Mailbag: Is the NL the fastball league?

From reader Kevin K.:

In my lifelong love of baseball and its history, there is one axiom that I have never understood: The National League is a fastball league and the American League is a breaking ball league.

I have only heard two explanations for this axiom. #1 is that the National League has weaker hitters at the bottom of the order – especially in the 9 hole. This hitters are much more likely to receive a steady diet of fastballs than the bottom of the order hitters in the American League. Obviously, this is connected to the DH rule. #2 is that years ago, most NL cities had spacious pitchers parks while AL cities had cozy hitters parks. Smaller parks meant that whenever it was a hitters count, pitchers would throw nothing but junk to cut down on the risk of the homerun. In the larger NL parks, pitchers felt more comfortable to use the fastball in a hitters count with the confidence that a mistake would still stay in the park.

I have no idea if either of these explanations is the real basis for the saying I have heard since I was a kid. More importantly, I am interested to know if there is any truth to the axiom. Has there EVER been any truth to saying?

Steve Treder: I don’t know how much truth there is to it, but it’s been commonly said (so, presumably, commonly believed) for a very long time, certainly before the Designated Hitter rule created a difference between the leagues beginning in 1973.

Whatever truth there is to it is probably a function of ballparks, as you suggest. But it may also have gained its footing in the decade of the 1930s, when the two leagues deployed baseballs of significantly different resilience: the American League’s ball was much livelier, which would prompt pitchers to be more hesitant to challenge hitters.

As with any other aggregation of humans, over the years the leagues have tended to develop and maintain their own cultures, ways of seeing themselves as special and different from the “other.” One suspects the idea of the NL as “the fastball league” and the AL as “the breaking ball league” is one of those things that rests on a small foundation of factual truth, and a large remainder of colorful yarn.

John Walsh: I had a quick look at the percentage of different pitch types thrown in the two leagues, using pitch f/x data from 2007 (the only year available). This is what I found:

+--------+--------+------+------+------+------+
| League | NP     | FB%  | SL%  | CB%  | CU%  |
+--------+--------+------+------+------+------+
| NL     | 134744 | 0.60 | 0.17 | 0.11 | 0.11 |
| AL     | 132635 | 0.58 | 0.17 | 0.11 | 0.13 |
| IL     |  28396 | 0.59 | 0.15 | 0.13 | 0.12 |
+--------+--------+------+------+------+------+

It looks like AL pitchers throw a few more changeups and slightly fewer fastballs. Of course, the imprecision of identifying pitch types may be larger than the 2% difference between the two leagues.

(IL is for inter-league games).