 When outcomes depend on what several agents do, one agent’s best choice may depend on what other agents choose. Although the principles of rationality governing individual choice still apply, arguably there are further principles of rationality governing expectations of the actions of others (and of their expectations concerning your actions and expectations, and so forth). Game theory occupies an increasingly important role within economics, and it is also relevant both to inquiries concerning rationality and inquiries concerning ethics. For further discussion see the entries on  game theory,  game theory and ethics, and  evolutionary game theory.