John Hibbing, a leading researcher in political physiology, explains it like this: “What we’ve found is pretty clear and consistent—that conservatives tend to have more reaction to negative things. We like to see not just if they report in a survey-type format whether they are bothered by that, but actually physiologically if there has been a change.”

In his experiments, Hibbing often attaches electrodes to liberal and conservative participants’ skin and then shows them disturbing images, such as a man eating a handful of worms. In these tests, conservatives sweat more (i.e., have a stronger gut reaction) in response to the disgusting stimulus. And when Hibbing hooks participants up to eye-tracking machines, he finds conservatives monitor more closely the things that make them squirm. So they are more readily provoked and more vigilant. These differences between liberals and conservatives are likely deep seated in the brain: scientists have found that conservatives tend to have larger amygdala, a region of the brain involved in fear processing, than liberals do.

And when people become fearful, they’re more likely put distance between their group and others. “Since out-group members are more likely to carry pathogens to which members of the in-group have not yet developed immunity, avoidance of out-groups can be adaptive when the threat of the disease is salient,” UCLA researchers wrote in a 2006 paper.