The research used nine data sets from varying time periods. The analysis showed that the percentages of homicides in which victims and offenders are members of the same family varies widely from nation to nation, from one time period to another within a nation, and from one geographic area of a nation to another. Nearly all the cross-sectional and time-series empirical tests supported the hypothesis. Findings supported a 'primary group lag theory' which holds that primary groups, especially the family, have characteristics which both engender a certain minimal rate of violence and restrict serious violence. The crucial family characteristics are the conflict that grows out of the organizational structure of the family, constraints against resolving these conflicts by leaving, and implicit cultural norms that tolerate a certain level of intra-family violence, some of which becomes lethal. Discussion of implications for research and prevention, figures, tables, and footnotes.