On the SABR listserv this week there's been a lively discussion on the increase in homeruns since 1992. One poster asked whether perhaps a contributing factor is the fact that teams carry more pitchers these days and so give a greater percentage of their innings to inferior pitchers.
In thinking about this question it occurred to me that one way to possibly test this would be to look at those pitchers that threw the most innings (on the assumption that generally the "best pitchers" get the most innings) and see if their homerun rate had differed over time or held constant. If it held constant, then the increase in homeruns could be attributed to the other, presumably inferior, pitchers who are now getting more innings. If it had increased along with the league average, then the best pitchers are getting victimized by the homerun along with everybody else.
What I did was to select the top seasons in innings pitched for each of the last four spans of 11 years (60, 72, 78, and 90 pitchers in each group to track with the increase in number of teams). Then I calculated their homeruns per 9 innings and compared that with the weighted league average during that span. The results:
AVGIP HR/9 LgHR/9 Pct of Lg
1993-2003 243.9 0.81 1.07 76%
1982-1992 266.6 0.74 0.82 90%
1971-1981 308.5 0.65 0.73 89%
1960-1970 302.4 0.67 0.83 81%
The average innings pitched of the top pitchers in each set definitely decreased and their homeruns per 9 innings increased (the bump in the 1971-1981 period can be attributed I think to knuckleballers Wood and Niekro who took 7 of the top 21 slots). So their rate did not remain constant which would tend to support the idea that inferior pitchers pitching more innings does not account for the increase in homeruns. However, relative to the league, the top pitchers in the period 1993-2003 gave up fewer homeruns than did those of the previous 3 periods. So tentatively, it looks like there is some support for the argument although it appears to some degree that a rising tide of homeruns has lifted all boats.
My personal opinion is that this may be a factor to add to the confluence of contributors that include:
* Greater strength by hitters through weight training that has increased the speed at which they swing the bat and therefore the distance they hit the ball
* The development of thin-handled bats that allow for whipping the bat through the strike zone at higher speeds
* The use of aluminum bats at lower levels that train pitchers to work the outside rather than the inside corner
* The crackdown on bean balls and fights that works to the advantage of the hitter since the pitcher is the one getting ejected, and because it allows hitters to dive out after the ball on the outside corner and hit opposite field homeruns (a rarity as you’ll remember in the 1980s and before)
You'll note that not included in this list is smaller ballparks and expansion. The reason the latter is not included is that expansion effects should only be evident for a year or two. The former isn't included because I don't think there's any evidence that the newer parks are any smaller in terms of dimensions. Certainly, the newer parks have smaller foul territory which would increase offense since balls that would normally be outs turn into second chances. I'm not sure it could account for much of a difference in homeruns per game although I'm willing to add it to the list of contributors.