Criminologists and other scientists that study crime have been offering a lot of differing reasons why crime is down. I've seen quite a number of articles that looked specifically at the decline of crime in New York City and offered varying explanations on why crime dropped. The Atlantic Cities had a piece on a recent study published in Justice Quarterly that casts doubt on a number of theories behind the decline.
That conclusion, based on a paper recently published in the journal Justice Quarterly, is a pretty jarring one. It challenges widely held narratives of how New York won its war on crime. But it also raises awkward questions about the efficacy of certain police tactics everywhere, particularly "broken windows." ("It’s a curiosity," Greenberg adds, "that this name got attached to what the New York Police Department was doing, because the police never went after broken windows").