A dramatic spike in the number of Americans with permits to carry concealed weapons coincides with an equally stark drop in violent crime, according to a new study, which Second Amendment advocates say makes the case that more guns can mean safer streets.

The study by the Crime Prevention Research Center found that 11.1 million Americans now have permits to carry concealed weapons, up from 4.5 million in 2007. The 146 percent increase has come even as both murder and violent crime rates have dropped by 22 percent.

“When you allow people to carry concealed handguns, you see changes in the behavior of criminals,” said the center’s president, John R. Lott, a Fox News contributor. “Some criminals stop committing crimes, others move on to crimes in which they don’t come into contact with victims and others actually move to areas where they have less fear of being confronted by armed victims.”

Increasing gun ownership, litigation and new state laws have all contributed to the rise in concealed carry permits. In March, Illinois became the 50th state to begin issuing concealed weapons permits. But the cost and other requirements for obtaining the permits varies greatly, from South Dakota, where a permit requires $10, a background check and no training, to Illinois, where the cost of obtaining a permit comes to more than $600 when the fee and cost of training programs are taken into account.

Six states don’t require a permit for legal gun owners to conceal their weapons, and Lott notes those states have some of the lowest violent crime rates in the nation.

The real measure of the deterrent effect of concealed carry permits, according to Lott, is not laws on the books, but the percentage of a given state’s population that holds the permits. In 10 states, more than 8 percent of adults hold concealed carry permits, and all are among the states with the lowest crime rates. Lott claims his group’s analysis shows that each one percentage point increase in the adult population holding permits brings a 1.4 percent drop in the murder rate.

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