Psychology is traditionally used in political science to explain deviations from rationality. Lost in the debate between rationalists and their critics, however, is a sense of whether the kinds of strategic self-interested behavior predicted by these models has psychological microfoundations: what would homo economicus look like in the real world? We argue that strategic rationality varies across individuals and is characterized by a pro-self social-value orientation and a high level of epistemic motivation. Testing our argument in the context of international relations, we employ a laboratory bargaining game and integrate it with archival research on German foreign policy-making in the 1920s. We find in both contexts that even among those interested in maximizing only their own egoistic gains, those with greater epistemic motivation are better able to adapt to the strategic situation, particularly the distribution of power. Our results build a bridge between two approaches often considered to be antithetical to one another.