It refers to research, heavily publicized by Freakonomics, which claims that the reduction in crime rates in the US is not because of police interventions etc.

It refers to research, heavily publicized by Freakonomics, which claims that the reduction in crime rates in the US is not because of police interventions etc. but because more people are having abortions and abortions are legal in more states and so fewer people are born in dysfunctional environments that tend to produce more criminal behavior.