This longitudinal study examined effects of family socialization on sociomoral reasoning in the context of the peer and parent-child relationships. Subjects were 121 urban Icelandic children. Social class was constructed as a multinominal measure defined by the nature of work, education, authority, and responsibility of the parents in the work system. Family socialization was assessed when subjects were age 7 to include sources of parental support and control techniques. Sociomoral development was assessed through reasoning about a friendship dilemma, including issues about close friendship and promise keeping assessed at ages 7, 9, 12, and 15; and reasoning about an authority dilemma, especially parent-child and sibling relationships, assessed at ages 12 and 15 years. Results of a loglinear multivariate analysis identified two family socialization factors: (1) a supportive factor indicating a discursive, culturally oriented, person-centered, and communicative family climate; and (2) a restrictiveness factor indicating power assertive control techniques. Regression analyses revealed that sociomoral reasoning in the context of peer relationships was significantly affected by both family socialization factors at age 7, 9, and 12 years. Supportive family socialization fostered sociomoral development in the peer domain for children and early adolescents. In middle adolescence, sociomoral reasoning in the context of peer relationships was largely independent of family socialization. For 12-year-olds, sociomoral sensitivity in the context of parent-child relationships was not affected by family socialization. However, in middle adolescence, supportive family socialization appeared to foster sociomoral development and restrictive socialization tended to hinder development. (Contains 11 references.) (Author/KB)