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Let’s begin by assuming that the ball isn’t juiced. Over the past two years, we’ve seen a rather obvious spike in the home run rate, such that suddenly Ryan Schimpf and Yonder Alonso are getting mentions in articles about home runs. In 2014, runs scored per game (4.07) had dipped to their lowest rate since 1980, and the game, according to people who watch it for a living, had become un-watchable.

This season, thanks in large part to the aforementioned spike in the home run rate–they’re being hit at a rate around the all-time high set in 2000–runs are back in fashion. The 2016 season featured a home run rate that was just off the all-time record set in 2000, and as of the time I write this, if the 2017 rate to date holds, it would best 2000. Not bad considering that in 2014, home runs were at their lowest rate since 1992.

Thus was born the great juiced baseball conspiracy. With other forms of juicing now being tested for via blood and urine, perhaps MLB was quietly altering the ball ever so slightly. Not to the point where 100-foot pop-ups would suddenly become 500-foot moonshots, but just enough to encourage a few more balls to go over the right field wall rather than into the right fielder’s glove. It wouldn’t take much to put a thumb on the scale. Even the increase from 2014 to 2016 rates going from record lows over the past 25 years to record highs only represents an extra home run every three games for each team.

Like most conspiracies, it consisted of leveling a charge at someone that they at least would have had motive and opportunity to do, while having no definitive proof that they actually did it, but demanding that they prove that they had not done it. Last week, MLB proved it. They provided a document to Ben Lindbergh of The Ringer (and my former boss here at BP) which detailed the regular testing done by the league to ensure its game balls are indeed meeting their standards. Lindbergh was not allowed to release the report, but was allowed to release one graph and a summary of what the report said.

According to Lindbergh, the message over and over again was that the balls showed little change over time on any physical specification imaginable and always well within the official guidelines. I suppose that the conspiracy theorist’s response would be to point out that MLB did the testing itself and provided the report, and could have (theoretically) doctored the data. (Here we are back to leveling a charge and demanding proof of a negative!) While it’s possible that MLB went to the trouble to fake some data, perhaps it’s possible that the ball hasn’t really changed?

Let’s assume that the ball isn’t juiced. The problem is that we still have to explain this:

That’s HR/FB, by month, over the last five full seasons (2012-2016). While a good amount of the speculation about the ball was initially started by an article written by Lindbergh and BP alumnus Rob Arthur in which they noted that exit velocity jumped at the All-Star break in 2015, and the HR/FB chart shows a big jump from July to August of that year. The thing is that there was an almost identical jump from September 2014 to April 2015. Maybe teams and players came to spring training looking to try something new out?

Perhaps there’s a perfectly logical, non-conspiratorial reason that home runs have spiked?

Warning! Gory Mathematical Details Ahead!

Before we get into the home runs, we need to first talk about strikeouts, because they hold the key to all of this. Much has been made about the strikeout epidemic in baseball, and the numbers show that despite the increase in home runs, strikeouts have continued to climb, unabated over the past few years.

There’s another trend that’s actually been re-born of late as well. In the latter part of the last decade, teams began to develop a strategy in which they baited the starter into 14 pitch at-bats, seeing that most starters came into the game with a 100-pitch limit anyway. Well …

As shown above, the upward trend in pitches per plate appearance is back, but for a different reason than in the past. In the past, swing rates on the first pitch of a plate appearance had been very low, with batters effectively saying to pitchers, “I’m not going to swing until you throw me a strike.” Pitchers eventually figured this out and responded with a collective, “OK, if you insist.” After 2013, that changed.

But here’s the thing: While batters were willing to swing a little more early in the count, they weren’t exactly hitting it more. In fact, contact rate on those swings fell.

And it isn’t just the first pitch of a plate appearance either. While overall swing rates haven’t changed all that much over the past decade, rates of swinging and missing have steadily crept upward. The hot new thing to do isn’t to take pitches. It’s to swing and miss. In some sense, it’s perfectly logical. If you’re willing to accept a strike, why not swing really hard in case you hit it? If you miss, two-thirds of the time, they just throw you another pitch.

***

Engaging in behavior that leads to strikeouts might seem counterintuitive, but maybe there’s a strategy behind it. It’s generally accepted that there’s a trade-off to be made between power and contact. It’s not the same trade-off for every hitter, but the harder you swing, the more likely it is that you will miss, but also the more likely that the ball will go farther, and the home run numbers suggest that when batters do hit the ball, it is going farther.

What’s interesting is that fly-ball rates haven’t been particularly high during the spike, so despite all of the discussion of launch angles, batters (as a whole) aren’t hitting the ball upward more often. Just harder. Fly balls aren’t increasing, but HR/FB sure has been. It’s worth saying that there are cases where individual players seem to be fine tuning the whole launch angle thing, but league-wide it seems that the big idea has been hitters getting more out of the fly balls that they were already hitting.

