A theory regarding the effect of legalized abortion on crime is a controversial hypothesis about the reduction in crime in the decades following the legalization of abortion. Proponents argue that the availability of abortion resulted in fewer births of children at the highest risk of committing crime. The earliest research suggesting such an effect was a 1966 study in Sweden. In 2001, Steven Levitt of the University of Chicago and John Donohue of Yale University, citing their research and earlier studies, argued that children who are unwanted or whose parents cannot support them are likelier to become criminals. This idea was further popularized by its inclusion in the book Freakonomics, which Levitt co-wrote.