**7. Autism and ADHD**

There is a very high comorbidity between autism and ADHD. Child psychiatric disorders have a comorbidity more than expected by chance [34]. Attention and hyperactivity are common in many disorders and, indeed, many more disorders in child psychiatry, and there is clearly poor separation of condition at a clinical level. Measurement issues are common. In relation to comorbidity, there are shared risks factors and one disorder creating an increased risk for another disorder. Neil et al. [35] pointed out that there are correlated liabilities where the risk factors of the two disorders correlate. There are social deficits in both ADHD and autism with overlap from an etiological point of view, but with ADHD the social deficits are more impulsive, and with autism, the social deficits can again be impulsive, but also, they can be due to lack of social know-how and theory of mind deficits. There is no sharp division here.

In a study of ADHD combined type, with one or more siblings, the diagnosis of autism was excluded at the beginning, and siblings with ADHD were compared with siblings without ADHD by Mulligan et al. [36]. They wrote that phenotypic correlation of ADHD and autism symptoms was 0.71 and that 32% of this correlation was due to shared familial characteristics but with a higher percentage for male ADHD probands. There was a trend for children with high ADHD symptoms to have high autism symptoms, as measured by the Social Communication Questionnaire. ADHD probands with definite language disorder or motor disorder had significantly higher symptoms of autism than those without. This study showed that autism symptoms as part of the ADHD phenotype were partly true. These were familial. Probands with autistic traits tend to have siblings with autistic traits, and probands without autistic traits tend to have affected siblings without autistic traits. Finally, latent class analysis of SCQ symptoms in probands with ADHD combined type showed the following clusters of autism symptoms: 31% with few or no symptoms of autism, 22.5% with repetitive and stereotyped behaviour, 21% with communication domain symptoms, 18.5% communication and reciprocal interaction domains and 7% who had symptoms in all three domains.

The percentage of phenotypic correlation due to shared familial influences (autistic symptoms and ADHD) was 35% for the whole group and 62% for males and 12% for females. In a family with a male child with ADHD and comorbid autistic symptoms, a second child with ADHD is also likely to have comorbid autistic symptoms (not so female), which suggest a different aetiology according to sex. Gillberg's [37] disorder of attention, motor control and perception would be showing similar findings. Fifty percent of children with DAMP had autistic features.

Children with oppositional defiant disorder and conduct disorder have more autistic traits than children without these comorbid disorders and ADHD [38]. Children with ADHD have more subthreshold symptoms of autism. Children with combined ADHD and social communication deficits are at increased risk of motor and language disorders. Overall, this shows the massive heterogeneity that is evident in child psychiatry classifications, and clearly, ADHD is not a homogenous disorder. Forty two percent of children with ADHD had few symptoms of autism. That autism symptoms are part of the ADHD phenotype is partly true. Autistic traits in ADHD are familial. This again supports the lack of a sharp overlap between neurodevelopmental disorders, here, autism and ADHD.
