Both the high propensity of young people to commit homicide and the social and financial problems experienced by large cohorts during their youth suggest that increases in the proportion of the population ages 15-29 years contributes to higher national homicide rates. Time-series studies in the United States confirms this relationship, but the expected macro-level relationship does not emerge in cross-national studies of other high-income countries. These researchers argue that the diverse results stem from the varied development of national institutions for collective social protection across high-income countries and that these institutions mitigate the impact on homicide of changes in the percentage of young people. Results of the multi-country analysis supported the hypothesis that national context, as measured by a scale of collectivism, reduced the otherwise positive effects of the percentage of youth on homicide. Findings suggested the need for theories to integrate demographic, political, and criminological perspectives in understanding social diversity among otherwise economically similar high-income democratic countries. Tables, notes, and 42 references (Author abstract modified)