Abstract
Although there is clear evidence that low verbal ability is a risk factor for conduct disorder (CD), some researchers have questioned whether this association is due to the common comorbidity between attention-deficit/hyperactivity disorder (ADHD) and CD. The present study examined the association among verbal ability, ADHD, and CD in a genetically informative sample in order to examine the role of genes and/or environmental influences shared in common with ADHD on the covariation between verbal ability and CD. Participants were 2744 adolescents from the Center for Antisocial Drug Dependence (CADD), and included 360 monozygotic (MZ) female twin pairs, 221 dizygotic (DZ) female twin pairs, 297 MZ male twin pairs, 220 DZ male twin pairs, and 274 opposite-sex DZ twin pairs. The Diagnostic Interview Schedule for Children (DISC-IV) was used to assess lifetime symptoms of ADHD and CD. Verbal ability was assessed via the Vocabulary subtest of the Wechsler Adult Intelligence Scale III (WAIS-III) for individuals over the age of 16 and the Vocabulary subtest of the Wechsler Intelligence Scale for Children III (WISC-III) for individuals under the age of 16. There was a small but significant negative covariance between verbal ability and CD and between verbal ability and ADHD. Results also suggest that the covariation between verbal ability and CD is due to influences shared in common with ADHD.
Highlights
Conduct disorder (CD) is defined by the Diagnostic and Statistical Manual of Mental Disorders as a repetitive and persistent pattern of behavior in which the basic rights of others or major age-appropriate societal norms or rules are violated (American Psychiatric Association, 2000)
We investigated the etiology of CD, examining whether low verbal ability is a significant independent risk factor for CD after controlling for attention-deficit/hyperactivity disorder (ADHD)
The longitudinal twin sample (LTS) and community twin sample (CTS) are two samples recruited into the Center for Antisocial Drug Dependence (CADD) from the Colorado twin registry (CTR), a population-based registry housed at Institute for Behavioral Genetics (IBG) at the University of Colorado
Summary
Conduct disorder (CD) is defined by the Diagnostic and Statistical Manual of Mental Disorders as a repetitive and persistent pattern of behavior in which the basic rights of others or major age-appropriate societal norms or rules are violated (American Psychiatric Association, 2000). Several lines of evidence suggest that low verbal ability is a risk factor for CD (e.g., Wechsler, 1944; Moffitt and Silva, 1988a). A review paper reports that delinquent individuals have average full scale or verbal intelligence quotient (IQ) scores 8–10 points lower than the population average (Nigg and Huang-Pollock, 2003). Quay’s (1987) review found that delinquent adolescents have lower IQ scores than adolescents from general population samples, and that the delinquents’ deficits were stronger in the verbal domain. Longitudinal studies find that early weakness in verbal learning and verbal reasoning ability predicts later persistent offending, CD, and antisocial outcomes (e.g., Moffitt, 1990a; Farrington and Hawkins, 1991). The verbal deficit in youth with CD appears to be at least partially independent of such potential confounds as ethnicity and socioeconomic status (e.g., Hirschi and Hindelang, 1977; Wilson and Herrnstein, 1985; Petee and Walsh, 1987; Lynam et al, 1993; Moffitt et al, 1994)
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