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.

As...