Today on the Fox Sports Broadcast of the Cubs/Brewers game Steve Stone and Chip Carey were discussing the relative difficulties of reaching 500 homeruns and 300 wins. Steve mentioned that in his view virtually all the changes in baseball since 1968 have favored the hitters. Specifically, he mentioned...
1. Lowering the mound
2. Playing in smaller ballparks
3. Allowing hitters to pad up their lead elbow
Although I think he left out the most significant reason, that of weight training and increased bat speed, the first and third reasons he cites I agree with. The second is debatable.
Contrary to Steve's opinion the general consensus is that the new ballparks are not smaller in terms of their outfield dimensions although this is difficult to calculate since newer parks are not as symmetrical and distances are not well-marked in most parks. However, there is agreement that the decrease in foul territory does contribute to the increase in offense. This also is a factor in some older parks by the way as teams in recent years have added premium seating behind the plate and down the lines thereby decreasing foul territory. Fenway Park, Wrigley Field, and Kauffman Stadium among others have these changes made.
But how big is the effect of decreased foul territory?
In February of 2003 Tom Tippett of Diamond Mind did some quick analysis of the question. In looking at play-by-play data he found that there was an average of 138 foul outs per park per season from 1999-2002. 121 of these were on foul outs in the infield area and 17 on foul fly balls in the outfield. After Fenway Park added some new seats prior to 2002 it decreased the infield foul outs from the previous average of 128 to just 111. So assuming that the seats cost 17 outs, that's a little over 1 per month per team - certainly not enough to make much of a difference in run scoring or homeruns.
But what about the decreased foul territory in new parks? Network Colisuem and Dodger Stadium, both older ballparks (pre-1993) saw averages of 186 and 178 foul outs per season respectively. Assuming newer parks (post-1993) are configured more like Fenway than like Network Coliseum the new parks could have cost pitchers 30 to 50 outs per season per park using a generous estimate. Since 1993 seven of the twelve existing NL teams and six of the fourteen AL teams have moved into new ballparks. These thirteen parks therefore may have cost as many as 520 outs per season (13*40) on the high side and therefore about 21 homeruns per season (the major league average during this period was 1.07 HR per 27 outs so 520 outs would produce (1.07/27)*520 = 20.6). Once again, a total too small to effect the general trend.
So I doubt that decreased foul territory has played a significant role in the increase in offense since 1993.
Note: On another note the Cubs today signed Niefi Perez to a minor league contract. Apparently, the Cubs are trying to see how many of the worst offensive players in history they can sign having already run through Rey Ordonez this season.