You see the stories everywhere. “Baseball teams are striking out more than ever,” blared an NBC Sports headline back in April. In 2015, The Hardball Times wrote a story entitled “The Strikeout Ascendant.” And the year before that, friend of the program Ben Lindbergh hosted a “rising strikeout rate symposium” on his popular podcast, Effectively Wild. Clearly, the baseball world is well aware that strikeout rates are up — way up.
What’s less well-known, but equally true, is this: Baseball’s recent rise in strikeout rates has little to do with how good batters are at making contact. That’s a bit counterintuitive, I know, because strikeout rates have increased (up nearly 26 percent since 20021), and when you think of a strikeout, you usually imagine a batter taking a mighty hack and missing. And indeed, swinging strikes are up as a percentage of all pitches.
But keep in mind that this isn’t only about the batters — pitchers also have a big say in the matter.
And in 2009, pretty much all of a sudden, they started throwing way fewer pitches in the strike zone, as measured by Fangraphs’ zone percentage statistic.2 From 2002 (the earliest year for which we have data) through 2008, pitchers put the ball through the zone about 52 percent of the time, year in and year out. In fact, over the seven seasons from 2002 to 2008, the league-wide rate of pitches in the zone never dropped below 50.5 percent or rose above 54.2 percent. When it came to tossing strikes, MLB hurlers were a model of consistency.
But during the 2009 season, pitchers threw balls in the zone just 48.3 percent of the time. In 2010, that number kept falling — to 45.4 percent! — and by 2011, the league-wide rate of pitches in the zone was just 44.6 percent. In just three years, about one out of every nine pitches that had previously been thrown in the zone started missing its mark. (Since then, MLB’s zone rate has basically leveled off: Last year, it was exactly the same — 44.6 percent — as it was in 2011.)
But despite that drastic change, batters haven’t really changed how they react to pitches inside the zone. Batters still swing at basically the same proportion of pitches in the zone as they did in 2002, and they still make contact with those pitches at essentially the same rate.
Instead, the big changes have come outside the zone.
The trend here is striking. As pitchers started throwing outside the zone more and more — again, the really big decline in strike-throwing started around 2009 — hitters not only started swinging at more pitches outside the zone, they also started getting much better at making contact on those wayward pitches. Add it all up, and you see that instead of making contact on just 10 percent of all balls thrown outside the strike zone, as they did back in 2002, hitters made contact on nearly 20 percent of such pitches in 2016.3 That’s double the rate! That’s unbelievable!
Let’s step back for a second, because this is a pretty counterintuitive finding. Batters are striking out more often, but they haven’t gotten any worse at their core task: hitting the ball. They’re also not getting as many good pitches to hit as they used to, but they’re about as good as they ever were at making contact on balls inside the strike zone, and far better than they used to be at making contact on balls outside the zone. Normally that should add up to swinging and missing less often, not more often. But because more of the balls they’re swinging at are outside the zone, and those balls are fundamentally harder to hit, the effect on overall contact is just about level. It’s a classic Simpson’s Paradox.
So if MLB hitters suddenly started channeling their inner Vladimir Guerrero, making better contact on bad pitches, why are they still suffering so many strikeouts? We don’t really know for sure, and there’s a lot more digging to be done before anything can be said conclusively. For one thing, it’s not entirely clear which came first: pitchers throwing more outside the zone or hitters swinging at those pitches. I tested whether one month’s zone rate predicted the next month’s swing rate and vice versa, but I found almost no case for either.4 This suggests that the complex interactions between batters and pitchers are happening on a much smaller scale than a month, and that they deserve more granular research.
But here’s one possible (as-yet-untested) hypothesis for the big-picture story: Sometime in the late 2000s, pitchers began throwing more breaking pitches outside of the zone — hence the decline in zone percentage.5 At the same time — possibly out of necessity — hitters became increasingly willing to swing at pitches outside the zone, even finding some reasonable success doing it. But pitchers had another weapon: The fastball on the corner of the strike zone. Perhaps the adjustment that hitters made in order to hit breaking stuff outside the zone also made them vulnerable to hard stuff inside it — for which they were not mechanically prepared — and they started getting called out on strikes by the boatload.
Again, that’s only a theory. But the fact is that batters are striking out more on called strikes in the zone even as they’re getting better at hitting pitches outside of it. So the next time you see a hitter preparing for a breaking ball out of the zone, remember: It’s not the pitches you swing at that get you. It’s the ones you don’t.