The Effectively Wild podcast gets a good number of emails every day, and a recent one was sent along by Aidan Jackson-Evans. He was asking about a trend that he noticed, and wrote about several days ago. I’d like to point you to his post, at High Heat Stats. You should read it, because this thing is quickly just building off that. I wouldn’t have noticed this trend if not for the email, and he’s already done a lot of the work.

I’ll get to the heart of the matter. At FanGraphs, we have splits going back to 2002. That’s not very long, but it’s long enough for research purposes. In this plot, you simply see league wOBA by leadoff hitters, and overall league wOBA. Nothing complicated, at least if we’re all to the point where we no longer think of wOBA as being complicated.

Last year, leadoff hitters were better than average. The same was true the year before that, and the year before that. And so on. Nothing earth-shattering. This year, the numbers have been exactly equal. Here’s where things get cooler. In here, leadoff-hitter wOBAs, split by whether it’s the first plate appearance of the game, or a subsequent plate appearance.

Allow me to translate that! Between 2002 – 2015, leadoff hitters averaged a .323 wOBA in their first plate appearances. Over all the rest of their plate appearances, they averaged a .325 wOBA. Essentially the same. Nothing weird. But since the start of last season, leadoff hitters have averaged a .343 wOBA in their first plate appearances. Over all the rest of their plate appearances, they’ve averaged a .323 wOBA. Leadoff hitters, in other words, have started performing a lot better in the top and bottom of the first inning. This is what Jackson-Evans found.

So what’s going on? As with everything in baseball, the answer is presumably complex. Yet, using data from Baseball Savant, I bet I can simplify. Firstly, let’s look at the rate of pitches thrown within the Gameday strike zone. Here you see two rates: the league overall rate, and the rate for first plate appearances.

The first batter of the game sees more strikes. But this might not seem very dramatic. It’s not! This is dramatic. Here’s the same as above, but showing fastball rates instead:

Over the past decade, over all plate appearances, hitters have seen an average of about 57% fastballs. However, leadoff hitters in the first inning have seen an average of about 73% fastballs. The margin so far this season is 72% to 55%. It’s particularly extreme if you look at the first pitch of the first plate appearance, where pitchers routinely throw fastballs more than nine-tenths of the time. Pitchers have historically been aggressive to start off, trying to get into the flow of things. If the results are any indication, leadoff hitters have started to ambush more often. That is, instead of letting pitchers get away with free strikes, those leadoff hitters in the first inning are going up more prepared to take a swing or two. It’s as if the hitters are getting ready before the pitchers are.

It’s weird to see it happen so suddenly, but this is where we are after thousands upon thousands of plate appearances. It’ll be interesting to see if pitchers try to adjust by folding in more secondary stuff at the beginning. It’s the obvious apparent correction, but it might not make them feel very comfortable. Maybe they want or need to settle in with a few early fastballs. It’s something for all of us to try to monitor, I suppose. Kudos to Jackson-Evans for identifying this curiosity.