I ran across this op-ed in the New York Times on Monday. The premise of the article is that the recent increase in home runs in baseball is due to expansion, not to steroids. In the 1990s, MLB added four expansion teams; the theory is that these expansion teams diluted the talent in baseball, allowing the very best players to excel and hit more home runs. Now I don't know much about baseball, so I don't feel that I can judge this theory very well, but my gut feeling is that this seems unlikely.
First of all, the expansion only increased the size of the league by 15%. Are the top players in the minors really that much worse than major-league players that they could cause such a significant increase in home runs? By comparison, 15% is approximately the population growth of the U.S. between the 1977 expansion and the 1993 expansion. I would think that the pool of major-league-quality baseball players would have grown similarly during that time period.
I can understand that there might be a short-term effect, especially if all the "bad" and inexperienced players were initially clustered in the four expansion teams. But I would think that it would have evened out by now with trades and attrition. (The Marlins won the World Series after only 5 years in the majors, so clearly the expansion teams weren't stuck at the bottom with inferior players.)
Now if you look at the home run statistics, there's a significant jump in home runs in the NL in 1993, when the NL gained two teams. Before 1993, the NL seemed to be averaging around 1300 or 1400 home runs per season. Since 1993, it has never had fewer than 1900 (excluding 1994 because of the strike). Of course, some of that increase is due to the fact that there's more teams in the NL now. If you look at home runs per team in the NL since 1977, this is what you get:
(Again, 1994 was special, although given that they were about 70% of the way through the season, they were on course for more home runs than in 1993 or 1995.)
Unfortunately, this graph is inconclusive to me. Sure, the number of home runs per team has gone up about 50% in the past 15 years, and the increase happened around the time of the expansion. There was even a suspicious jump in home runs in 1993. But 50% seems like quite a lot to me, and I still wonder whether the increase is better explained by other factors. It could be drugs. It could also be a rational shift in baseball strategies. (Maybe trying to hit home runs helps you win games? Or maybe home runs attract spectators?)
As I said, I don't follow baseball very closely, so I'm interested to hear what other people think.