Is it even necessary to go over the background? I will quickly go over the background. Back in 2014, there was mounting concern that baseball no longer featured enough offense. Pitching had taken over, and we saw run-scoring plummet. Strikeouts, as you know, have only continued to go up over time, but that’s been countered by a sudden spike in home runs. There were 723 more home runs in 2015 than there were in 2014. There were 701 more home runs in 2016 than there were in 2015. There were 495 more home runs in 2017 than there were in 2016. Compared to 2014, the number of home runs hit in 2017 was up by a staggering 46%. That’s what baseball became — a showcase for the three true outcomes. Strikeouts and homers were in.

The surge in power made some people happy. It made other people sad. The remaining people were indifferent observers of a changing game. No matter your opinion, we were all left to wonder: now what? Would the power surge continue? Would the ball continue to fly? Would more and more batters continue to swing up, enthusiastically trading contact for dingers? It’s easy to observe a trend that’s already in the books. It’s more difficult to know where it’s going. Most of the time, anyway.

And in this case, I’ve given it away with the headline. What’s become of the home-run spike in 2018? There’s been, to be sure, no shortage of dingers. But the home-run count isn’t going up. It’s actually taken a step backward. Whether it’s signal or whether it’s noise, the home-run spike isn’t spiking.

Here’s the easiest way of looking at things. It doesn’t make sense to compare partial-season data against full-season data, so in the following table, you’ll see league-wide numbers for the last three seasons, through the early part of June:

To-Date League Averages Through This Date HR/Batted% HR/FB% GB% BB% K% wOBA 2016 4.2% 12.5% 45.2% 8.3% 21.1% 0.315 2017 4.7% 13.4% 44.5% 8.7% 21.5% 0.317 2018 4.4% 12.4% 43.3% 8.6% 22.4% 0.314

Strikeouts have gone up almost a percentage point. That’s unsurprising — the strikeout trend continues unabated. Walks have hardly budged. Offense overall is slightly down, as reflected by wOBA. We have seen a slight shift toward hitting more fly balls. And yet, consider those fly balls. The rate of homers per fly ball has come back down a full percentage point. The rate of homers per batted ball has also, of course, come back down somewhat. It’s not like the league is back to its 2014 level. But it’s also not particularly close to its recent 2017 level, standing closer to 2016 instead. Perhaps, in time, when it comes to the home-run surge, we’ll see 2017 as the apex. It’s too early to arrive at such a sweeping conclusion, but it’s something to pay attention to.

For the sake of showing more information, I’ve broken a few of these numbers down. Using our splits-leaderboard feature, it’s possible to look at league-wide numbers on a week-to-week basis. Here’s how the last three seasons have begun, in terms of league grounder rate:

You can see things shifting downward. Whenever you reduce sample sizes, you increase the relative influence of noise, so on the week-to-week level, numbers bounce around. Still, we can say that more balls are being hit in the air. This is a trend with a variety of causes. And it’s hard to blame hitters, considering how fly balls have been rewarded.

Now here’s how the three years have begun, in terms of homers per fly ball:

And, very similar to that, here’s the plot of homers per batted ball:

Earlier this season, there were a lot of cancellations, and there was a lot of cold, inclement weather. That seemed to me like a reasonable explanation for why the early numbers were where they were, and as you can see, by week five or so, the home runs reappeared. For a few weeks, you can see the 2017 and 2018 lines mirror one another. But then there’s another observable separation. Over the past month, home runs have slipped well behind the 2017 pace. They’re even a little behind the 2016 pace. The home run is very much still alive in the game today, but something has reeled the trend back in.

As it happens, I’m not the first person to get to this. You probably heard about MLB’s scientific committee that investigated the properties of the baseball itself. That committee included one Jim Albert, and Albert has already written on the early numbers in the 2018 season. Here’s an analysis post from May 28, and here’s another from June 4. Albert makes use of some of the available Statcast information, and of particular note is that launch angles and exit velocities are up. If anything, we’d expect there to be *more* home runs, since hitters seem to be hitting the ball a little harder and a little higher. Clearly, that’s not what’s happening. And in the second post linked, Albert folds in the potential influence of lower temperatures. Temperature explains a part of this, but not nearly the whole.

Here’s how Albert sums up his first post:

There is clearly a decrease in home run hitting in 2018 and balls hit with specific launch angles and launch speeds are less likely to be home runs. As I noticed, 2018 hitters are actually hitting with higher launch angles and higher launch speeds, but the balls appear to have more drag. Of course, the big question is why this is happening, and this motivates further exploration.

And here’s a takeaway from his second:

By using a new model using three variables (including temperature), I find that the 2018 home run still is about 300 behind what one would expect based on the 2017 history. So temperature explains some, but not all of the drop in the 2018 home runs.

Home runs are down. That much we know. They’re not down by a dramatic amount, and it’s possible, as a fan, you hadn’t even noticed. It’s not like hitting for power is in any way endangered. Still, consider what the scientific committee found. In their collective judgment, home runs took off because the baseballs themselves experienced lesser drag. They didn’t have a definite answer for why that would be. Now, the early numbers suggest something has changed again. It seems as if the balls must be encountering more drag, relative to 2017, which only makes things all the more confusing. Plenty more analysis will have to be done, and I doubt I’ll be the one doing it. For now, know the baseball itself just continues to fascinate, with minor changes leading to broad repercussions.