Re the "mens rea" legal defense

Excerpts from an article at the Wall Street Journal:

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 a result, what once might have been considered simply a mistake is now sometimes punishable by jail time...

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...

Over time, lawmakers have devised a sliding scale for different crimes. For instance, a "willful" violation is among the toughest to prove... A lower threshold, attorneys say, involves proving that someone "knowingly" violated the law. It can be easier to fall afoul of the law under these terms...

Earlier this year, Justice Antonin Scalia, in a dissent from a Supreme Court decision upholding a firearms-related conviction, wrote that Congress "puts forth an ever-increasing volume" of imprecise criminal laws and criticized lawmakers for passing too much "fuzzy, leave-the-details-to-be-sorted-out-by-the-courts" legislation...

The growing number of federal laws with weakened mens rea safeguards is making the venerable legal principle that ignorance of the law is no defense a much riskier proposition for people. That principle made sense, says University of Virginia law professor Anne Coughlin, when there were fewer criminal laws, like murder, and most people could be expected to know them.

But when legislators "criminalize everything under the sun," Ms. Coughlin says, it's unrealistic to expect citizens to be fully informed about the penal code." With reduced intent requirements "suddenly it opens a whole lot of people to being potential violators."

Specific examples and further discussion at the link.