Maybe that's the point. Maybe the way we evolved to demonstrate group loyalty and therefore, trustworthiness, was to forfeit our rationality as a rite of initiation. We agree to be irrationally loyal.

If so, can't we dispense with that by now?

I like Drescher's derivation of a logical foundation for the Golden Rule even when it benefits no one to abide by it. It's essentially the same logic that leads one to forgo the $1,000 in Newcomb's problem.

If I know you're committed to rationality, then I can trust you to abide by the golden rule.