Bryce Harper is in the midst of a "slump" that most hitters could only dream about. Harper's season peaked on April 26, at which point his batting line stood at .328/.430/.844 with a 218 Weighted Runs Created Plus mark. (wRC+ is an advanced offensive metric that is adjusted for ballpark and league and sets 100 as "league-average.") Since then, he's hit .217/.401/.348, with a wRC+ of 100 -- meaning that for the past six weeks, Harper has been exactly an average hitter.
Of course, for many hitters, being average is a step up. Harper's six-week run of being average, a stretch that has looked like a near-collapse only when compared to his usual greatness, is still better than what sluggers like Jose Abreu, Hanley Ramirez, Adam Jones and Troy Tulowitzki have put up all year. A struggling Harper is still a pretty good hitter.
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But this is Harper we're talking about, and average isn't what anyone expects. So what's been tripping him up for the last month-plus? Good news, Nationals fans: There are signs that the tide is turning.
The narrative on Harper's recent struggles pretty much writes itself. In early May, the Cubs decided not pitching to Harper at all was a viable strategy, and so they set a record by walking him 13 times in a four-game series -- including a stunning six in one 13-inning game. Harper had just one hit, a single, and the Cubs emerged unscathed with a sweep. Thus, the "don't give Harper anything to hit, ever" strategy was born. Frustrated by the lack of hittable pitches, Harper had to expand his zone and go after bad pitches -- or so the story goes.
But that misstates both the timing and the cause, because this didn't start in Chicago. In a nine-game stretch comprising series against the Philles, Cardinals and Royals leading up to that Cubs series, Harper had hit only .156/.308/.250 with four singles and a home run, so what happened in Chicago just exacerbated a pre-existing condition. And it's not about what's happening outside of the strike zone, it's about what's happening inside of it -- or not.
Gif: Harper Home Run and Dugout High Fives
Let's run down the possible issues.
Is it because he's getting fewer hittable pitches in the strike zone?
Maybe a little. Harper saw 45.7 percent of pitches inside the zone through April 26, and he has seen only 41.7 percent since. That's a meaningful difference, though by itself it's not enough to throw him off track -- last year, when he had one of baseball's best-ever seasons, his season average was 41.5 percent of pitches in the strike zone. Pitchers avoiding Harper is not a new thing.
Is he expanding his zone to compensate?
Somewhat. Harper was going after 25.5 percent of pitches outside the strike zone through April 26, and that's up to 27.8 percent since. We know that contact with bad pitches can cost a hitter a considerable amount of production, but Harper has actually made contact less often outside the zone, going from 66.7 percent before the Phillies series to 64.3 percent since. Last year, however, that was just 60.6 percent, so this is a reasonable point.
Is he doing less damage on hittable pitches?
Sort of. Harper's Statcast™ exit velocity on pitches in the strike zone early in the season was 97.3 mph, which is excellent and one of baseball's 10 best. Since the Philadelphia series, that's down, but only to 95 mph, which is still very good (the Major League average on balls hit 95 mph or higher is .527.
Harper has, however, seen far less success on those pitches. His batting average on pitches in the zone has fallen by 140 points. That seems somewhat attributable to bad luck, and somewhat to the fact that Harper's launch angle on these batted balls has fallen from 18.7 degrees to 9.0 degrees -- that is, he's hitting fewer liners and more grounders.
So it's a little about seeing fewer strikes, a little about going outside the zone and somewhat more about doing less damage inside the zone. Here's the real reason, though:
Harper isn't swinging at nearly as many strikes.
All those other things add up, but this is the real issue:
Zone swing percentage
Through April 26: 76.7 percent
Since April 27: 57.9 percent
That's a massive drop of nearly 20 percentage points, and it's the primary reason for Harper's recent difficulties. Remember, it's really, really good to swing at hittable strikes. Last year, he did so 68 percent of the time. When Harper was red-hot early in the season, that was up by nearly 10 percentage points. Now it's down, by a lot, and it's that lack of aggressiveness (combined with lack of elevation on batted balls in the zone when he does attack) that's behind all of this.
But there is some small evidence that Harper is changing that back. It's easy to note that he has seven hits over the past three games, though that alone isn't enough. Take a look at this graph that shows Harper's 15-game rolling zone swing percentage, dating back to the start of last year, from our friends at FanGraphs.

As that clearly shows, Harper's swings on in-zone pitches really bottomed out just after the midway point of the 2016 season (to date), and recently he has managed to get back above his season average. He's swinging at strikes again, and the past few days have been his best since early April. It's no coincidence that Harper's two most recent homers, blasts off Mike Leake and Adam Wainwright, were both him the result of going after hittable in-zone pitches.
"Be more aggressive" isn't something you'd usually think about Harper, but that certainly appears to be the root of his slump. It sure seems like he's starting to, again. It sure seems like he's about to start hitting like Harper, again.