Chromosomal deletions and ADHD
A study in the Lancet seemed to imply that large chromosomal deletions could cause ADHD - children with ADHD were found more often to have such deletions than control subjects. However there is a flaw in the argument because according to the paper the children with ADHD also had lower IQ than controls. It is already known that large chromosomal deletions are associated with intellectual impairment. So a plausible explanation is that by selecting children with ADHD one obtains a sample with low IQ and such subjects can be expected to have more large deletions by virtue of their lower intelligence rather because of any specific relationship between deletions and ADHD. Of course, at one level it is true that ADHD is associated with large deletions. But if the effect is mediated by IQ then this is somewhat different from the notion that "deletions cause ADHD". To test this, one would need to study a sample of subjects with ADHD compared with controls who had been matched for IQ.