For centuries, a bedrock principle of criminal law has held that people must know they are doing something wrong before they can be found guilty. The concept is known as mens rea, Latin for a "guilty mind."
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As a result, what once might have been considered simply a mistake is now sometimes punishable by jail time. When the police came to Wade Martin's home in Sitka, Alaska, in 2003, he says he had no idea why. Under an exemption to the Marine Mammal Protection Act, coastal Native Alaskans such as Mr. Martin are allowed to trap and hunt species that others can't. That included the 10 sea otters he had recently sold for $50 apiece.
The law requires that animals sold to non-Native Alaskans be converted into handicrafts. He knew the law, Mr. Martin said, and he had thought the buyer was Native Alaskan.
He pleaded guilty in 2008. The government didn't have to prove he knew his conduct was illegal, his lawyer told him. They merely had to show he had made the sale.
"I was thinking, damn, my life's over," Mr. Martin says.
Federal magistrate Judge John Roberts gave him two years' probation and a $1,000 fine. He told the trapper: "You're responsible for the actions that you take."
Mr. Martin now asks customers to prove their heritage and residency. "You get real smart after they come to your house and arrest you and make you feel like Charles Manson," he says.
The U.S. Attorney's office in Alaska didn't respond to requests for comment.
Back in 1790, the first federal criminal law passed by Congress listed fewer than 20 federal crimes. Today there are an estimated 4,500 crimes in federal statutes, plus thousands more embedded in federal regulations, many of which have been added to the penal code since the 1970s.
One controversial new law can hold animal-rights activists criminally responsible for protests that cause the target of their attention to be fearful, regardless of the protesters' intentions. Congress passed the law in 2006 with only about a half-dozen of the 535 members voting on it.
Under English common law principles, most U.S. criminal statutes traditionally required prosecutors not only to prove that defendants committed a bad act, but also that they also had bad intentions. In a theft, don't merely show that the accused took someone's property, but also show that he or she knew it belonged to someone else.
Over time, lawmakers have devised a sliding scale for different crimes. For instance, a "willful" violation is among the toughest to prove.
Requiring the government to prove a willful violation is "a big protection for all of us," says Andrew Weissmann, a New York attorney who for a time ran the Justice Department's criminal investigation of Enron Corp. Generally speaking in criminal law, he says, willful means "you have the specific intent to violate the law."