Last November’s presidential election ended friendships because of opposing political views, and now an MRI study shows why people’s political beliefs just won’t budge.

In the study published in Nature, Jonas Kaplan, a professor at the University of Southern California, studied how the brain reacts when participants firmly held political beliefs are challenged.

“We used neuroimaging to investigate the neural systems involved in maintaining belief in the face of counterevidence, presenting 40 liberals with arguments that contradicted their strongly held political and non-political views,” wrote Kaplan.

Kaplan et al. found that the region of the brain that lit up controls deep, emotional thoughts about personal identity.

“Challenges to political beliefs produced increased activity in the default mode network—a set of interconnected structures associated with self-representation and disengagement from the external world,” they wrote.

Participants’ brains basically went into defense mode and shut down without allowing counter arguments.

“These results highlight the role of emotion in belief-change resistance and offer insight into the neural systems involved in belief maintenance, motivated reasoning, and related phenomena,” wrote Kaplan and colleagues.

The team only assessed liberals in this study but are planning to scan the brains of conservatives to see if their neurological responses follow the same pattern.