widening arsenals and providing pitchers, especially starters, with more targeted ways of attacking hitters.

“I think pitch design has probably helped a lot with just rethinking the game entirely as two arsenals – one against lefties, one against righties,” Langin said.

Traditionally, left-handed pitchers have focused on developing fastballs and changeups to counter the high frequency of right-handed batters they face. Several standout southpaw performances, including Bubic’s breakout, have arrived in tandem with a bifurcated approach that encourages throwing sliders or sweepers to left-handed batters.

“If you look at slider usage, and especially left-on-left slider/sweeper usage, you’re seeing that get exploited a bit more,” Langin said.

Indeed, lefty slider usage is at a peak in the pitch-tracking era (back to 2008), with 22.5% of southpaw pitches being sliders or sweepers in 2025. That’s up from 17.8% in 2021 and 13.2% in 2015. In left-on-left matchups, it’s up to 33.6%, with a .197/.235/.313 batting line that’s sure to prompt even more emphasis on sliding and sweeping.

Sometimes this is a subtle way to keep hitters off balance, by adding in a pitch that tunnels well with a sinker. That’s one notable change Fried has made in racing out to a 2.13 ERA with the Yankees, mixing in a healthy dose of sweepers to flummox batters waiting for his sinker. To wit, almost 59% of all left-on-left pitches in 2025, league-wide, have been sinkers, sliders or sweepers.

The Nationals’ MacKenzie Gore (86 RV-, career-best 3.09 ERA) is perhaps the most dramatic example of the slider spike. In 2024, he threw his cutter-ish slider only 27% of the time to lefties, and breaking pitches about 20% of the time to them. In 2025, a more traditional slider (see the velocity difference below) is his primary offering against same-sided batters. He’s throwing it about 45% of the time and holding them to a .135 batting average on the pitch.

Left-handers are also throwing more sliders and sweepers in the zone, challenging hitters with breaking stuff and seeking chase with offspeed pitches. This season, 47% of sliders and sweepers have landed in the zone, the highest rate of the pitch-tracking era.

This is admittedly an advancement right-handers would theoretically be taking advantage of as well, but Langin said that phase might be over for righties, where lefties are “still squeezing out some value and still seeing the uptick in that occurring each year.”

Theory: Never seen anything like it

One of the reasons lefties are so overrepresented in baseball is that novelty can be an advantage. Kids don’t grow up seeing as many left-handed pitches, so they don’t have the same eye training to see and react to what’s coming, so left-handed pitchers start out ahead.

That effect, as evidenced by the years of evidence the 2025 split is cutting against, doesn’t tend to be very large once you reach the major-league level – though it’s notable that lefties are often slightly better even with consistently lower average velocities.

This theory amounts to: What if the slimmed down minor leagues and generally faster promotion schedules have amplified the edge? What if more hitters are reaching the big leagues without fully acclimating to the idea of facing professional lefties?

It’s virtually impossible to prove, but there’s at least one manager who appears to be basing some decisions on this concept. Boston Red Sox manager Alex Cora has raised eyebrows by holding top prospects like Roman Anthony out of the starting lineup or pinch-hitting for them against left-handers.

“I think the lefties here (in the majors) are real lefties, there’s stuff here,” Cora told The Athletic. “Especially the guys in the bullpen, I truly believe that if you got stuff, you’re going to be here, you’re not going to be in Triple-A or Double-A. So what they (rookies) see here is different. And we have two of the best (pinch hitters). That’s the reality of our roster.”

The evidence I can muster suggests there might be something more to investigate here. Batters in their age-26 season or younger have a .298 wOBA against left-handers this season, about 10% worse than league average by 2025 standards. This is the wOBA you’d expect to get from a defense-first utility player or catcher.

That’s the worst mark in the pitch-tracking era and, notably, 19 points worse than their .317 wOBA against right-handers.

That gap between young hitters’ performance against righties and lefties would be the largest since 2008, besting 2024’s 13-point gap. No other season in the span has had wider than an eight-point gap. Hitters 27 and older, for the record, have an 11-point gap this year. As recently as 2023, that group hit left-handers better than righties.

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Theory: Randomness

As anyone who has tracked an extreme baseball fact knows, the unlikely is often short-lived. Langin pointed out lefties have enjoyed a more favorable rate of fly balls staying in the ballpark than righties.

Because it’s only July, because adjustments will be made, because an 11% edge on the league average is so extreme. These are all reasons to wait and see rather than pursue a lefty-only pitching staff.

The smart money is on left-handers broadly coming back to the pack alongside obvious individual regression candidates like Abbott and Bubic. It’s already happening to David Peterson, the New York Mets’ pitch-to-contact lefty who threw a shutout against Washington in early June and then in late June allowed five runs in less than five innings to the Pittsburgh Pirates.

PitchingBot, a pitching model that attempts to grade true talent by drilling down on stuff and command, gives left-handers a 3.99 ERA and righties a 4.10. Langin called it evidence of “a measurable talent edge, even before factoring in outcomes.”

Taken together, though, the marked changed in strategy and the drastic nature of the results warrants more than a wave of the hand. This left-handed superiority might not be what is appears, but it’s certainly more real than witchcraft.

Research support provided by Stats Perform’s Jake Coyne and Emory Brinkman. For more coverage, follow along on social media on Instagram, Bluesky, Facebook and X.

A Show of Sinister Force? Left-Handed Pitchers Are Dominating MLB Like Never Before Opta Analyst.

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