A cross-sectional test of the similar-trajectory hypothesis among adults with mental retardation.
Adults with ID follow the same cognitive aging curve as everyone else, so look elsewhere when you see unexpected score drops.
01Research in Context
What this study did
Facon (2008) gave the WAIS-R IQ test to adults with and without intellectual disability. The team wanted to see if both groups age the same way on verbal and performance scores.
They used a cross-sectional design. This means they compared different age groups at one point in time, not following the same people for years.
What they found
Adults with ID showed the same age-related pattern as adults without ID. Verbal and performance IQ scores moved in parallel across the adult lifespan.
The data supported the "similar-trajectory hypothesis." In plain words, getting older affects thinking skills in much the same way for both groups.
How this fits with other research
Bailey et al. (2010) seems to disagree. They found large working-memory updating deficits in adults with ID when compared to younger children matched for IQ. The key difference is who they compared. Bruno used age-matched adults; B et al. matched for IQ with younger kids. The contradiction fades when you see one study tracks aging while the other highlights developmental gaps.
Simpson et al. (2001) and Chaplin (2009) cast a wider net. The 2001 review says most adult-ID research lacks detail on social factors. The 2009 review argues general mental-health services are not enough for adults with dual diagnoses. Bruno’s clean cognitive data supply a missing puzzle piece for both reviews.
Together, the papers show: cognitive aging looks typical, but service systems and specific skills like working memory still need targeted support.
Why it matters
You can stop assuming that every dip in test scores is "just the disability." Age matters, but the slope is similar to typical adults. Use this when you explain assessment results to families or when you set long-term goals. Pair cognitive tasks with working-memory drills if the client struggles with updating, and keep pushing for specialized mental-health services when mood or behavior issues show up.
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02At a glance
03Original abstract
The similar-sequence and the similar-structure hypotheses are the two mainstays of the developmental approach to mental retardation. In the present study, a third way, the similar-trajectory hypothesis, is described and illustrated using the WAIS-R results of adults with and without mental retardation aged from 20 to 54 years. The whole sample (N=633) comprised 306 participants with mental retardation and 327 without mental retardation. Hierarchical regression analyses comparing the two groups showed similar evolutions of scores with increasing age for verbal and performance scales. These results seem to validate the similar-trajectory hypothesis, at least for the present samples and for the aspects of cognitive development considered here. Some weaknesses and implications of the study are considered in the discussion.
Research in developmental disabilities, 2008 · doi:10.1016/j.ridd.2006.10.003