Why are runs so scarce in the 2008 American League?

Last month, J.C. Bradbury showed that comparing April home run rates to average April temperatures showed a surprising correlation: the two curves moved together almost in unison.

So is this year's drop in home run power due to the weather? Apparently not. In a follow-up post, Bradbury shows that if you look more closely at the relationship between game-time temperature and home runs (outdoor games only), only 4% of this year's decline can be explained by the cold.

Also, and more interestingly, home runs are down a lot more in the American League than in the National League. In fact, AL offense is actually lower than NL offense so far this year: at time of writing, 4.38 runs per game vs. 4.60 runs per game. That's a big difference, especially considering the AL, with its DH rule, is normally about half a run *higher* than the NL. So the American League is about 0.72 runs below where it "should" be, although you have to adjust for interleague games, in which the DH advantage disappears. Call it, say, 0.6 runs per game.

Could it be that AL cities have been colder than NL cities so far this year? Nope. Bradbury checked that too, and it turns out that the temperature in AL parks has been pretty typical this year.

And to put one more nail in the temperature coffin, Zubin Jelveh looked at domed stadiums, and found the same sharp dropoff in slugging percentage as in other parks.

So what's causing the drop? One theory is that it's not a dropoff in hitting, but, rather, an improvement in pitching. Supporting that theory is that, in interleague play to date, the AL is three games above the NL, suggesting that it's still the better league. Of course, that could just be random chance, because there haven't been that many interleague games so far this year. (I couldn't find exact interleague records, but the AL as a whole has three more wins than losses, and the NL vice-versa.)

Another theory is that it's the PED clampdown causing the drop in power, but, as others have pointed out, steroids testing actually started several years ago, and, in any case, it doesn't make sense that users would be so heavily concentrated in the American League.

Over at the Sporting News, David Pinto suggests that it's an age thing. He notes that AL hitters are significantly older than their NL counterparts, 29.5 to 28.8. That's come as a reversal from 2005, when then AL was actually 0.4 years *younger* -- a four-year change of 1.1 years.

But there's no hard evidence that younger players are actually better than older players – looking at Pinto's (very interesting) charts, you note that the NL has better younger players, and the AL has better older players. But the AL old-guy advantage is smaller than the NL young-guy advantage. As Pinto writes,

"... at the ages where the NL OPS is higher, it tends to be much higher than it is at the corresponding AL age. Where the AL OPS is higher, the gap is not quite as large."

So it seems to me that it's not as simple as an age thing.

It's an interesting problem, and I don't know what the answer is. But any theory would have to explain:

-- why the sudden drop
-- why the drop is so much larger in the AL
-- and why the AL is ahead in interleague play, despite its anemic offense.