This research develops a theory and presents empirical evidence of a link between economic outcomes and genetic evolution. Important properties for successful analysis of such a link are found in the adaptive immune system and particularly in the major histocompatibility complex (MHC), a genetically encoded complex involved in the defence against infections. The theory incorporates properties of the MHC in a model of mutual dependence and exhibits a trade-off in which every agent who is better off having an immune response different from that of others is also part of the protecting belt of others in a population, in which mounting similar immune responses is optimal. The data are based on large numbers of blood samples from 63 different populations. The cross-country estimates show a robust negative association between economic and health outcomes and MHC diversity and between average offers in ultimatum and trust games and MHC diversity. The analyses suggest that societies incorporating externalities from mutual dependence are economically more successful, and that the incorporation of such externalities is evident at the gene level.