INTUITIVE theories are often easier to believe in than to prove. For instance: conventional wisdom says that the crime rate should rise during a recession. When people are out of work and out of money, the thinking goes, they turn to crime. But the evidence backing this theory is at best equivocal. There seem to be some links between crime and economic conditions, but they are neither as direct nor clear as one might assume. Crime rose during the Roaring Twenties then fell in the Depression. America's economy expanded and crime rates rose in the 1960s. Rates fell throughout the 1990s, when America's economy was healthy, but they kept falling during the recession in the early 2000s (see chart).

And during the current downturn, the unemployment rate rose as the crime rate fell. Between 2008 and 2009 violent crime fell by 5.3% and property crime by 4.6%; between 2009 and 2010, according to the preliminary Uniform Crime Report released by the FBI on May 23rd, violent crime fell by another 5.5% and property crime by 2.8%. Robberies—precisely the crime one might expect to rise during tough economic times—fell by 9.5% between 2009 to 2010. The decline in violent crimes was sharpest in small towns, where the rate dropped by more than 25%, and among regions sharpest in the South, which saw a 7.5% decline. Only two cities with more than 1m people—San Antonio and New York—saw their crime rates rise. And some perspective is warranted there: in 1991 around 2,200 people were murdered in New York. Last year just 536 were. Overall, America's violent-crime rate is at its lowest level in around 40 years, and its murder rate at its lowest in almost 50.

According to the social scientists, this was not supposed to happen. In 1995 James Wilson, who came up with the “broken windows” theory of crime prevention widely credited with making New York safer, warned that by 2000 there would be “30,000 more young muggers, killers and thieves than we have now. Get ready.” One year later John DiLulio, another political scientist who studies crime, warned of a wave of “juvenile super-predators” wreaking havoc by 2010. Yet even as they wrote, the violent-crime rate had already begun to fall. Except for a bit of a rise from 2004 to 2006, it has fallen every year since 1991.

Although nobody predicted the striking decline in crime during the 1990s, in hindsight theories explaining it abound. Some give credit to smarter police tactics: particularly quantitative methods and “broken windows” policing. Others point to the increased availability of legal abortion in the 1970s, resulting in fewer children born to teenage, unwed and poor mothers: precisely the sorts of children who commit crimes at high rates during adolescence. There is also the waning of violence associated with the crack market, and the increased incarceration rate, which keeps more criminals off the street for longer (though at tremendous cost).

Although these factors explain the drop since the late 1980s, they do not explain the sharp drop in the past two years. For that Al Blumstein, a criminologist who heads the National Consortium on Violence Research, posits an “Obama effect”, in which the election of America's first black president inspires a significant number of young black men away from violence. And indeed between 2008 and 2009, the numbers of blacks arrested for murder and robbery each declined by over 2%, though this theory has more narrative than evidentiary appeal.

Another theory concerns lead. Exposure to lead in childhood has been linked to aggression and criminal behaviour in adults. Jessica Wolpaw Reyes, an economist at Amherst College, argues that the decline in American children's exposure to lead since it was phased out of gasoline in the 1970s and removed almost entirely by 1985, accounts for much of the decline in violent crime in the 1990s. It may account for even more, as more of America's unleaded children enter adolescence and their early 20s. And then there are those perennial bogeymen, video games and the internet, affordable forms of entertainment that keep people inside, and away from real crime and drugs.