We initially hypothesized that the increased rates of ADHD found in TS families would be due to bilineal mating between individuals with ADHD and individuals with TS or OCD. However, we did not find evidence of non-random mating for ADHD, or for ADHD with either TS or OCD. Thus, we cannot easily explain the strong environmental correlation and lack of genetic correlation between TS and ADHD. It is possible that the observed association is related to specific environmental exposures (such as prenatal maternal smoking) that predispose an individual to both TS and ADHD, either alone or in combination with additional environmental or genetic environmental influences53, 54.
