This longitudinal study examined the multiple influences of social class and family socialization on intelligence at age 7, and on the development of cognitive competence from age 7 through age 15 in a sample of 121 urban Icelandic children and adolescents. Socialization condition was defined as sources of parental support and control strategies, with a supportive factor indicating highly discursive, culturally oriented, person-centered, and communicative family climate; while a restrictive factor indicated authoritative control. Social class was based on nature of work, education, and income. Intelligence was assessed with the Raven-IQ at age 7. Cognitive competence was measured at 7, 9, 12, and 15 years by aggregated scores on a battery of Piagetian tasks. Results showed a significant correlation between intelligence and social class that was strongly mediated by family socialization conditions. There was no significant effect of social class for cognitive competence; however, there was a general trend of increasing competence with increasing class position, mediated by family socialization conditions. In lower social classes, restrictive socialization conditions suppressed cognitive competence while supportive socialization conditions in upper classes fostered cognitive competence. With increasing age, the variance in cognitive competence increased. Until 12 years, the influence of family socialization showed additive or partly mediated influences of intelligence at age 7. Intelligence at age 7 had the strongest impact on cognitive competence; at later ages the impact was mediated by the developmental process. Both intelligence and cognitive competence were directly and indirectly affected by class specific socialization conditions. (Contains 11 references.) (KB)