Mental retardation in a birth cohort, 1976-1980, Rochester, Minnesota.

The cumulative incidence of mental retardation in a birth cohort of children born from 1976 through 1980 in Rochester, Minnesota (n = 5,919) was estimated. Rochester is the site of the Rochester Epidemiology Project, which captures virtually all medical care delivered locally. Passive follow-up through school and community medical records for criteria diagnostic of mild or severe mental retardation was undertaken. Thirty children were classified with mental retardation. The cumulative incidence of mental retardation by age 8 years was 9.1 per 1,000 (95% confidence interval = 6.2, 13.0) and was similar for boys and girls (8.3 vs. 10.0 per 1,000, respectively). The cumulative incidence for severe mental retardation in girls was more than twice that in boys; for mild retardation it was the opposite. The cumulative incidence among boys was 1.7 times greater than the cumulative incidence for girls. Results suggest that although the overall incidence of mental retardation by gender was similar, the severity may differ somewhat.