Damien Broderick, <email@example.com>, replied:
> Well, that there are significant numbers of other people who are stubbornly
> irrational *in a certain domain*.
> Presumably this is the means whereby people in a (fairly) peaceful
> pluralist society treat the religious and (mainstream variant) political
> views of their neighbors. The polite rule against discussing politics and
> religion at the dinner table (or whatever it is) acknowledges this working
> assumption. It has the convenient side effect that people with novel
> approaches to the world actually can get on and try them out, up to a
> point, which sometimes leads to fresh science, art, technology...
You also see embarrassing situations where someone who is respected within a group also holds a belief in another domain which differs from that held by most group members. I recall reading an obituary in The Mathematical Intelligencer which went to some lengths to mention that although the deceased mathematician hadn't been a Socialist, he was well liked and considered a good fellow even given his peculiar lack of support for Socialism.
> Presumably adherence to sporting claques (as non-playing supporters) is a
> contrived version of this domain-construction, or maybe an adaptation of
> earlier tribalisms which saw competitors not so much as irrational but as
> worthy and feared Others, to be belittled by scornful shouts but envied for
> their prowess and possessions.
As you say, it's not clear that this is quite the same thing as the belief that other people are irration, but it does seem that we are comfortable in "us vs them" scenarios. We go so far as to set them up intentionally, for entertainment. Perhaps some aspect of this is operating in factual and technical disputes, a love of taking sides for its own sake.