US crime levels have fallen to their lowest reported levels in nearly half a century despite major unemployment and the worst economic downturn since the Great Depression. Even more remarkably, the drop was steepest in America’s big cities – which are still popularly believed to be cauldrons of criminality. The question is: why?

Research from the Brookings Institution recently noted that the hubs of the 100 largest US metro areas benefited most from declining crime rates. But even in the suburbs, the results were surprising: “Older higher-density suburbs saw crime drop at a faster pace than newer, lower-density emerging and exurban communities on the metropolitan fringe.”

When crime rates first began to plummet in the 1990s, economists such as Freakonomics author Steven Levitt argued that legalised abortion had been responsible, since unwanted children were more likely to grow up to be criminals. Others suggested America’s astronomical incarceration rates could be responsible. Myriad other theories also emerged, with some experts even attributing much of the drop in violent crime to children’s reduced exposure to lead.