Some things in baseball simply come into existence and we accept them without question. Using Earned Run Average as the primary form of statistical evaluation of pitchers was practiced for almost 100 years. The idea that a team needs to hit home runs to be successful is still a popular notion among old-school baseball folks. And, currently, we have the AL/NL talent differential.
When Cliff Lee, for example, is traded from Cleveland to the Phillies, his tRA promptly drops by over 40 points. When Matt Holliday went from Oakland to St. Louis, his batting average rocketed from .286 in Oakland to .351 with the Cardinals. And how could we forget about CC Sabathia, who racked up an astounding 5.1 WAR and 2.38 tRA in less than half a season for the Milwaukee Brewers?
The examples of successful National League players – most notably pitchers – who have moved to the AL and performed poorly seem innumerable. Everyone from Adrian Beltre to Rich Harden seems to have either resurrected their career at least somewhat by moving to the NL or started sucking after moving to the AL.
Matt Klaassen of Fangraphs.com points out that the better GMs tend to be in the AL, which could have a reasonable effect on the balance between the two leagues. Some people think that the AL simply has better hitters and better pitchers than the NL, which could also be true. But, overall, AL teams have destroyed NL teams in Interleague Play over the past couple of years, showing that the former tend to be just plain better than the latter. I mean, a 56-44 win percentage split is pretty substantial. However, we can approach this in a different way. People tend to forget that the the apparent NL/AL talent difference is largely built into the system.
The most basic fundamental difference between the American League and the National League is the presence of the designated hitter in the former. In the NL, the pitcher usually bats 2 times per game, and is replaced by a usually mediocre pinch hitter for his last at-bat. In 2009, the average pitcher hit .136/.176/.174, whereas the average AL DH hits .256/.338/.444. The differential between these two stat lines, over 400 at-bats, comes out to be around 48 hits and a huge difference in power, which is a fairly significant total.
When CC Sabathia, for example, pitches in the American League, he faces hitters of the ilk of Hideki Matsui, Nick Johnson, Adam Lind, and Milton Bradley, who reach base around 35% of the time. When CC pitches in the National League, he faces a pitcher instead of a DH who only reaches base around 16-17% of the time. That is a massive difference. And although one hitter is only 1/9th of the entire lineup, facing a hapless pitcher waving his bat around to no apparent rhythm with 2 outs and the bases full is much preferable to facing David Ortiz in the same situation.
If you’re not yet convinced, designated hitters accounted for over 380 home runs in 2009, while pitchers hit a total of 26. And the league wOBA for the DH-less NL was 10 points lower than that of the AL last year.
In summary, pitchers who’ve been pitching in the National League aren’t necessarily worse than those from the American League. They just had the advantage of facing an extremely inept batter twice or three times a game, which is essentially two or three more times than they would in the AL. It is true that the AL currently holds the edge in talent – see the aforementioned Matt Holliday’s league splits – but for the most part, the miraculous devolution suffered by NL pitchers in the AL is a product of the system.