Taken together, the results of these family studies support the hypothesis that immunological factors may play a role in the etiology of at least some individuals with OCD and/or TD/CTD, but these findings require replication in large population-based samples. Furthermore, the mechanisms of such association, if confirmed, are yet to be established. One possibility is that OCD and TD/CTD share genetic risk factors with AD. If the shared genetic risk hypothesis were correct, we would expect that the link between AD and these neuropsychiatric syndromes is not limited to mothers but also other first-degree relatives. Further, we would expect that the risks decrease with increasing genetic distance (for example, the risks for first-degree relatives should be higher than those for second- and third-degree relatives). An alternative – and perhaps not mutually exclusive – hypothesis is that mothers transmit antibodies to their offspring via the placenta.22, 23 If the maternal transmission hypothesis were correct, we would expect that the risk for mothers would be significantly higher than the risk for fathers and other first-degree relatives.
