The American League dominated Interleague Play again this year, winning 149 of 261 games. Playing against the National League has no doubt fueled the Tigers each of the past three years, save for a few games one October. I have heard and read plenty of explanations as to why this disparity exists, but way too many of them tip toe around the true answer: The American League has better players.
You may remember the first two months of the season. Run scoring was way down in the American League. Guys simply weren't hitting. Lineups with nine professional hitters per lineup were being outscored by the lineups from the other league with eight professional hitters and a designated bunter. Through May 22, Jack Cust's .888 OPS was good for fifth in the American League. Something wasn't right. The prevailing sentiment was that the talent had finally shifted from the American League to the National League. Then the leagues started playing eachother.
American: 4.58, 4.47, 4.96
National: 4.45, 4.55, 4.02
Those are runs per game averages for each league. From left to right, this shows the total runs scored per game this year, the runs scored per game within the league, and the runs scored per game in interleague games. You can see that the American League has surged ahead in runs scored over the past month, but you can also see that more runs are being scored in games where neither team has the designated hitter. Then you have the fact that the American League has completely trounced the National League in Interleague play this year. The American League scored about half a run per game more against National League teams than they did against themselves, while the National League scored about half a run per game less. This seems to suggest that it is much easier to score runs against National League teams than American League teams. It also suggests that despite the offensive struggles of the first two months, the American League still has better hitters than the National League.
One possible reason I have been hearing a lot, especially on XM 175, is the designated hitter rule. The theory is that American League teams are better built for interleague play because they are putting a real hitter out to DH instead of a bench player. This would seem to make sense, and looking at the American League's DH splits against the National League's, you can see that American League DH's do indeed hit better.
American League: .248/.334/.419 (BA/OBP/SLG)
National League: .242/.314/.402
So, the American League seemingly has an advantage here, but did it help them in interleague play, when American League teams have to bench their designated hitter when visiting National League parks?
American League w/ DH: 5.13 runs per game
American League no DH: 4.80 runs per game
National League w/ DH: 4.25 runs per game
National League no DH: 3.80 runs per game
The American League teams scored .33 runs less per game during Interleague play in National League parks, while the National League scored .45 runs more per game in American League parks than at home. I'm not quite sure what to make of that other than the National League does benefit fairly significantly from the designated hitter, thereby suggesting that this probably doesn't have nearly as much of an effect on the American League dominance as superior talent.
While interleague play has it's flaws, one of it's main benefits is that we have an easier time comparing the two leagues. While National League apologists will try to come up with justification for why this is, we know the truth. American League teams simply have better hitters, better pitchers, and better fielders.
All numbers were gathered from baseball-reference.com, with the home/road interleague splits derived from the game logs.