The new study, conducted by researchers in Texas, Missouri, Massachusetts and Pakistan, covers the years 2001 through 2019 and examines age-adjusted mortality rates—the number of deaths per 100,000 people each year—from the top 10 leading causes of death, as recorded in 2019. These include heart disease, cancer, lung disease, unintentional injuries and suicide. The researchers then analyzed county-level results in each of the five presidential elections that took place during their study period, identifying counties as Republican or Democratic for the subsequent four years. They found the gap in mortality rates between Republican and Democratic counties increased for nine out of 10 causes of death. (The gap for cerebrovascular disease, which includes stroke and aneurysms, remained but narrowed.) Political environment, the authors suggest in the paper, is a “core determinant of health.”

What is it about conservative areas that might lead to this disadvantage in health outcomes? Multiple factors probably contribute to the gap. Previous research has found differences between Republican and Democratic regions in health-related behaviors such as exercising or smoking. Those findings were nuanced. For example, Democrats had higher odds of smoking, and Republicans were less likely to exercise. But people living in Republican states, whatever their own political leanings, were more likely to smoke.

And an analysis of the new study’s data by subgroups supports the idea that individual choices play a role. Hispanic Americans everywhere saw significant improvements in their risk of death. Black Americans still have the highest mortality rates of any racial group, but they saw relatively similar improvement. “It didn’t really matter where they lived,” Warraich says. For white Americans, however, the difference was profound—a fourfold increase in the mortality gap between those living in Republican and Democratic areas.