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.

In an effort to sort things out, Ohio’s state legislature now requires greater precision when its members draw up new statutes. At the end of last year John Kasich, the governor, signed a first-of-its-kind law that requires new crimes to specify a threshold level of intent, or be declared void. So, for example, if legislators want an action to be criminal regardless of intent, they must state that in the law. Where the existing code is unclear, the threshold for guilt becomes “reckless” behaviour. The bill passed with unanimous support.

Most states, though, are ignoring the problem, even as a flood of new laws means more will be unwittingly broken. Take Michigan, where a bill similar to Ohio’s failed last year. Lawmakers there create, on average, 45 new crimes each year, according to the Manhattan Institute, a think-tank. The state now has over 3,000 crimes, many of which lack intent requirements. The penal code ensnares people like Lisa Snyder, who was accused of running illegal day care after watching over her neighbours’ children as they waited for the school bus. (A legislative fix helped her avoid prosecution.)

Guilty mind? Never mind

At the federal level, Justice Antonin Scalia on the Supreme Court has lamented the increasing volume of “fuzzy, leave-the-details-to-be-sorted-out-by-the-courts legislation”. Federal statutes contain some 4,500 crimes, and there are thousands more in the federal regulatory code. In 2009 the Heritage Foundation, a think-tank, and the National Association of Criminal Defence Lawyers (NACDL) found that two-thirds of the non-violent criminal offences enacted by the 109th Congress (which sat from 2005-06) lacked an adequate intent requirement. A rule change that lets the House Judiciary Committee review all new federal criminal laws is intended to address these problems.

Business has often been the victim of Congress’s shoddy lawmaking. According to the NACDL, most crimes created by the Wall Street Reform and Consumer Protection Act, better known as Dodd-Frank, fail to account for intent. But ordinary citizens are also finding it hard to obey poorly-written federal laws. In his book “Three Felonies a Day” Harvey Silverglate, a lawyer, estimates that the average professional unknowingly commits several crimes daily. The erosion of mens rea requirements makes that a little less amusing.