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### Environmental risk factors  Researchers used data from the Avon Longitudinal Study of Parents and Children to identify maternal factors that increase the risk of tic disorders in offspring \citep{Ben_Shlomo_2015}. The Avon Longitudinal Study is an ongoing, prospective, pre-birth cohort study of all children born in Avon, United Kingdom, between April 1, 1991, and December 31, 1992. Maternal questionnaires were administered throughout pregnancy and mothers also completed questionnaires about themselves and their children's development every 6 months from the child's birth to the age of 7 and then yearly thereafter. In the final multivariate model, chronic maternal anxiety, evident both before and after the child's birth, was associated with TS or chronic tic disorder in children. This association may reflect shared genetic susceptibility or prenatal exposure.