But … those strikeouts.

Let’s consider strikeouts for a moment, not as we often do instinctively from a moral perspective (a strikeout gets its own line in the box score with an implied tsk tsk at the end, but no one counts 4-3 ground outs or fly outs to left), but for what they functionally do. If a batter is going to make an out, in a lot of cases, it really doesn’t matter whether it’s a strikeout or a ground out.

If there are two outs or there are no runners on base, then functionally, a strikeout does the same thing as a grounder to second. In 2016, that described nearly three-quarters of plate appearances league-wide. In addition, it’s rare for an out to actually move a runner up. In 2016, when there was a runner on and fewer than two outs and a ball was put into play, it only resulted in the runner moving up about 20 percent of the time. Yes, strikeouts have the disadvantage of giving no chance to provide a “productive” out, but it’s not really common that grounders to second do either. Plus, strikeouts have the advantage of avoiding double plays.

So, let’s trade some contact for power. Assume for a moment that in this trade-off, we could maintain a steady on-base percentage (i.e., the same number of outs), we just change the shape of those outs. Changing to a strikeout isn’t going to be that big a penalty, and anything that we pull out of the single or double or walk basket to put into the home run basket is going to be very valuable. It’s enough that even if we have to give back some OBP, it might actually be worth it.

It might not be great baseball to watch, but it makes sense.

***

Now that we can justify the extra strikeouts, the patterns that we’re seeing make sense, but they’re not quite enough to explain all that’s happening. There’s another chart worth seeing. This is the percentage of balls hit into play (or over the fence) that were fastballs.

Here’s another showing the percentage of home runs hit that were hit on fastballs.

Starting in 2015, around the time of the current power spike, hitters seemed to be sitting fastball more. And why not? Even though fastball usage has actually declined among pitchers, it still accounts for more than half of all pitches. As a batter, you know you’re eventually going to get one. So, why not wait for one and swing really hard? You might miss and eventually you might strike out, but if you do, so what?

Sit Fastball, Swing Hard, Strikeouts Don’t Matter

It’s not surprising that we’ve seen a big jump in home runs, and we don’t need a juiced ball to explain it. We’ve got evidence that the game is just evolving toward an approach that favors a lot of home runs. There’s no grand conspiracy other than evolution.

Consider the lament that started 10 years ago. Baseball was being over-run by pitchers, relievers especially, who threw really fast fastballs. Well, if you know that a lot of the pitchers whom you will be facing are going to be throwing heat, you might as well sit on the heat. You’re probably also going to be brought to the majors more quickly if you show some ability to hit that heat. This is not rocket science.

There’s always been an incentive to hit the ball hard in the game, and in this case Statcast just confirmed what was long known that harder hit balls were better hit balls. The advantage that Statcast has wrought might not even be in its measurement, but in that it brought words like exit velocity and launch angle into the baseball vernacular. Players now talk with the media and perhaps with each other about these concepts, and sometimes having a word to call something is the missing ingredient in changing it.

Being able to say “I can change my launch angle” or “I can focus on adding more exit velocity” might be what gives a player the mental space to actually change his swing in a way that he wouldn’t have been able to before. It may also be that Statcast and other radar and tracking technologies allow players to do work in the batting cages where they can see in real time what the effects of a little tweak here and there are in a nice, quantifiable way. You don’t need a juiced ball if you swing harder.

And finally, there’s been an embrace of the strikeout in baseball. The game has been trending toward more strikeouts since Chester Arthur was president, but lately the pace has been accelerating a bit more. There will always be some base level of strikeouts in the game, but at some point players stopped asking how to avoid striking out and instead focused on the best approach to generate value for the team. If living with an increased chance of a strikeout buys you something even more valuable, then it’s worth living with the increased chance of a strikeout. This again is not rocket science. It’s algebra.

It doesn’t have to be every hitter in every plate appearance. In fact, if you look at the graphs, you see that the changes I’m pointing out are on the order of a couple of percentage points. But that’s all it takes. It’s a couple of guys a game picking a pitch or two that they’re just gonna sit fastball on and decide that if they see it, they’re gonna swing real hard in case they hit it. It might not be obvious in the moment what’s going on or that there’s anything different going on. It’s the sort of thing that you only see in the aggregate data, and as I pointed out earlier, we’re talking about one of the greatest home run spikes in baseball history, and it’s really only a couple of extra home runs per week for a team.

It’s entirely possible that my theory here isn’t the whole story. There may be other things going on that are contributing, but I think that there are perfectly reasonable explanations for what’s going on that don’t involve shadowy plans made in the dead of the night.