Subjects with intellectual disability (ID) or autism were excluded to remove individuals with perseverative behaviors and stereotypies that might mimic tics. Autism and ID were defined based on a review of medical and school records as described previously.24 As record-review data for ID were only available for a subset of ∼900 subjects, ID was also defined based on the results of age-appropriate standard neuropsychological assessments administered at ages 4 and 8 (Wisconsin Preschool and Primary Scale of Intelligence [WPPSI] and WISC-IV, respectively). Age 4 data were examined only if age 8 data were unavailable. Subjects with full-scale IQ ≤80 were excluded. When IQ data were not available at either age (n = 1,437), the presence of a Special Educational Needs (SEN) statement for any reason other than “sensory or physical needs” was used as a proxy for low IQ; as a result, an additional six subjects were excluded (Figure 1). A total of 267 subjects had no IQ, autism, ID, or SEN statement data available. These subjects were no more likely to receive subsequent tic diagnoses compared with subjects in
