Lewin, K., Lippitt, R., & White, R. K. (1939). Patterns of aggressive behavior in experimentally created "social climates." Journal of Social Psychology, 10, 271-299.
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Lewin and his colleagues examined the impact of different leadership styles on children's aggressive behavior. Lewin randomly assigned 10-year-old boys to different activity groups led by an adult and observed their behavior over the course of 5 months. The groups differed in the adults' leadership styles: autocratic (authoritarian), democratic (collective rule), or laissez-faire (no structure or guidance). The results indicate that the boys in the autocratic groups exhibited more spontaneous aggression and hostile behaviors than did boys in the other two groups. Lewin explains the group conditions necessary to elicit aggression: 1) tension, 2) narrow space of free movement at as a source of tension, 3) aggression as the effect of tension, 4) rigidity of group structure, and 5) style of living (culture).
This article is considered a classic in psychology because it demonstrates that group processes influence aggressive behavior. Prior to this study, research primarily focused on individual differences as the cause for aggression.