Recent research suggests that psychological states have an impact on the incidence and outcome of physical illness. A fundamental question arises out of this research. Is the psychological state-morbidity relationship an accident of nature or it is biologically adaptive? I propose that the archeological and psychological data are consistent with the hypothesis that illness was functional during the Paleolithic, that although illness ceased being functional from Neolithic times onwards, the modern genotype reflects that earlier function. This hypothesis is supported by (a) considering two ways in which death is functional, (b) examining the different selective forces operating during Paleolithic and Neolithic/postneolithic times, (c) showing that the psychological data is consistent with morbidity evolving a function during Paleolithic but not Neolithic/postneolithic times, and (d) examining a possible mechanism for the psychological state-morbidity relationship.