We propose a modal logic that enables to reason about
different kinds of rationality in strategic games. This logic integrates the
concepts of joint action, belief, individual preference and group
preference. The first part of the talk is focused on the notion of
individualistic rationality assumed in classical game theory: an agent
decides to perform a certain action only if the agent believes that this
action is a best response to what he expects the others will do. The
second part of the talk explores different kinds of social rationality
such as fairness and reciprocity. Differently from individualistically
rational agents (alias selfinterested agents), social rational agents
also consider the benefits of their choice for the group. Moreover, their
decisions can be affected by their beliefs about other agents' willingness
to act for the well-being of the group.