“The denial of science—I’ll just state it straight up—is dangerous,” said Michael Myers, the managing director of the Rockefeller Foundation.

The “denial of science” of course is a nuanced phenomenon, and not one that is as simple as flatly denying science as a concept. People who don’t believe in climate change, or evolution, or who mistrust vaccines all come to those conclusions for different reasons. A lot of it has to do with tribalism—you believe what the people in your group believe, because membership in that group is more important to you than the truth.

“What I think is part of the challenge here is that people’s hardening belief systems are going against the science,” Myers said Saturday at the Aspen Ideas Festival, which is co-hosted by the Aspen Institute and The Atlantic. And that’s true—one of the biggest reasons that it’s so hard for facts to change people’s minds is that people have an incentive to keep believing what they already believe, as I’ve written before, especially if it’s a belief that’s deeply tied to their identity. The mental gymnastics they do to achieve this are known in psychology as “motivated reasoning.” It’s something that’s extremely hard to get around. If people change their minds on these things, it happens slowly, and it often has to be something they want to change their mind about.