Abstract The investigation of interpersonal conflict and violence requires a sound theory of the nature of self‐interests. The requisite theory is Darwin's: Motives and emotions evolved to promote fitness in ancestral environments, and we thus experience our interests as conflicting in situations where one party's expected fitness can be enhanced at the expense of another's. Fundamental overlaps and conflicts of interest are relationship‐specific. Genetic relatedness is an obvious and primary determinant of the degree to which fitness interests overlap, and is thus expected to mitigate conflict and facilitate forgiveness. Sexual relationship is more fraught, since the powerful overlap of interests that derives from reproducing together can be undermined by cuckoldry, defection, investment in distinct kindreds, and shirking. Treating homicide as an assay of intense interpersonal conflict, I review how these and other such insights have generated novel predictions about homicide rates and patterns. Evolutionary thinking has motivated numerous discoveries about the demography and epidemiology of violence that had escaped those whose imaginations were not informed by Darwinism.