Our ancestors evolved in small groups, where cooperation and persuasion had at least as much to do with reproductive success as holding accurate factual beliefs about the world. Assimilation into one’s tribe required assimilation into the group’s ideological belief system. An instinctive bias in favor of one’s “in-group” and its worldview is deeply ingrained in human psychology.
A human being’s very sense of self is intimately tied up with his or her identity group’s status and beliefs. Unsurprisingly, then, people respond automatically and defensively to information that threatens their ideological worldview. We respond with rationalization and selective assessment of evidence – that is, we engage in “confirmation bias,” giving credit to expert testimony we like and find reasons to reject the rest.