After almost four decades of decline, homicide among romantic partners is now on the rise, according to recent research by Northeastern University criminologist James Alan Fox. The worrisome uptick has been primarily driven by gun violence, he told HuffPost in an interview.

Fox’s paper, which was authored with Ph.D. student Emma Fridel, analyzed gender differences in homicide patterns over a 42-year period, using data obtained from the FBI.

He discovered that intimate partner homicides ― when a person murders their spouse or romantic partner ― increased each year between 2014 and 2017.

In 2014, 1,875 people were killed by an intimate partner, the majority of victims female. In 2015, the death toll rose to 2,096. In 2016, it inched up to 2,149. And in 2017, there were 2,237 such deaths.

Domestic violence groups often repeat the statistic that three women a day are killed by domestic violence. But according to Fox’s most recent data, it is four.

He also found that since 2010, gun-related murders of intimate partners have increased by 26 percent ― a significant rise ― while those involving other weapons like knives have continued to decline. Most of the increase in gun deaths has occurred since 2014, he said.

Fox said he did not want to speculate about the cause of the spike.

“Regardless of the whys, efforts to disarm abusers and stalkers are critical,” he said. “All too often, guns that are purchased and kept in the home, ostensibly for the purpose of self-defense, end up being used against a family member.”