4.1. Phenotypic analyses
4.1.1. Zero-order correlations
All nine composite measures were significantly correlated with all other measures, but the magnitude of the correlations varied significantly (). As expected, word reading had the highest correlation with phoneme awareness (r = .71). Correlations between word reading and the other five cognitive composites were also significant and medium to large in magnitude (r = .35 - .61), although the correlation between reading and response inhibition was significantly smaller than the correlations with the other four composites. Analyses of the ADHD composites provided further support for the distinction between inattention and hyperactivity-impulsivity. Although both symptom dimensions were significantly correlated with all six cognitive composites, correlations with inattention were significantly higher than correlations with hyperactivity-impulsivity on all six cognitive composites.
Phenotypic correlations (95% confidence interval) between composite measures of reading, ADHD, and neuropsychological functioning
4.1.2. Categorical analyses
summarizes analyses that compared the performance of groups with RD, ADHD, both disorders, or neither disorder on the six neuropsychological composites. Groups with RD or ADHD exhibited significant weaknesses on all 6 neuropsychological composites compared to the control group without RD or ADHD. Both groups with RD were more impaired than the group with ADHD alone on measures of phoneme awareness, verbal reasoning, working memory, and naming speed, and the comorbid group was more impaired than the groups with either disorder alone on measures of response inhibition and processing speed.
Performance of groups with and without RD and ADHD on the six cognitive composites.
Because RD and ADHD were associated with significant weaknesses on each of the cognitive composites, separate multiple logistic regression analyses were conducted to test which cognitive constructs independently predicted RD or ADHD. In the first model the six cognitive constructs were included as simultaneous predictors of RD status, and the model was then repeated with ADHD as the dependent measure (). RD was independently predicted by lower scores on all cognitive composites except response inhibition. In contrast, overall ADHD status was predicted by response inhibition and processing speed only, and results were similar when the inattentive and combined subtypes were analyzed separately. Taken together, these results suggest that processing speed is the most promising candidate for a shared cognitive weakness in RD and ADHD.
Multiple logistic regression models predicting RD and ADHD status from scores on the six cognitive composites
4.2. Univariate twin analyses
The first set of behavior genetic analyses tested the univariate etiology of individual differences and extreme scores on the three diagnostic measures and six cognitive composites. MZ correlations were significantly higher than DZ correlations for all nine composites, suggesting that individual differences on all measures are influenced by genes (). Similarly, in the DF analyses of extreme scores the mean of the MZ cotwins regressed less toward the population mean than the mean of the DZ cotwins on all measures (). Indeed, high heritability estimates were obtained for both individual differences and extreme scores on the measures of inattention and hyperactivity-impulsivity (h2 = .72 - .74; h2g = .84 - .86), and moderate heritability estimates were obtained for the word reading composite and the six neurocognitive composites (h2 = .41 - .67; h2g = .44 - .65; Tables and ). Shared environmental influences were significant for all measures except the ADHD and response inhibition composites, but the point estimates for shared environment influences were smaller in magnitude than the estimates of genetic influences (c2 = .01 - .40; c2g = .00 - .35).
Univariate twin analyses of individual differences on the nine composite measures in the entire sample
Univariate twin analyses of extreme scores
Similar estimates of genetic and shared environment influences were obtained in the DF analyses of extreme scores and the Mx analyses of individual differences (mean difference between h2g and h2 = .05; mean difference between c2g and c2 = -.03). Although these analyses cannot test definitively whether the same etiological influences act on extreme scores and individual differences, the similarity of these and results is consistent with this hypothesis.
4.3. Multivariate twin anayses
After testing the univariate etiology of each composite measure, we next examined the genetic and environmental associations among the nine constructs. Genetic correlations were significant between measures of reading, inattention, and hyperactivity-impulsivity (Tables and ), confirming previous results that suggested that the association between reading difficulties and ADHD is due in part to common genetic influences. On the other hand, the confidence intervals for the genetic correlations do not include unity, indicating that unique genetic influences contribute to each of the three diagnostic phenotypes.
Genetic and shared environmental correlations among the nine composite measures in the overall sample
Bivariate heritabilities and genetic correlations of extreme scores on the diagnostic measures and the cognitive composites
Genetic correlations were significant and moderate to high between reading and the six cognitive measures (rg above the diagonal in , and rg[sel] is included in ). Although the largest genetic correlations with reading were with phoneme awareness and processing speed, the significance and magnitude of the genetic correlations with the other cognitive composites are consistent with a multiple deficit model of RD.
Moderate and significant genetic correlations were also observed between inattention and all of the cognitive composites, with the strongest genetic associations with processing speed and response inhibition. Genetic correlations with hyperactivity-impulsivity were lower and only marginally significant for several measures, although once again the highest genetic correlation were with processing speed and response inhibition.
Shared environmental correlations with reading were significant and large for phoneme awareness, working memory, processing speed, and verbal reasoning, suggesting that the majority of the shared environmental influences on each of these measures act on all of the measures (below the diagonal in ). In contrast, although the point estimates for shared environmental correlations with inattention or hyperactivity-impulsivity were often high, none were significant due to the negligible effect of shared environmental influences on ADHD symptoms ().
4.4. Slow processing speed as an explanation for comorbidity between RD and ADHD
Based on our previous and current results, we conducted a final targeted analysis to test if common genetic influences on processing and naming speed accounted for comorbidity between RD and ADHD. A genetic Cholesky decomposition analysis was used to estimate the shared and independent genetic influences on reading, inattention, hyperactivity-impulsivity, and the processing and naming speed composites ().
Genetic Cholesky decomposition analysis of ADHD, reading, and processing and naming speed. Solid paths are significant (p < .01).
The significant path loadings on the first genetic factor (A1) indicate that common genetic influences account for significant covariance among the five measures. Because the Cholesky model is hierarchical, paths from genetic factors A2 - A5 test for additional genetic influences that are independent of those included in A1. A separate genetic factor contributes significantly to covariance between inattention and hyperactivity-impulsivity that is independent of word reading (A4), and there were unique genetic influences on naming speed (A2), word reading (A3), and hyperactivity-impulsivity (A5) that were not significantly associated with any of the other composites. Finally, the most important result for the primary question in this paper is the absence of any additional shared genetic influences on reading and either ADHD composite after accounting for the genetic influences that are shared with processing speed.
These results suggest that comorbidity between reading difficulties and ADHD is primarily attributable to common genetic influences that lead to slow processing and naming speed. To test the specificity of this result, additional analyses were run in which working memory or inhibition was entered first in the Cholesky model rather than processing and naming speed. In each model there were significant shared genetic influences between reading and inattention that were independent of the genetic influences that are shared with inhibition or working memory.