The controversial book “Freakonomics”, by the economist Steven D Levitt, lays the onus of unexplained drop in crime rate in the US on:

Freakonomics argues that legalized abortion in the 1970s led to fewer unwanted children born in difficult circumstances, which in turn reduced crime rates in the 1990s as those children would have been in their peak crime-committing years. The controversial thesis links demographic changes (fewer births likely to lead to criminal behavior) with crime reduction, not economic factors, policing, or population aging.