And then … something happened. Between 1991 and 2015, rates of criminal victimization dropped by half, with the most violent offenses—murder and rape—declining the most precipitously. An American citizen is less likely to be a victim of crime today than at any time since good records began in the 1970s. New York City is almost certainly safer today than at any time in its history. And the weird thing is that nobody quite knows why it all happened. Ever tougher criminal sentencing helped. A single criminal will commit many crimes over his career. The more of that career he spends behind bars, the fewer crimes he’ll have opportunity to commit.

More sophisticated policing surely helped. “Broken windows” and community policing got cops out of their cars and onto their beats. CompStat systems directed police resources where they’d do the most good.

Many other technological and social factors played their part too. Some have nominated the aging of the population, or the phasing out of lead paint, or new immigration flows, or improvements in educational levels, or the decline in use of crack cocaine. Some hardcore proponents credit more abortion of unwanted children or more concealed carry of private firearms. Yet the uncomfortable truth is that there’s remarkably little clarity about the causes of this hugely important social improvement. Almost nobody predicted it beforehand. Even after it happened, social scientists cannot agree on why.