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, and so on.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. 

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. 

In the 1980s, the federal government switched from paper money to electronic benefit transfers. 

They didn't switch all at once.They switched one county at a time within states.This created a kind of randomly controlled environment for the researchers, who studied Missouri's counties to establish whether the areas that switched from welfare cash to electronic transfers saw a concurrent decline in crime. 

The results were striking: The shift away from cash was associated with “a significant decrease in the overall crime rate and the specific offenses of burglary and assault in Missouri and a decline in arrests.” In other words, the counties saw a decline in specific crimes when they switched away from cash welfare. 

Perhaps most interestingly, they found that the switch to electronic transfers reduced robbery but not rape, suggesting that the move away from cash only had an impact on crime related to getting and spending cash. 

The move toward cashlessness in the U.S.continues quickly.Google now lets you attach money to emails to send to friends, which means that for some shoppers, pulling out your credit card could become as rare as finding exact change in your coin purse.It might seem absurd to imagine Visa, Square, and Google Wallet as crime-fighting technologies.But with a better understanding of how cash's availabili-ty affects crime, perhaps the government should consider killing more than just the penny.