ADHD in the Northern Finnish Birth Cohort

The Northern Finnish Birth Cohort (NFBC) study of Attention Deficit Hyperactivity Disorder (ADHD) was initiated in 2001 to identify risk genes and gene–by–environmental interactions underlying ADHD in a population-based cohort of adolescents who have been followed prospectively since their birth in 1986. The cohort is drawn from northern Finland (pop. 630,000), where the population consists primarily of two groups—an early settlement and a late settlement. The late settlement descended from a small number of families (or founders) who migrated there in the 17th century and expanded locally. The small number of founders in a region that is fairly geographically isolated has resulted in a “population isolate” that is invaluable to genetic studies of single gene (i.e., monogenic) disorders as well as disorders which are thought to be due to several genes (i.e., polygenic). The first set of five papers on the NFBC study of ADHD was recently published in the December 2007 issue of the Journal of the American Academy of Child and Adolescent Psychiatry, and this article will summarize those detailed papers to present the main conclusions drawn about ADHD, based on one of the largest population-based studies of ADHD. Readers are directed to that journal for the methods, detailed results, and a more extensive discussion of the conclusions. The NFBC provides a unique opportunity to examine the developmental trajectory of ADHD in a social setting where medication is not the primary approach to treating ADHD. In fact, very few of the almost 457 (ADHD = 188, Controls = 269) adolescents assessed during the study had ever received any pharmacological treatment. Despite striking differences in treatment approaches between northern Finland and the United States, the clinical characteristics, psychiatric comorbidity, and cognitive deficits associated with ADHD are very much alike. Thus, one of the first major conclusions from this set of studies is that, despite the controversy regarding the validity of the ADHD diagnosis, ADHD is a clearly recognizable condition with a relatively stable prevalence worldwide. Using DSM–IV diagnostic criteria in a population-based cohort of adolescents (ages 16–18) in northern Finland, the prevalence is 8.5%. The clinical characteristics of ADHD in this population are strikingly similar to other ADHD samples drawn around the world with more boys than girls being affected, similar rates of psychiatric comorbidity, and similar rates of persistence of ADHD from childhood into adolescence (~2/3). Description of the ascertainment method and clinical characteristics of the sample were described in the paper by Smalley and colleagues (Smalley et al., 2007). One goal of the study was to examine ADHD symptoms within a large popu