A new study has found that states with mandatory background checks on gun purchases are about half as likely to have a school shooting than those with no background checks. Further, researchers said that states in which people must submit to a check before buying ammunition were even less likely to experience a school shooting. The study, published in Injury Prevention, examined 154 such incidents in the U.S. between Jan. 1, 2013, and Dec. 31, 2015. A school shooting was defined as “an incident when a firearm was discharged inside a school building or on a school campus or grounds.” More than half of those incidents happened on a K-12 campus. Among the findings: Sixty-six percent were intentional, and 99 percent were perpetrated by boys; 36 percent of the assailants were students. Those school shootings were spread among 39 states. Between 2013 and 2015, Georgia had 15 school shootings, while Texas and Florida each had 14. North Carolina had 12 school shootings, and Tennessee had 10. But there were 11 states with no school shootings in that time period, including Alaska, New Hampshire, New Jersey, Connecticut, Hawaii, New York, North Dakota, Rhode Island, Vermont, West Virginia, and Wyoming.