[B]irtherism is a shibboleth, that is, an affirmation that marks the speaker as a member of their community or tribe. (The original shibboleth was a password chosen by the Gileadites because their Ephraimite enemies could not say “Sh”.) Asserting a belief that would be too absurd to countenance for anyone outside a given tribal/ideological group makes for a good political shibboleth.
It’s clear, as Dave Weigel points out, that beliefs of this kind are a marker for partisanship, as witness the high correlation between stated birtherist beliefs and approval of Palin. But that doesn’t necessarily mean that the statement isn’t actually believed. Rather this is an open question and an important one for agnotological understanding of the emergence of comprehensive culturally induced ignorance as a marker for the Republican tribe.