WHEN asked why he robbed banks, Willie Sutton, a prolific bank robber (pictured, after he retired), is said to have replied: “Because that's where the money is.” Sutton reportedly pinched $2m during a lifetime of crime, but most robbers don't do nearly as well. In Britain the average haul from a successful robbery is £20,330 ($31,610), according to Barry Reilly of the University of Sussex, whose team of economists crunched data on the 364 robberies in 2005-08. American robbers do worse, scooping $10,025 on average, according to the FBI. They boast a higher success rate, however: 90% of attempted heists in America netted cash—compared with 66% in Britain.

Despite what you see in the movies, removing large bundles of cash from a bank is rather difficult. In America, most robbers get no further than the counter; only 5-10% make it inside the vault. Banks could make it even harder for the men in masks by installing better security. Metal screens, driven by compressed air, can seal off staff and money in half a second. In Britain, only a third of robberies of banks with screens succeed. But since they cost a hefty £4,500 per bank teller, installing just five works out at more than the average loss from a robbery. This may explain why only 12% of bank branches in Britain bother with them.

If banks invest in beefier security, robbers may invest in more guns. According to Mr Reilly and his colleagues, armed robberies yield £10,300 more than unarmed ones (the penalties for being caught are much stiffer). Another popular tactic for robbers is to team up. As in other trades, many hands make lighter work for the lightfingered: each additional gang member boosts the expected take by £9,033. The trouble is, the loot has to be split. Since the average successful British bank robber gets £12,707, too many crooks spoil the broth.

Given the average loot and the chances of being caught, the authors reckon a fine of around £34,000 might deter British bank robbers. That is roughly how much income a minimum-wage worker would forgo during a three-year jail term, which is the average sentence for bank robbery in Britain. This assumes, however, that criminals are rational cost-benefit calculators. They aren't. In his autobiography, Sutton denied having ever used the phrase for which he is famous. “Why did I rob banks?” he asked. “Because I enjoyed it.”