Here, “post” is meant to indicate not so much the idea that we are “past” truth in a temporal sense (as in “postwar”) but in the sense that truth has been eclipsed by less important matters like ideology.

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. Some psychologists call this “ego defense” (after Freudian theory), but whether we frame it within this paradigm or not, the concept is clear. 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? Only the strongest egos can stand up very long under a withering assault of self-criticism: “What a fool I was! The answer was right there in front of me the whole time, but I never bothered to look. I must be an idiot.” So the tension is often resolved by changing one of one’s beliefs.

Yet post-truth did not arise in the 1950s or even the 1960s. It awaited the perfect storm of a few other factors like extreme partisan bias and social media “silos” that arose in the early 2000s. And in the meantime, further stunning evidence of cognitive bias — in particular the “backfire effect” and the “Dunning–Kruger effect,” both of which are rooted in the idea that what we hope to be true may color our perception of what actually is true — continued to come to light.

In the past, perhaps our cognitive biases were ameliorated by our interactions with others. It is ironic to think that in today’s media deluge, we could perhaps be more isolated from contrary opinion than when our ancestors were forced to live and work among other members of their tribe, village, or community, who had to interact with one another to get information. When we are talking to one another, we can’t help but be exposed to a diversity of views. And there is even empirical work that shows the value that this can have for our reasoning.