Since the 1960s many epidemiologists and psychiatrists have commented on the relative decline in incidence of anxiety disorders and the concomitant rise in diagnoses of depression. Most commentators have explained this change through reference to the determinative power of the pharmaceutical market. This paper argues for a broader explanation of this transformation. It demonstrates how the meanings of anxiety and depression were transformed through the rise of animal experimentation and evolutionary psychology. Examining the work of a group of psychiatrists and clinical psychologists centred around the Maudsley Hospital in London, I try to show how the discovery of an evolutionary dimension to mental illness has made possible new kinds of psychiatric intervention and new forms of political action.