For centuries western cultural traditions in religion, language, and law conveyed an image of suicide and homicide as morally, ethically, and legally equivalent. Self-murder was distinguished from other felonious homicide only by the fact that the victim was also the perpetrator. It was within this cultural context that the earliest empirical analyses of the relationship between suicide and homicide began to be produced by members of an intellectual movement known as the cartographic school of criminology, or, more informally, as the moral statisticians. Ferri and Morselli came late in the development of this school. By 1880 the moral statisticians had mapped the distributions of homicide and suicide rates for several European countries and had begun to develop analyses of fairly long time series. Over time, they argued, homicide rates had fallen and suicide rates increased with "the progress of civilization." These empirical studies and others like them formed the basis for a theory of suicide and homicide that emerged in the 1870's and 1880's from the research of Enrico Morselli and Enrico Ferri. Morselli emphasized the inverse relationship between suicide and homicide in ecological data. Ferri noted that the inverse pattern noted by Morselli also held in both short-term and long-term time series. In rejecting Ferri's and Morselli's theses in favor of the view that suicide and homicide are opposing social currents, Durkheim followed a strategy similar to Tarde's (1886), focusing primarily on exceptions to the law of inversion drawn from an array of European sources. Durkheim, however, was unable to destroy the notion that suicide and murder are different manifestations of the same basic phenomenon. Sigmund Freud shared with Morselli and Ferri the conceptualization of suicide and homicide as two channels in a single stream of violence. Indeed, the equivalence of suicide and homicide was a central tenet of Freud's theorizing on aggression, which had considerable influence on clinical perspectives of both suicide and homicide. In 1939 an interdisciplinary team of scholars at Yale University -- John Dollard and his colleagues -- undertook the task of systematizing Freud's ideas on aggression and integrating them with materials from empirical psychology, leading to the formulation of the frustration-aggression hypothesis, which implicitly or explicitly guides most recent thinking on the relationship between suicide and homicide. The sociological study of suicide in the interval between Durkheim and the mid-twentieth century was largely dominated by Chicago-school ecological studies of suicide in individual cities. This school linked suicide to social disorganization, defined in terms of such indicators as population density, social mobility, anonymity, impersonality, and instability, all of which reduce the effectiveness of social control. The school generally found a positive relationship between suicide and homicide rates in the metropolitan spatial structure.