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Why was AL BABIP falling before the DH?

Not sure what good this will do, but I have a graph and questions.

Babip_1950_to_2010_medium

One of those things that comes up when you look at Jim Palmer and Nolan Ryan's career lines is how low their BABIPs were.  Ryan's came in at .269, and Palmer at an astonishing .251, both unheard of in the modern era, but really not too much of a stretch in the 1970s.

Arbitrarily beginning with 1950, I plotted BABIP over time for the NL and AL (the NL in orange, the AL in blue).  There are two very notable jumps - in 1973, the DH was introduced to the American League, and in 1993, MLB added the Florida Marlins and Colorado Rockies to the National League.  There are a few things that do not make sense, though.

1) Why did BABIP drop in the AL from 1950-1972 while it rose slightly in the NL during that period?  And how is it that adding a DH to the AL managed to equalize BABIP with the NL?  Did AL umpires have drastically different strike zones from NL umpires?

2) Why did BABIP rise for both leagues in 1993 after one league's expansion?  Did the expansion draft make that much of a difference?  And even then, why didn't any other expansion seem to have that effect?  Does this mean we should assume everyone took steroids in 1993?

Maybe you know.  Even if you don't, I probably won't know the difference.

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