Brain reserve theory: Are adults with intellectual disability more vulnerable to age than peers with typical development?

Background: Life expectancy is on rise and the intriguing question is: When does cognitive decline occur among adults with intellectual disability, compared to adults with typical development? This cross-sectional study examined cognitive performance of crystallised/fluid intelligence, working and long-term memory of adults with intellectual disability of etiologies other than Down syndrome (IQ 50–68) and adults with typical development (IQ 85–114) in four age cohorts (30–39; 40–49; 50–59;60–69).Method: The WAIS III HEB and the Rey-AVLT were administered to both groups .Results: Four patterns of cognitive performance were found: (a) Vocabulary (crystal-lised intelligence), Spatial Span Forward and Retention yielded similar scores across all four age cohorts in participants with typical development and with intellectual dis-ability. (b) Similarities, Raven and Digit Span Backward exhibit lower scores only in50–59 or 60–69 compared to the 30–39 age cohort in both groups, (c) Digit Span Forward, Spatial Span Backward and Total Leaning (LTM) yielded lower scores in the50–59 or 60–69 age cohorts in the typical group, but similar scores in participants with intellectual disability along the age cohorts, (d) Block Design (fluid intelligence)yielded a lower score in the 50–59 cohort versus lower scores only at ages 60–69 in participants with typical development. Conclusions: Our findings suggest a possible parallel trajectory in age-related cognitive performance for individuals with and without intellectual disability in six measures, and a possible more preserved trajectory in fluid intelligence and some memory measures in adults with intellectual disability compared to their peers. Caution should be exercised regarding Digit and Spatial Span Backwards, which yielded a floor effect in participants with intellectual disability. The Cognitive Reserve Theory, the Safeguard Hypothesis and late maturation might serve as explanations for these findings.