IQ decline in mentally retarded adults: a matter of fact or methodological flaw.
While it has generally been assumed that intelligence in the retarded decreases with age, the institutional research supporting this premise, whether cross-sectional or longitudinal, is methodologically flawed by selective sample attrition. This paper demonstrates that in an institutional population those leaving, at every age level, are indeed the brightest. When the IQ data is viewed cross-sectionally there is the appearance of decline, but when the same population is studied semi-longitudinally, not only is there no decrease in IQ with age but there are regular increments, particularly in performance scores.
D000293:Adolescent; D000328:Adult; D000375:Aging; D006801:Humans; D008607:Intellectual Disability / Q000523:psychology*; D007360:Intelligence; D008137:Longitudinal Studies; D012107:Research Design