The disciplinary history of International Relations (IR) has been marked by confrontation between those who believe that the study of politics can and should be modelled on the natural sciences, a position defended most forcibly in the United States, and those who have dissented, viewing this ambition as methodologically unjustified and ethically undesirable. But the scientific template against which to judge such claims is constantly shifting. In this article it is suggested that mainstream IR theorists are likely to turn increasingly to the biological sciences for inspiration and intellectual legitimacy. Some of the possibilities and problems involved in this move are explored, focusing in particular on the prominent role played by evolutionary psychology in the social sciences. A variety of reasons are offered, political and theoretical, as to why IR scholars should be extremely wary of looking to the biological sciences to provide universalistic accounts of human behavioural patterns.