This paper examines how selection affects trust and altruism in a Trust and Modified Dictator Game. Past Trust and Dictator game experiments not allowing partner selection show substantially more trust and altruism than equilibrium predicts. We predict partner selection will cause sorting in which behavior across partner types without selection will be positively correlated with partner choice. This selection pattern will cause trust and altruism to be higher with selection and the increase will be proportional to a maximum possible gain. We find selection has all these effects. We also find greater gains in the Trust than Modified Dictator game consistent with larger possible gains in the Trust game. The results imply that theories ignoring selection will underestimate trust and altruism in markets with selection.