Researchers have long-sought explanations for why America's crime rate dropped in the 1990s.  Academics have suggested a laundry list of theories: including larger police forces, increased prison populations and changes in the crack cocaine market.  New research out of the University of Pennsylvania suggests an answer could be in your pocket or purse.

"The crime decline really started in 1993 and that's coincident with when mobile phone technology started coming online but really penetration started sometime in the mid-90's," said criminologist John MacDonald, an Associate Professor at the University of Pennsylvania.

MacDonald and his co-researchers suggest cellphones are at least partially the reason for the crime decline.  MacDonald and his fellow researchers analyzed whether mobile phones could have contributed to the decline in crime — in particular, violent crimes such as assaults and rapes. 

"We looked at the increase in mobile phone use in states between 1999 and 2007 and what we found was that states where there were more subscribers coming online there were significantly larger drops in violent crime overall," he said.

The theory goes that both potential victims and bystanders could more easily call for help when they didn't have to find a payphone.  MacDonald said when you give people greater access to a device like mobile phones it raises the cost of committing crime.  "Which is very much both an economic argument for why you might have fewer of these, but also fits into criminological models for offender motivation," he said.  When the chance of being detected and caught goes up, people are more likely to avoid committing that kind of crime.

MacDonald said this is only a contributing factor, not enough to keep these crimes from rising again, even as mobile phones become even more ubiquitous.

Thomas Stratmann, a professor of economics and law at George Mason University, conducted the research with the two University of Pennsylvania professors.  "If someone has a cell phone, it potentially increases the cost - or the price of committing a crime for the criminal.  Because the criminal is more likely to be caught if the potential victim is going to pull out a cellphone and call the police or calls the police while he or she is being assaulted."  Stratmann said he believes current crime rates would be much higher if there were fewer phones around.  He adds that current phones' photo and GPS abilities give the devices even more potential to reduce violent crime.