It happened again. This time, gunmen in El Paso, Texas, and Dayton, Ohio, murdered 31 people and injured at least 50 more in separate mass shooting attacks within 13 hours of each other Saturday night and Sunday morning. It was, in many ways, just another weekend in America, the only nation in the developed world where horrific gun massacres regularly occur. Though nothing new, the frequency of such public mass shootings appears to have accelerated over the past five years, along with larger and more tragic death tolls. According to one recent analysis by The Washington Post, a mass shooting event has claimed the lives of four or more people every 47 days since June 2015. In the mid-’90s, such attacks happened just twice a year, on average.

But this surge in public executions has not swept across all corners of the country equally. Hawaii, for instance, hasn’t seen a mass shooting since 1999. Florida, on the other hand, has had six such incidents, defined by the US government as four or more people killed by a single individual, in the past three years alone, according to data from the nonprofit Gun Violence Archive. And like other forms of gun violence—including homicide, suicide, and unintended accidents—researchers are finding that mass shooting events happen more often in states with looser gun laws.

Because while Congress may not have passed any national gun laws in the aftermath of past mass shootings, individual state legislatures have. And as the disparity between states with weak gun laws and those with tough ones has widened, so too has the gap in mass shootings. Which means that terrorist acts like those committed in El Paso and Dayton over the weekend are more likely to keep happening to people who live in places where it’s easy to buy, sell, and carry guns. The country is splitting into the gun law haves and the gun law have-nots, and deadly statistics are now revealing the impact those policy decisions have on people’s lives.

Studying mass shootings, which make up only a tiny fraction of all gun deaths, has long been tricky, because of their historical rarity and a general dearth of data on guns or gun deaths. (That’s because of research-stifling federal legislation that was only recently overturned.) But one ironic effect of there being more mass shootings lately is scientists now have enough data to start to see trends emerging.

In a paper published earlier this year in BMJ (previously the British Medical Journal), epidemiologists at Columbia University looked back at the Federal Bureau of Investigation’s crime database from 1998 to 2015 to calculate annual rates of mass shootings in each state. Then they matched that up against each year’s edition of the Traveler’s Guide to the Firearms Laws of the Fifty States—an annual report that tracks any changes to gun laws in all 50 states and rates each one on their permissiveness. Published by a Kentucky attorney and arms dealer for a gun-toting audience, the guide is frequently promoted by the National Rifle Association. States are scored zero (for completely restrictive) to 100 (for completely permissive) based on 13 factors, including the right to carry guns in the open, limitations on the types of guns state residents can own, and whether out-of-state gun permits are recognized.

What the researchers found was that over time states have dug themselves into a bimodal distribution. That is, they’ve self-clumped into two distinct groups—a smaller one made up of eight states scoring between 5 and 25, and another, much larger, one clustered around scores from 70 to 100. “One of the most interesting things about this data is that we aren’t seeing a full spectrum, because there just aren’t that many states directly in the middle,” says Paul Reeping, the study’s lead author.