text: Tics might be one of the most common behavioural problems in childhood, but frequency estimates vary greatly. Community surveys indicate that between 1 and 13 per cent of boys, and between 1 and 11 per cent of girls manifest frequent 'tics, twitches, mannerisms or habit spasms' (27) Race and socio-economic status do not seem to influence the frequency of tics, while urban living may be associated with elevated rates. There are virtually no general population studies of transient tic disorder or chronic motor or vocal tic disorder. For Tourette's disorder, several studies have been conducted among school-age children in the United States, yielding widely divergent prevalence estimates, ranging from 2.87 per 10 000 in Monroe County, New York,(28,) and 5.2 per 10 000 in the North Dakota Survey, (1J> to 105 per 10 000 boys and 13.2 per 10 000 girls in a California school district. ^i) These data indicate that children are five to 12 times more likely than adults to have a tic disorder.
  In adolescents, epidemiological studies using strict diagnostic criteria and structured clinical interviews have been conducted in several parts of the world, estimating the prevalence of OCD between 1 and 4 per cent, and the prevalence of Tourette's disorder at 4.3 per 10 000 ( Table 1). In the largest epidemiological study on juvenile OCD, conducted in a North American population of 5596 high-school students, (7) the current prevalence rate of OCD in adolescents was estimated to 1 (± 0.5) per cent, and its lifetime prevalence rate to 1.9 (±0.7) per cent. The study showed that the disorder was clearly underdiagnosed and undertreated in this age group, none of the cases identified having been previously diagnosed, and only 20 per cent ever treated for (comorbid) psychological problems. In a later study, (8) examining 562 consecutive inductees into the Israeli Army, the point prevalence of OCD was 3.6 (±0.7) per cent. Of note was the high proportion of subjects with obsessions only (50 per cent), potentially less disruptive of everyday functioning. If the prevalence of OCD was estimated excluding those individuals with only obsessions, the point prevalence dropped to 1.8 per cent. Among the OCD cases, there was a significant elevation of tic disorders (Tourette's disorder 5 per cent, chronic multiple tics 10 per cent, transient tics 10 per cent, versus 0 per cent, 0.9 per cent, and 1.1 per cent respectively in individuals with no diagnosis of OCD). In two longitudinal studies following cohorts of children in the community up to the age of 18 years, one from the United States found a lifetime prevalence rate for OCD of 2.1 per cent,(29) and the other from New Zealand found an overall 1-year prevalence rate of 4 per cent, but only 1.2 per cent when subjects with obsessions only were excluded/9) Th us, it appears that OCD might be as frequent in adolescents as it is in adults (see ChapterJ.^).
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  Table 1 Epidemiological studies of OCD and Tourette's disorder in adolescence (studies using structured diagnostic interviews)
  A study conducted amongst the total population of inductees into the Israeli Defence Force over a 1-year period estimated the point prevalence of Tourette's disorder at 4.9 per 10 000 males and 3.1 per 10 000 females. (30) The point prevalence of OCD among subjects with Tourette's disorder was significantly elevated (41.7 per cent) compared with those without (3.4 per cent).