In the 1950s, studies of this kind led Leon Festinger and his colleagues at Stanford University to develop the theory of cognitive dissonance. This holds that when a person is facing contradictory cognitions there is a driving force that compels their mind to acquire or invent new beliefs, or to modify existing beliefs, in order to reduce the dissonance between these thoughts. Cognitive dissonance is thus the uncomfortable feeling of being confronted by things that 'should not be, but are'. The need to avoid this causes people to create rationalisations to help dismiss the disconfirming evidence, a situation rendered even more problematic when people have committed to a belief publicly.
To reduce cognitive dissonance, people will either adopt other people's beliefs (peer pressure), adopt a view that makes the person who believes differently significantly distinct from themselves (discrimination), or apply pressure to those who believe differently (persecution). The same mechanism can account for "myside" bias (or confirmation bias), whereby people find it easier to accept information compatible with their prior beliefs, and harder to acknowledge contradictory observations. There are dozens of other phenomena that psychologists have observed which show the same general trend.
These cognitive biases may seem to reopen the case for egoism, but nothing in their working suggests all behaviour is self-interested, let alone selfish. Rather, they show that our minds work subconsciously towards maintaining a consistent set of beliefs. I'm inclined to suspect that it's just as well they do, otherwise any kind of rationality or reason would be nearly impossible. The surprising thing is how easily we adopt new ideas when they fit with what we already believe, and how vehemently we oppose them when they do not.
Part 17 of 23 in the Pentenary series.