People have been saying for years that cash will disappear. So far, they have been spectacularly wrong. Over the past decade, the face value of U.S. dollar paper currency in public hands has doubled. Today, there is nearly $1.6 trillion in banknotes outstanding, more than 80 percent of which is in $100 bills (see chart)! In fact, there are thirty-nine $100 bills in circulation for each of the 326 million residents of the United States.

Why is 90 percent of the U.S. increase in circulation accounted for by $100 bills? One possible explanation is that, with nominal interest rates near zero, the opportunity cost of holding cash has dwindled, reducing the incentive to deposit rising inventories of cash in a bank. The second, and more compelling, reason for the big increase in large-denomination notes is more troubling: it facilitates illicit activity. Money laundering, tax evasion, drug dealing, human trafficking, and a whole host of other criminal activities run on cash. Big banknotes are a convenient way to transfer funds anonymously with finality. A $100 bill weighs less than a gram, so $1,000,000 weighs roughly 10kg and is small enough to fit in a medium-size briefcase.

To put it simply, most of the U.S. currency in circulation is almost surely being used by criminals....