Two mechanisms of the hypothesized social causation of psychopathology--differential incidence and cumulative prevalence--were tested over 9 years in a nationally representative sample of 1,075 children and youths, ages 8-17 at Time 1 (1986). Analyses using parental responses on behavior checklists at 4 time points showed significant increases in clinical elevations for those of the lowest socioeconomic status (SES) on anxious/depressed, somatic complaints, thought problems, delinquent, and aggressive syndromes. This SES-linked differential incidence supports the social causation hypothesis that factors associated with SES contribute to variations in levels of psychological problems. SES-linked differential cumulative prevalence was found for withdrawn and somatic complaints; this finding indicates that low-SES cases do not improve as much as do middle- and high-SES cases, which results in greater accumulation of low-SES cases.