Since a 26-year-old shot and killed at least 9 people and injured several others at Umpqua Community College in Oregon on Thursday, the debate about gun control is back in the news.* In his speech after the shooting, President Obama reprimanded lawmakers for not tightening gun control laws. Do strong gun control laws prevent gun violence? The chart below, based on data from the Law Center to Prevent Gun Violence, offers some evidence.

Before you sound off in the comments section: Correlation does not mean causation, of course, but that’s not where the debate ends. We can’t start a society in a vacuum and tweak the gun control laws to see what happens; we can only rely on messy real-world data. Other studies have shown that states with more guns have more suicide and homicide; that suicides (which account for about three-fifths of gun deaths) decrease when gun control is tightened; and that countries with more guns have more homicide. Among advanced countries, the U.S. has the highest rate of gun ownership and the highest homicide rate. What explains all these patterns? So far, gun control critics have only provided weak theories about culture or mental illness. Maybe it’s the guns.