William D. Hamilton postulated the existence of 'genes underlying altruism', under the rubric of inclusive fitness theory, a half-century ago. Such genes are now poised for discovery. In this article we develop a set of intuitive criteria for the recognition and analysis of genes for altruism, and describe the first candidates for altruism genes from social insects and humans. We also provide evidence from a human population for genetically-based tradeoffs, underlain by oxytocin-system polymorphisms, between alleles for altruism and alleles for non-social cognition. Such tradeoffs between self-oriented and altruistic behaviour may influence the evolution of phenotypic diversity across all social animals.