The canary in the coal mine, for me, was Bryce Harper. Although just 20, Harper is one of the game’s bright lights, a potentially transcendent talent. He has top-of-the-scale power, but also a firm grasp of the strike zone and a swing that (while not immune to whiffs) has promise from a sheer hit-tool perspective. He should be to the National League as Mike Trout has been to the American League, a generational player, a treasure and a perennial MVP candidate. He might even get there.

Something is holding him back right now, though. Something strange and glaring and somewhat perturbing is pinning down Harper’s value relative to, say, Yasiel Puig, Paul Goldschmidt or Wil Myers. It’s something that, of that group, affects only Harper.

It’s Lefty-Lefty Disease, and it’s becoming an epidemic.

Just to give a flavor of what’s going on, here are Harper’s splits for 2013:

Bryce Harper, Batting, 2013

And Mike Trout’s:

Mike Trout, Batting, 2013

Now, let me widen the lens. Since 1969 (when the mound came down out of the sky), these are the seasons in which left-handed batters facing left-handed pitchers have had the worst tOPS+, meaning, the years in which lefties facing lefties had the worst OPS relative to the league-average figure, adjusting for park factors:

Worst tOPS+ for LHB Facing LHP, National League, 1969-2013

Notice that this is only for the National League. There’s a reason. No iteration of the American League has posted a tOPS+ south of 80 since 1970, when it was 72. The AL is seeing the same trends, but on a more muted basis. That signals to me that pitchers have an unusually large platoon split, which is not terribly surprising, is it? Anyway, that’s the situation. The AL will be mostly left out of this discussion going forward, because the phenomenon I’m talking about is affecting primarily the NL.

As the above table shows, it’s not a good time to have lefties going up there against other lefties. Along with the mid-1980s, this is the most platoon-friendly slice of the five-man rotation era. Therefore, before I get into what’s behind the problem, I’ll stop and offer this table as an answer to whether or not managers throughout the league are adapting to their environment and employing those tandem approaches to lineup spots.

Most Plate Appearances, LHB Facing LHP, National League, 1969-2013

Not only was 2012 the season in which lefty batters saw more southpaw pitching than at any other point in the period under study, but the gap is enormous, over 8,000 PA, over 10 percent. This year doesn’t look like it will break that record, but it could still be the second-highest by more than 1,000.

Check out how much better the league once protected itself, at a time when the platoon split was nearly as wide as it is now:

Worst tOPS+ for LHB Facing LHP, National League, 1969-2013

So perhaps overexposure is feeding this. Maybe some guys who should just never be facing this many lefty hurlers are getting substantial play against them, and it’s showing.

It’s not that, or at least, not entirely that. Guys hitting left-handed took 41,029 plate appearances in 2012, the fourth-highest figure of the last 40-plus years, but it’s not an outlier, and most years since 1998 fall in that general range.

No, the number of times lefties are seeing lefties is up mostly because lefties are on the mound more.

Most Games Started, LHP, National League, 1969-2013

 This is strange, for two reasons. First of all, from a pure talent-dilution perspective, the more lefties there are making starts across the league, the less we would expect them to dominate. This is particularly true because lefties are relatively rare, within the broader population, relative to righties, but it would be true in any case. More quantity means less quality. That’s usually a fairly immutable truth of production.

Secondly, one theory (one I think we might be debunking right here and now) of the greater disparity between lefties’ splits and righties’ splits has always been the rarity with which lefties face other lefties. It’s harder to adjust to their disadvantage because they get fewer opportunities to work on it. Yet, the most left-leaning pitching seasons of the modern era are turning out to be the ones that feature the widest platoon disparities.

Let’s talk about the shape of the splits, because this is something I’ve been saying for a while, and it may be a big part of why this is happening now.

It’s not about BABIP. You can toss that idea out right now. Lefty-on-lefty BABIP in the NL was among the 10 highest since 1969 last season, and this year is in the middle of the pack. Shifting might be hurting lefties in general, but if anything (since most shifts claim pulled grounders as victims and batters pull the ball more against opposite-handed pitching), it’s doing it when they face righty arms. Nor is the diminished role of speed in the game, which of course has always favored lefty batters, apparently doing much damage.

It’s strikeouts. Strikeouts are the hydrogen bonding of baseball since 2010, the answer to every question of how the game works, or at least part of the answer.

Most Strikeouts, LHB Facing LHP, National League, 1969-2013

Again, last year’s record is out of reach, but second place is all but assured. Yes, these are all seasons that have seen plenty of plate appearances of these types, but the order would basically hold even if I showed you rates instead of raw totals.

The platoon advantage—the intrinsic, inevitable part—lives chiefly in controlling the strike zone. You’ll often see pitchers with reverse platoon splits overall, but whose strikeout-to-walk ratios still favor the situations in which they have the normal advantage. That’s how it works. Power and BABIP are predicated on the location of a pitch and the speed, torque and lift of a swing. It’s picking up a guy a split-second earlier, getting a better look at the spin of the ball and lining up the swing more easily that lends the platoon advantage, and those things show up in to whom the strike zone belongs.

This might be the biggest reason for the leaps forward in platoon gaps over the past few years. As strikeouts become more prevalent than ever before, as pitchers claim victory in the battle for the area floating over home plate between a batter’s knees and the letters of his uniform (and often, for the territory just below the knees, just to each side of home plate, too), the platoon advantage is becoming more important to hitters. Indeed, lefties, at least, are having a harder time than ever surviving without it.

We should ask why this is happening mostly to lefties. I think I can answer that. The phenomenon whereby umpires call pitches an extra inch or two off the outside corner strikes with lefties at bat, relative to the same pitches for righties, has become well-established. I think that’s the main culprit, and I think it signals that that problem is both more ubiquitous and more insidious than we imagine.

So it comes down to this: Lefty starting pitchers are more common than they used to be, but not worse. Lefty relievers are far, far more specialized and carefully selected than they used to be, and because they pitch in such short bursts, are better. Lefty batters can avail themselves less of their speed than they once could, and suffer more at the hands of umpires than their counterparts. Platoons are disappearing because of the constraints modern bullpen-building place on roster construction, and so guys who might normally have shelter from those tough match-ups have lost their safety net. Maybe most importantly, the thing that makes same-side matchups most dangerous for batters (strikeouts) is skyrocketing across the league, while walks (which help mitigate that problem, are relatively static.

I’m not sure what can be done to reverse this. An automated, or just better-called, strike zone would be a strong step. Slow, natural selection of left-handed hitters with better contact skills might help. In the meantime, though, it’s harder to be a good left-handed hitter than it has been in a long time, and righties rule at the plate.

Thank you, Sean Forman’s mother, for having Sean Forman. Thank you, Sean Forman, for creating Baseball-Reference.com. And thank you, Baseball-Reference.com’s Play Index, for making it easy to search league-wide split stats and line them up historically, among a thousand other things.