What you don't know can hurt you: the peril of vague criminal statutes.

• Author: Silverglate, Harvey A.

THE SOVIET UNION enacted an infamous law in 1922 that criminalized "hooliganism." The crime was in the eye of the beholder, the beholder of consequence being the Soviet secret police. Because it was impossible for dissidents to know in advance whether they were violating this prohibition, they were always subject to arrest and imprisonment, all ostensibly according to law.

In the United States, we have legal safeguards against Soviet-style social controls, not least of which is the judicial branch's ability to nullify laws so vague that they violate the right to due process. Yet far too many federal laws leave citizens unsure about the line between legal and illegal conduct, punishing incorrect guesses with imprisonment. The average working American adult, going about his or her normal life, commits several arguable federal felonies a day without even realizing it. Entire lives can change based on the attention of a creative federal prosecutor interpreting vague criminal laws.

Mail Fraud for Art Supplies

Consider the federal prohibition of "mail fraud," which mainly describes the means of a crime ("through the mails") rather than the substantive acts that violate the law ("a scheme or artifice to defraud"). In 2004, Steven Kurtz, an art professor at the State University of New York in Buffalo, was indicted on mail fraud charges for what boiled down to a paperwork error. Federal agents, after learning that Kurtz was using bacteria in his artwork to critique genetic engineering, launched a full-scale bioterrorism investigation against him. Finding nothing pernicious about the harmless stomach flora, they resorted to a creative interpretation of the marl fraud statute. Because Kurtz had ordered the bacteria through a colleague at the University of Pittsburgh Human Genetics Laboratory, his "scheme" to "defraud" consisted of not properly indicating on the order form that the bacteria were meant for his own use.

Or consider the Computer Fraud and Abuse Act, a 1986 law whose prohibitions--accessing a computer "without authorization," for example--have been stretched to cover a wide swath of activity never envisioned when the bill was passed. In 2008, federal prosecutors in Los Angeles won a conviction in an online harassment case based on the theory that violating a website's "terms of service" is a crime under this law. Thankfully, the judge rejected this interpretation and threw out the jury's conviction.

The most dangerously far-reaching...