Los Angeles police investigate a shooting last month. 

Alan: My completely unsubstantiated hypothesis is that people now are so entertained/preempted by their cellphones that they are neither motivated to participate in crime both because they lack time but also because a cellphone offers the entertainment that crime used to provide... or pay for.

No one can figure out why crime is so low

The late-20th-century crime wave has crested and ebbed, writes Eric Eckholm, but where it came from and where it went remains a mystery:

The reasons for the broad drop in crime remain elusive. It has confounded both those from the right who had predicted that waves of young predators would terrorize communities and those on the left who watched crime fall even through ups and downs in poverty and unemployment. ...

Various experts have also linked the fall in violence to the aging of the population, low inflation rates and even the decline in early-childhood lead exposure.

But in the end, none of these factors fully explain a drop that occurred, in tandem, in much of the world.

That's why today's chart of the day, from The New York Times, is something of a Rorschach test. Everyone sees what they want to see in it. New York Police Commissioner William Bratton puts the decline down to the "broken windows" theory of aggressive policing. Advocates of harsh sentencing laws can argue that more criminals were spending more time in jail, and that others, threatened with long terms, gave up their accomplices once apprehended.

As politicians on the right and the left look for alternatives to being tough on crime, there will be plenty of somber warnings about keeping the streets safe, but the evidence is limited.