Democrats and Republicans may sit together for Obama's speech, but partisanship won't budge.

Partisanship is also like racism in a third way: Studies have shown that racism is so socially proscribed that people exhibit it nowadays only when they can plausibly deny—to themselves and to others—that they are biased. One meta-analysis of studies, for example, found that "discrimination against blacks was more likely to occur when potential helpers had more opportunities to rationalize decisions not to help" by invoking "justifiable explanations having nothing to do with race."

Munro, Lasane, and Leary found the same pattern of behavior in partisanship. The partisan college evaluators were willing to acknowledge that applicants they chose who shared their political loyalties had lower test scores—an objective fact—but they selected the candidates anyway by inflating the importance of the recommendation letters that came with applications. Accepting candidates merely on the basis of low test scores would have shown the evaluators were biased. Accepting candidates on the basis of recommendation letters—and arguing the letters were more important than scores—allowed the evaluators to plausibly deny that they were biased.

If partisanship and racism are both tied to social identity, then a post-partisan America is about as likely as a post-racial America. Our views on issues may change, but our identities remain stable over decades. Democrats and Republicans sitting together in Congress will no sooner put an end to partisanship than gay men, black women, and Alabama hunters will give up their tribes.