The interest thing about the decline in crime circa 1930 is that there are at least two different candidates for a causative change, and they are both Federal laws: the 21st Amendment and the 1924 immigration law. I suspect they were both important. 
 
Oh, I forgot to mention, the 1924 immigration law was racist (even though it didn't mention race) and anti-semitic (even though its effects never differed between Jews and gentiles), as well as pre-figuring the current immigration debate (though the 1924 law did not regulate immigration from Mexico or Central America at all). 
 
Okay, sarcasm completed.