Down syndrome results when all or part of the 21st chromosome occurs in triplicate at or shortly after conception. Triplication of this chromosome results in small changes in the formation of certain biological structures. Initial structural differences constitute one important influence on development. This thesis explored development of a small number of cognitive abilities across the ages 4 years to 30 years for individuals with Down syndrome using data from the Stanford Binet (fourth edition) (SB:IV) test of intelligence. The approach used in this thesis reflected the understanding that domains underpinning this assessment tool (derived through factor analysis with a general population sample) may not have provided sufficient detail for understanding development for this population. Using the multilevel model for change, growth curves were derived to describe development of abilities at the sub-test level, allowing a more fine grained interpretation of the data.
The dataset for this study came from test proformas and information files of individuals who had participated in one or more studies of the Down Syndrome Research Program at the Fred and Eleanor Schonell Special Education Research Centre at the University of Queensland. Two hundred and eight individuals were assessed with the SB:IV between 1987 and 2004. Individuals had been assessed on one to seven occasions within this timeframe providing a total of 545 assessments on which growth curve predictions were based.
The SB:IV has six core subtests which were included for analysis: Vocabulary, Comprehension, Quantitative, Pattern Analysis, and two Short-Term Memory subtests, Bead Memory and Memory for Sentences. Both group trends and individual differences in development on these subtests were examined successively. Eight predictors were built into models for change to explore differential influences on change in subtest scores. These predictors included: gender, year of birth, school
experience, severity of medical conditions, mothers’ education, and teacher rated temperament scores from middle childhood. Temperament scores provided information on general activity level, the individual’s tendency toward negative moods, and their general persistence.
Results of initial growth curve analyses indicated quadratic age-related change with continual development to early adulthood (18 to 20 years) on all subtests with the exception of Pattern Analysis for which declines associated with quadratic change were not indicated. Scores for the Pattern Analysis subtest demonstrated the greatest variability between individuals. Peaks for Vocabulary and Comprehension scores appeared premature relative to normative patterns of development. Variation in performance between individuals with Down syndrome was large and significant for all subtests except Memory for Sentences. Age equivalent (AE) scores for the Pattern Analysis subtest converged with chronological age for some individuals in the study.
Effects of predictors on subtest scores were analysed using a subset of test results from 89 of the original 208 individuals included in the study. Inclusion in this subset was determined by availability of information on the predictors included in the analyses. No effects of gender or birth-year were detected for any subtest. Negative mood was associated with poorer performance on the Pattern Analysis subtest only. Persistence was associated with enhanced performance on the Pattern Analysis, Vocabulary, Quantitative and Bead Memory subtests. On all subtests except Vocabulary the effect of persistence comprised consistently higher scores from age 4 to 30 years. For the Vocabulary and also Quantitative subtests persistence was associated with an increased rate of development in subtest scores.
Higher education levels of mothers were associated with higher general ability levels for all subtests. Mothers’ education was the only significant predictor for performance on the Memory for Sentences subtest. Higher maternal education was associated with both higher performance generally from age 4 to age 30 and greater rate of change with age for this subtest. The only other subtest for which mothers’ education level influenced rate of change in performance was for the Comprehension subtest.
Results confirmed the importance of understanding development at sub-domain levels of cognitive ability for individuals with Down syndrome. Hypotheses regarding the meaning of individual and group trends in trajectories and implications of findings in relation to education and future research were considered.