Morality is serious yet it needs to be reconciled with the free play of alternatives that characterizes rational and ethical agency. Beginning with a sketch of the seriousness of morality modeled as a constraint, this paper introduces a technical conception of play as degrees of freedom. We consider two ways to apply game theory to ethics, rationalist and evolutionary game theory, contrasting the way they model moral constraint. Freedom in the rationalist account is problematic, subverting willful commitment. In the evolutionary (...) account, freedom is also problematic, leading to an infinity of possible social norms with dubious normative force. While these two approaches complement each other, we argue that the evolutionary approach is superior on both theoretical and practical grounds. (shrink)