In the book entitled Freakconomics it theorizes that legalized abortion was the cause of crime reduction in New York in the 1990's. Abortion was legalized around 1970 which is 20 years before the crime rate fell. The claim is that unwanted babies are more likely to become criminals and because they were killed by abortion this reduced the crime rate by 50%. Now there are a lot of arguments for and against this idea but it is still interesting.
But I also have a theory. The pill was introduced in around 1960 in New York. The pill affected the New York population in that “responsible” families, those who could comply with the “wisdom” of fertility reduction, had fewer children. People who were not intelligent, inept, or responsible were not as successful with the cultural value of fertility reduction and had more children. This increase the ratio of children in the population who came from “dysfunctional”parents and this led to a higher crime rate, twenty years later, in the 1980's in New York.
This all reeks of eugenics but I would guess that the culture of contraception has had a more detrimental effect on society than the crime reduction ability of abortion can counterbalance. To bad that many Catholics in America have sold out to the wisdom of fertility reduction [i.e. culture of death] but that is a topic for another post.