The family analyses revealed significantly elevated risk of AD in first-degree relatives of both OCD and TD/CTD patients, but not in second- or third-degree relatives. The magnitude of the results was highest for first-degree relatives of TD/CTD probands. The results were unchanged when we excluded OCD and TD/CTD probands with diagnosed AD, as well as relatives with either OCD or TD/CTD, and in various sensitivity analyses. While our design does not allow us to confidently rule out shared environmental factors, the pattern of results is compatible with a possible shared genetic etiology between OCD and TD/CTD and AD, at least in some patients. First, the association was only statistically significant amongst first-degree relatives. Second, siblings of probands had similar (OCD) or lower (TD/CTD) risks of AD than mothers and fathers (they all share about 50% of the genes) despite siblings sharing more of the environment (siblings are born in the same family, during a similar period of time, and grow up in the same household). Furthermore, maternal and paternal half-siblings (both sharing 25% of genes but paternal half-siblings sharing less
