I Have Seen the Fastball of the Future, and It Is a Cutter

If you watch a random pitch from a major league game, there’s a better than even chance you’re going to end up picking out a fastball. The fastball is the core concept upon which pitching is understood, the theme upon which all variations, from changeup to knuckle-curve, are composed. Our society has three great establishments: “establish the fastball” in baseball; “establish the run” in football; and the Establishment Clause of the First Amendment.

They are all, to some extent, going out of style.

The thing about the fastball is that it is as hittable as it is fundamental. Few pitchers can just pump gas past major league hitters; the reason a pitcher must establish the fastball is so he can set a hitter’s expectations, then confound those expectations with something else. But a little less than a decade ago, a few pitchers figured out that conventional wisdom had a blind spot. Pitchers with an exceptional secondary pitch ought to just throw it all the time. This approach was pioneered by Rich Hill and a few others and reached its zenith in Game 7 of the 2017 ALCS. Lance McCullers Jr., architect of one of the sourest curveballs ever devised, threw 24 of them in a row in a four-inning relief appearance with a trip to the World Series on the line.

To hell with the fastball, then. Not for everyone, of course. The pendulum of trendy pitching theory swings back and forth, and plenty of pitchers have always been better off mixing different types of fastball than committing full-time to the breaking ball lifestyle. But on the aggregate, fastball usage is on a downward trend across the league, and has been since Statcast was introduced.

But if you’re talking about the fastball, you have to be specific, because they come in three broad flavors. There’s the four-seamer: high velocity, some arm-side run, so little vertical drop we sometimes talk about it having “rise.” The two-seamer, or sinker: moderate velocity, extreme arm-side movement, and some sink. And then there’s the cutter: lower velocity and glove-side movement, to the point that the line between a cutter and a slider can be blurred.

Below is the average movement for each type of pitch by year since 2015 — four-seamers in blue, sinkers in yellow, cutters in red:

Since 2015, everything has gotten harder. In life, generally, yes, but specifically: fastballs are thrown harder now. The average four-seamer in 2023 is 1.0 mph faster than it was in 2015; sinkers are up 1.2 mph, and cutters are up 0.7 mph. Sinkers have more movement than ever in both directions and are harder now than four-seamers were in 2015.

And yet the two-seamer has fallen out of favor. The decline of the fastball is not shared evenly among the three types:

No, wait, come back! The scaling on that graph makes the effect look less pronounced than it is. In 2015, sinkers accounted for 21.3% of all pitches. This year, that’s down to 15.1%, a drop of just over six percentage points but almost 30% in terms of total pitches. As a proportion of the 700,000-plus total pitches thrown in a typical MLB season, we’re talking between 30,000–40,000 fewer sinkers than we saw less than a decade ago.

Much of that drop is probably attributable to the narrative that developed during the swing plane revolution of the late 2010s. The downward action on a sinking fastball — previously thought to be desirable, as the ball would drop under hitters’ bats and lead to ground balls — now served to meet an uppercut swing flush, leading to hard contact and home runs. But as that revolution has cooled, or at the very least is no longer one of the sport’s dominant talking points, two-seamer usage has merely plateaued, not rebounded.

Cutters, the least common fastball, are on the rise. A pitch that got used in the high-5% range in the mid-2010s now accounts for 7.8% of total pitches in 2023. Near as I can tell, there is no real cutter revolution sweeping the nation. Corbin Burnes’ transformation is a few years old at this point; perhaps he’s the torchbearer for a whole new generation of pitchers, the third stage on the Mariano Rivera–Kenley Jansen rocket.

Of the top 100 pitches on Baseball Savant’s run value list, only four are cutters. Adjusting to run value per 100 pitches, the list gets a little more heavily populated: there are nine cutters in the top 100 in RV/100 (minimum 25 PA), but three of those nine are between 90–99 on the list, so maybe this is an arbitrary endpoints thing.

Acknowledging that some cutters are fastballs and some cutters are basically sliders, it’s hard to draw conclusions based on league-wide data, but here’s an ironic twist. You’d think that there would be a cause-and-effect relationship between the increase in cutter usage and hitters’ success rate against the pitch. In other words, it would make sense to throw more cutters if this were an unexploited market efficiency. In fact, hitters are getting better against cutters — a lot better. The league-wide wOBA against the other two types of fastball has stayed in a 10–15-point range over the past decade. Against cutters, hitters posted wOBAs between .304–.312 from 2015 to 2018. That went up into the mid-.320s by the turn of the decade. This year, it’s up to .336.

This season, the league is hitting .267/.332/.445 off the cutter; those slash marks are within a few points of what Wander Franco and Manny Machado are hitting this year. Would you line up to turn opponents into Manny Machado and Wander Franco? I think not.

Another way of framing that information is this: Opponents are slugging almost 40 points better against cutters than they were in the mid-2010s. This is the worst season for cutters in the Statcast era. And in spite of all that, a .336 opponent wOBA would be the stingiest season of the Statcast era for either four-seamers or sinkers.

Is the cutter a more effective pitch now than it was eight years ago? On a league-wide basis, no. But even at its low-water mark, it’s more effective than other fastballs, which by Hill’s Law means it should be thrown more and other fastballs thrown less. Which is exactly what’s happening:

So what makes the cutter so special? It’s the only fastball with glove-side movement. It’s the fastball, in other words, that doesn’t move like a fastball. The orthodox position on lateral movement is that moving the ball away from the batter makes it harder for the batter to hit. But when I broke out cutter usage by platoon advantage, I couldn’t find an effect to explain the league-wide uptick. Left-on-left cutter usage has risen and fallen; right-on-right cutter usage has gone up since 2015, but right-on-left cutter usage has gone up more. The thesis that increased cutter usage is a new way to gain the platoon advantage on fastballs doesn’t hold up.

I also noticed that the downward trend in fastball usage, which is even greater when you take cutters out of the equation, was not matched by an increase in changeups and splitters, which also have arm-side movement. In fact, we’re seeing quite a significant uptick in the percentage of pitches that have glove-side movement, but the percentage of pitches that have batter-side movement has fluctuated within a corridor of just 1.5 percentage points throughout the Statcast era. The percentage of pitches thrown where the pitcher has the platoon advantage has also gone up and down within a window of 1.5 percentage points, and that’s accounting for the introduction of the three-batter minimum in 2020.

All of this leads to a fascinating trend regarding the platoon advantage. This season, pitchers with the platoon advantage are throwing pitches with batter-side movement (abbreviated to BSM below, in the interest of fitting everything into the chart) just 58.0% of the time. That’s the lowest mark of the Statcast era, down from 64.6% in 2015 and 66.3% in the high-water mark of 2017. And that makes sense. If the point of the platoon advantage for a pitcher is to play keep-away with the breaking ball, pitchers are doing that more than ever. It’d be reasonable to expect that number to drop in years to come.

But that’s not the fascinating part. The fascinating part is that when pitchers don’t have the platoon advantage, they’re throwing more pitches with batter-side movement than ever — a lot more:

Same-handed hitters? You’re going to get a lot more breaking balls. Opposite-handed hitters? You’re going to get a lot more breaking balls, too. “Throw your best stuff more often” defies the traditional understanding of the fastball’s role in pitching. Apparently it defies the traditional understanding of the platoon advantage as well.