One of the deepest roots of post-truth has been with us the longest, for it has been wired into our brains over the history of human evolution: cognitive bias. Psychologists for decades have been performing experiments that show that we are not quite as rational as we think. Some of this work bears directly on how we react in the face of unexpected or uncomfortable truths.
A central concept of human psychology is that we strive to avoid psychic discomfort. It is not a pleasant thing to think badly of oneself… It just feels better for us to think that we are smart, well-informed, capable people than that we are not. What happens when we are confronted with information that suggests that something we believe is untrue? It creates psychological tension. How could I be an intelligent person yet believe a falsehood?
If we are already motivated to want to believe certain things, it doesn’t take much to tip us over to believing them, especially if others we care about already do so. Our inherent cognitive biases make us ripe for manipulation and exploitation by those who have an agenda to push, especially if they can discredit all other sources of information.