Norms only exist when there is a community of agents to enforce them. When there are only two agents, there’s no such thing as a norm, only cooperation or conflict. 1/

There can be Nash equilibria supporting cooperation with only two players, sure. But they’re fragile in practice. If defection yields a strong negative payout to the sole other player, its capacity to punish ex post and so deter ex ante will be weak. 2/

Norms can be thought of as Nash equilibria in very multiplayer games. They are strong if (and only if) defection can’t kneecap the whole community’s capability to punish. In practice that’s often the case. 3/

When there are broad communities, norms really can support cooperation! But much less so when the game collapses effectively to only two players. /fin