The theory: more guns, less crime

The case for: Here’s the logic: If a would-be robber thinks there’s a good chance the person he wants to rob has a gun and will shoot him, he might think twice about the robbery. And many states passed right-to-carry laws in the mid- to late 1990s, just as crime started to fall.

The “more guns, less crime” hypothesis is associated with economist John Lott, who wrote a 1997 study that showed dramatic results. The paper concluded that enacting a right-to-carry law in every state would prevent nearly 1,600 murders a year.

The case against: Lott’s paper is very influential, but it’s also become increasingly suspect. Two later papers looking at Lott’s data but adding more, or slightly tweaking the model he used in his analysis, have found it’s really hard to say that guns have had as much of an effect on crime as Lott suggests. And Lott’s done a terrible job of defending his integrity — at one point, when scholars started asking about a survey Lott had supposedly conducted but lost the data for, Lott created a sock puppet persona named Mary Rosh to attack his critics online.

Meanwhile, other research has suggested that, if anything, having more permissive gun laws — or increasing gun ownership, more generally — might increase crime levels.

The bottom line: No effect. Most researchers studying the link between gun laws and crime have found no reason to think that guns deserve credit for the crime decline.