children 18 years of age or less including population based twin data and birth registers; 2) the prevalence rates were based on DSM-III-R or DSM-IV criteria. If only preschool children were enrolled, the study was excluded. We reviewed 1,237 abstracts of the 8,209 abstracts identified. For all abstracts that met these or similar criteria, the full articles were read (115). Of these 115 articles 39 met our full criteria. However, for 14 of these 39 studies we were not able to locate the authors and the publication of prevalence rates did not include either standard errors or confidence intervals necessary for the meta-regression analysis. The information that appears in Table 1 was extracted for the 25 articles that met our full criteria including the availability of confidence intervals. If longitudinal data were presented, only results from the first wave were analyzed. If more than one prevalence rate was presented, the total rate was analyzed.
