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This study examined the cognitive abilities of 60 pairs of reading-disabled twins aged between eight and 18 years (at least one member of each twin pair had been diagnosed as having a reading disability). Principal component analysis of nine cognitive tests yielded three readily interpretable composites of cognitive abilities--reading, symbol-processing speed and sequential memory. Scores on these three cognitive composites were significantly lower for the disabled readers than for normal readers. Individual differences in these cognitive composites within the reading-disabled twin sample were found to be due to genetic differences. Furthermore, deficits in the reading composite for the reading-disabled sample appear to be heritable.

Longitudinal psychometric test data collected at two different ages (9.4 +/- 1.3 and 14.8 +/- 1.2 years) were utilized to compare the developmental rates of 69 pairs of reading-disabled and matched control children. Results of a two-factor (GROUP X TIME), mixed-model, multivariate analysis of variance indicated that reading-disabled children manifested deficits on measures of academic achievement, symbolic processing speed and spatial reasoning abilities at both ages. Although both reading-disabled and control children were found to improve significantly between the two testing sessions, the lack of a significant GROUP X TIME interaction on measures of academic achievement and spatial reasoning abilities indicated that the rate of development was similar for the two groups. For measures of symbolic processing speed, however, differences between reading-disabled and control children were somewhat greater at the later age.

Evidence for the heterogeneity of reading disability was sought in a family study. Psychometric test data on 125 reading-disabled children (probands) and their siblings and parents were used to identify four subtypes of reading disability, each with a distinctive cognitive ability profile. The validity of this profile analysis was then assessed by applying the same classification system to profiles of affected parents and siblings of the probands. Affected siblings of probands in subtype 3 (severely impaired reading ability but normal spatial/reasoning and coding/speed) were more likely to be of the same subtype than would be expected on the basis of chance. However, no such evidence for familial transmission of specific subtypes was found in the parental data. It is suggested that family studies may provide a strong test of the heterogeneity of reading disability, and that such studies should be used to assess the validity of alternative typologies.

A multifactorial diagnostic assessment battery was evaluated using data obtained from 105 reading-disabled children and their matched controls (total N = 210). A stepwise discriminant function analysis selected five nonreading measures from among 16 as the best diagnostic predictors of reading disability. These measures included WISC-R Information and Digit Span, self-reported family histories of reading problems, and tests of symbol-processing speed and letter fluency.

Bannatyne's proposed "genetic dyslexic" subtype of reading disability (RD) was evaluated using data obtained from 60 (30 monozygotic and 30 dizygotic) pairs of affected twins (in which at least one member of each pair has a reading disorder) and 60 pairs of control twins. The predicted spatial greater than conceptual greater than sequential pattern of means was found to be reliable and specific to the RD sample. While this study does not provide conclusive evidence on the genetic etiology of the proposed subtype, our results do indicate that individuals with this particular profile may constitute a specific subtype of reading disability.