The relationship between state gun laws and the flow of firearms between states can be measured using data from the Bureau of Alcohol, Tobacco, Firearms and Explosives, which traces guns’ origins and where law enforcement recovers them. An analysis of data from 107 pairs of bordering states throughout the country shows a relationship between the strictness of a state’s gun laws relative to its neighbor and the number of firearms recovered from that neighbor.

A 2014 report from the city of Chicago noted that 60 percent of guns used to commit crimes in Chicago from 2009 to 2013 originated outside of Illinois, and Indiana and Wisconsin were two of the biggest sources of recovered guns. And Illinois is not alone.

By analyzing these pairs, we found that the relationship between gun laws and firearm recoveries generally appears to be more pronounced in states with larger differences in the strictness of their gun laws. Illinois and Indiana are 28.5 points apart in our combined gun rankings, and in 2016, for every 100,000 residents of the two states combined, Illinois recovered nearly 6.5 more firearms originating in Indiana than Indiana recovered guns originating in Illinois.

Read the full article on guns crossing state lines by Jeff Asher and Mai Nguyen at FiveThirtyEight