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Existing research and media reports convey conflicting impressions of trends in the prevalence of mental illness. We sought to investigate trends in the prevalence of symptoms of mental illness in a large population-based cohort of Canadian children and adolescents.

We obtained population-based data from the National Longitudinal Survey of Children and Youth. Every 2 years, participants completed self-reported measures of mental illness indicators, including conduct disorder, hyperactivity, indirect aggression, suicidal behaviour, and depression and anxiety. We analyzed trends in mean scores over time using linear regression.

We evaluated 11 725 participants aged 10–11 years from cycles 1 (1994/95) through 6 (2004/05), 10 574 aged 12–13 years from cycles 2 (1996/97) through 7 (2006/07), and 9835 aged 14–15 years from cycles 3 (1998/99) through 8 (2008/09). The distribution of scores on depression and anxiety, conduct and indirect aggression scales remained stable or showed small decreases over time for participants of all ages. The mean hyperactivity score increased over time in participants aged 10–11 years (change per 2-year cycle: 0.16, 95% CI 0.02 to 0.12) and those aged 12–13 years (0.13, 95% CI 0.09 to 0.18). Over time, fewer participants aged 12–13 years (0.40% per cycle, 95% CI −0.78 to −0.07) and aged 14–15 years (0.56% per cycle, 95% CI −0.91 to −0.23) reported attempting suicide in the previous 12 months.

With the exception of hyperactivity, the prevalence of symptoms of mental illness in Canadian children and adolescents has remained relatively stable from …