MENTAL-RETARDATION IN A BIRTH COHORT, 1976-1980, ROCHESTER, MINNESOTA

The cumulative incidence of mental retardation in a birth cohort of ch ildren born from 1976 through 1980 in Rochester, Minnesota (n = 5,919) was estimated. Rochester is the site of the Rochester Epidemiology Pr oject, which captures virtually all medical care delivered locally. Pa ssive follow-up through school and community medical records for crite ria diagnostic of mild or severe mental retardation was undertaken. Th irty 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 sev ere mental retardation in girls was more than twice that in boys; for mild retardation it was the opposite. The cumulative incidence among b oys was 1.7 times greater than the cumulative incidence for girls. Res ults suggest that although the overall incidence of mental retardation by gender was similar, the severity may differ somewhat.