BOBBY UNSER, a former car racer, was snowmobiling with a friend in Colorado when a blizzard sent them off course. After their vehicles broke down the two men spent a night in the wilderness, nearly freezing to death. They survived, but two weeks later the Forest Service charged Mr Unser with operating a motorised vehicle inside a protected area, a federal crime. “I got lost,” said Mr Unser. “People shouldn’t be prosecuted for something they have no control over.” A judge disagreed, finding him guilty.
Under the common-law system that America inherited from England, a person performing a prohibited act (actus reus) must also possess a guilty mind (mens rea) in order to be convicted of a crime. In other words, intent matters. In keeping with this tradition, many states differentiate between criminal behaviour that is purposeful, negligent or something in between. But things have got fuzzy lately. New criminal laws often lack intent requirements—sometimes on purpose, often by mistake—and thus ensnare people, like Mr Unser, who inadvertently fall foul of them. Inconsistent courts have added to the muddle.