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Prevalence of mental illness
Epidemiological research has shown that between 3% and 18% of children have a psychiatric disorder causing significant functional impairment (reasons for these widely divergent prevalence rates are discussed below) and Costello and colleagues have proposed a median prevalence estimate of 12%. Using a different statiistical method, Waddell and colleagues propose a prevalence rate for all mental disorders in children of 14.2%.

Developmental epidemiology
Developmental epidemiology seeks to "disentangle how the trajectories of symptoms, environment, and individual development intertwine to produce psychopathology".

Socio-economic influences
Mental illness in childhood and adolescence is associated with parental unemployment, low family income, being on family income assistance, lower parental educational level, and single-parent, blended or step-parent families

Methodological issues
Epidemiological research has produced widely divergent estimates, depending on the nature of the disgostic method (e.g. structured clinical interview, unstrutured clinical interview, self-report or parent-report questionnaire), but more recent studies using DSM-IV-based structured interviews produce more reliable estimates of clinical "caseness". Past resarch has also been limited by inconsistent definitions of clinical disorders, and differing upper and lowe age limits of the study population. Changing definitions over time have given rise to spurious evidence of changing prevalence of disorders. Furthermore, almost all epidemiological surveys have been carried out in Europe, North America and Australia, and the cross-cultural validity of DSM criteria have been questioned, so it is not clear to what extent the published data can be generalised to developing countries.