Gen-Z Is Killing the Curveball

Friends, I come to you today to relieve my soul of a burden I’ve been carrying. I’ve been harboring a cranky, irrational, old man opinion, and worse still, I’ve been lying to you about it.

Time and again, while evaluating pitchers, I’ve praised the slider. Dylan Cease’s slider? Incredible. Andrés Muñoz, Chris Sale, whoever. In the kayfabe my position demands, I must praise a slider that gets outs. But my heart isn’t in it. I am awed by the slider’s effectiveness the same way I’m awed by the voraciousness of a swarm of locusts.

Deep down, I detest the slider. It is a crude instrument, with none of the curveball’s grace or the changeup’s playfulness. The curveball is a calligraphy brush, all swooping lines and fine control. The changeup is a Blackwing pencil, rich and precise, its marks here one moment and gone the next.

The slider is a crayon.

It is a blunt, imprecise tool, the perfect pitch for this moment in history, an age of postliterate maximalism in all areas of life, baseball included. There’s a (probably apocryphal) story about the Red Army at Stalingrad, depicted memorably in Enemy at the Gates: Soldiers would arrive at the front to find that there weren’t enough rifles to go around. So the Red Army officers would send unarmed men to the front and tell them to take a gun from a dead comrade.

The modern major league team must be restrained by statute to an eight-man bullpen, and the path to the playoffs frequently leaves that many exhausted arms on the injured list. If the GM is Marshal Zhukhov, the slider is the Mosin-Nagant. Do not be alarmed. A bullpen spot will become available soon.

Perhaps I might not find the slider so objectionable, so noisome, if it were not eating its more beautiful, venerable sibling: the curveball.

This season, just 8.12% of total pitches thrown have been curveballs, which is the lowest mark of the pitch tracking era. The league-wide curveball rate is down almost exactly a third from its all-time high, which was as recent as 2020. (These figures include all pitches in the curveball group, so true curves, knuckle curves, and “slow curves,” according to Baseball Savant, while the slider group includes sliders, sweepers, and slurves.)

Now, I called my hatred of the slider and love of the curveball an “old man opinion” because having a fierce normative opinion based on absolutely no empirical evidence at all has long been the province of the old man. (While writing this sentence, I ever so briefly imagined a version of Archie Bunker who lived during the age of 24-hour cable news and became so frightened I had to get up and go for a walk.)

But this is also an old man opinion because, more and more, the curveball is an old man’s pitch. Not only is the curveball decreasing in popularity, older pitchers are accounting for a greater percentage of those few curveballs that do get thrown. I took the population of pitchers and sorted it into five buckets by age: 23 and under, 24 to 27, 28 to 31, 32 to 35, and 36 and up.

Over the past 17 seasons, the average age of the major league pitcher, sorted by workload, has shifted slightly older. For the first 16 seasons, the most populous bucket in every season was the 24-to-27 age group; in the late 2000s, that group threw about 300,000 pitches a year, while the 28-to-31s threw less than 200,000 pitches a year. But that gap narrowed gradually throughout the 2010s and into the 2020s:

This season, we finally saw convergence. The 28-to-31s have thrown about 4,600 more pitches so far this season than the 24-to-27s. And while the percentage of pitches thrown by the 32-and-up group held steady at around 20% from 2008 to 2022, the past two seasons, these graybeards have thrown about a quarter of the league’s total pitches.

So cancel that botox appointment: 31 is the new 24.

Originally, I was looking at a pretty appalling shift in age profile for curveball users. In 2008, 52.6% of curveballs were thrown by pitchers 27 and younger; this year, that’s down to 37.7%. But those raw numbers exaggerate the trend, because the league as a whole is getting more work from older pitchers.

To get a more realistic appraisal of the age profile of curveball users, we have to compare the percentage of the league’s curveballs thrown by each age group to the percentage of total pitches thrown by each age group.

At this point, I realized that I was looking for something that could be expressed the same way as a plus statistic, like wRC+ or OPS+. So for each year, I took each age group’s curveball usage rate and compared it to the league-wide curveball usage rate, using 100 as a baseline. In 2013, 32- to 35-year-olds threw 13.52% of the league’s curveballs and 13.51% of its total pitches. So for that season, that age group gets a Curveball+ score (for lack of a better name, it’s not like anyone’s ever going to use this nomenclature again) of an even 100. Cohorts that throw more curveballs than average get a higher score; those that throw less than the league average get a lower score.

To make the outliers more obvious, I’ve marked scores under 90 in blue, scores between 111 and 120 in orange, and scores over 120 in red:

Old guys have thrown a disproportionate share of curveballs over the past six or seven years, and a hugely disproportionate share since the pandemic.

And here’s where things get really scary. Once we’re splitting hairs this fine — with 36-and-over pitchers in a single season, throwing a single pitch type — we’re not talking about that many pitches. That 178 in 2022 is just 5,823 curveballs, out of a total of 717,945 pitches thrown league-wide over the entire season.

So what made the curveball take off so much among old pitchers in the early 2010s?

I can actually pinpoint the specific date everything changed: November 12, 2019. That’s the day Charlie Morton turned 36, and aged into that last bucket. Even though Morton only started nine regular-season games in 2020 and went very light on the hook by his standards, he accounted for 16.4% of all curveballs thrown by pitchers 36 and over in 2020.

And what accounts for the sharp decline from 2023 to 2024? Rich Hill hasn’t pitched this season. Since 2020, Hill and Morton have accounted for about a third of the Old Guy Curveballs in the league every year:

These two are stemming the tide. And they’re not going to keep going forever. A one percentage point drop in the league-wide curveball rate is about 7,000 pitches a year. If Hill and Morton are good for 2,000 curveballs a year, replacing them with slider jockeys is going to have a measurable impact on this endangered species.

And the population is only getting older. Here are the 10 highest curveball rates (combined curve and knuckle-curve) among pitchers with at least 70 innings this year:

Lowering the innings threshold doesn’t make things much better; the league leader with a 50-inning minimum is 35-year-old Drew Smyly, who’s thrown his curveball 47.3% of the time this season.

Curveballs, then, are like ankle socks and reaction GIFs: They’re going to die out with the Geriatric Millennials, leaving us to suffer through a dystopia where one-inning fastball-slider guys are grown in vats by the thousand, then used up and discarded. The artists, the technicians, the Mortons and Hills, are not long for this league. I pity the generations to come.