What brought the authors together to write this book was a shared ambition (though we were not aware of this until we met in the summer of 1992) to contribute to the development of a coherent theory of human nature. For many years we had independently held the view that no theory in psychology or psychiatry could hope to possess any lasting value unless it was securely founded on knowledge of the evolution of our species. This shared view had emerged over a professional lifetime which followed a different course in each case, Price in psychological medicine, Stevens in psychotherapy and analytical psychology. We did, however, have one formative experience in common (in addition to a training in medicine and psychiatry): we both read ‘PPP’ (Psychology, Philosophy, and Physiology) at Oxford, Price between 1953 and 1955, Stevens between 1958 and 1959. Oxford – largely, we suspect, because of Niko Tinbergen’s presence in the Department of Zoology – was to the forefront in seeking to build a bridge between psychology and behavioural biology, and this gave us a theoretical orientation which has influenced our thinking ever since.