Why has crime in the U.S. declined so dramatically since the 1990s?

Economists and sociologists have offered a bounty of reasons, including more police, more security technology, more economic growth, more immigration, more imprisonment, less lead in paint and gas, and even, most controversially, more abortions.

The "real" answer is almost certainly a combination of these factors, rather than one of them to the exclusion of the rest. But a new paper adds a surprising variable to the mix. What if the decline of crime in America started with the decline of cash?

Cash is critical to the health of an underground economy, because it’s anonymous, nearly untraceable, and easily stolen. This makes it the lifeblood of the black market, and the types of crimes that have declined the most since the 1990s, like robberies, are focused on getting cash illegally to spend on other illegal activities, like drugs, guns, and prostitution. (Indeed, a recent Urban Institute study on these underground economies measured their size relative to the cash economy of their respective cities.)

But Americans are rapidly abandoning cash thanks to credit cards, debit cards, and mobile payments. Half a century ago, cash was used in 80 percent of U.S. payments. Now that figure is about 50 percent, according to researchers.