The idea that people follow trust norms when making trust decisions is developed in an evolutionary model of adaptive play by boundedly rational agents. Because it neither implies nor is it implied by cooperation, trust is not modelled as cooperation in a Prisoners’ Dilemma but as a coordination game with Stag hunt payoffs. The game’s two pure strategy Nash equilibria correspond to high and low trust norms, the former being Pareto optimal and the latter risk-dominant. The mixed strategy Nash equilibrium corresponds to medium trust, but is not a norm in the sense that it is not stable. Multiple equilibria explain the local conformity in trust decisions within countries, and the global diversity across countries. The model suggests that most countries are low trust because the low trust norm is stochastically stable; and that a collapse of trust in medium trust countries is prevented by exogenous forces, probably formal institutions.