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.”

This legal protection is now being eroded as the U.S. federal criminal code dramatically swells. In recent decades, Congress has repeatedly crafted laws that weaken or disregard the notion of criminal intent. Today not only are there thousands more criminal laws than before, but it is easier to fall afoul of them…

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…

Overall, more than 40% of nonviolent offenses created or amended during two recent Congresses—the 109th and the 111th, the latter of which ran through last year—had “weak” mens rea requirements at best, according to a study conducted by the conservative Heritage Foundation and the National Association of Criminal Defense Lawyers. The study, one of the few to examine mens rea, was extended to include the most recent Congress at the request of The Wall Street Journal.