Knowledge about which child psychiatric disorders precede criminal behaviour is important to delineate high risk children seen in child psychiatric services. Research has consistently demonstrated the long-term impact of childhood psychiatric problems on later antisocial traits, especially conduct problems that have been shown to be developmental precursors of later antisocial behaviour and criminality [1–8]. Recent studies conducted on prisoners in western countries have shown that about half of the imprisoned fulfilled the diagnoses of serious conduct disorder or antisocial personality disorder when incarcerated [9, 10].
Although conduct disorder is a well known antecedent of antisocial development, other childhood disorders as precursors of antisociality are more controversial. So far, long-term follow-up studies have demonstrated that attention-deficit/hyperactivity disorder (ADHD) combined with conduct disorder is a precursor of later antisocial behaviour [11–13]. There are, however, discrepant findings with regard to ADHD without conduct problems as an independent precursor of criminality. Based on the results of long-term epidemiological follow-up studies, Farrington , Babinski et al.  and Sourander et al.  found that hyperactivity-impulsivity, independently of conduct problems, predicted later criminality in males. In two long-term clinical follow-up studies, Mannuzza et al. [15, 16] similarly found that ADHD was a developmental precursor of antisocial behaviour in early- and mid-adulthood. Satterfield et al. , on the other hand, reported in his clinical follow-up study of hyperactive outpatient boys that only those individuals with ADHD combined with childhood conduct problems were at increased risk of criminality. Likewise, in a 10-year follow-up study of a birth cohort, Fergusson et al. found that children with attention deficits but no conduct problems were not at increased risk of juvenile delinquency. They were, however, at risk of later reduced academic success in a dose-response manner . Recently, Diamantopolou et al. , similarly, found that there were no direct association between ADHD symptoms and later antisocial personality problems. Neither did they find that the combination of internalizing and externalizing symptoms appeared to add to the prediction of later antisocial behaviour in adolescence.
During the last decades, there has been a growing interest in the interplay between internalizing and externalizing problems, but there have been no clear findings about the outcome for children with comorbid conduct and emotional disorders. Sourander et al. found that children with combined emotional and conduct problems had a higher risk of criminality compared with children who only had emotional problems, attention deficits and/or conduct problems . Their results provided only partial support from previous research. In two longitudinal clinical studies, Harrington et al.  and Fombonne et al.  found that children and adolescents with comorbid conduct and depressive disorders had a higher risk of later criminality and antisocial behaviour than those who only had emotional disorders. However, the outcome among children with comorbid disorders was similar to those with conduct disorders alone.
In sum, there are still no consistent findings from epidemiological or clinical studies whether ADHD alone is a precursor of later criminality; nor is it known whether children with combined emotional and conduct disorder are at higher risk of later antisocial behaviour than those with conduct disorder alone or in combination with hyperactivity.
Studying child psychiatric in-patients with excessive symptom load could enhance prediction of which disorders precede criminality. Previous research has shown that severity of symptoms increases the stability of a disorder , and clinical referred children have been found to have high diagnostic stability from childhood to adolescence . To our knowledge, there are, however, few long-term follow-up studies of seriously affected in-patient children with ADHD or comorbid emotional and conduct disorder. Only two of the previously mentioned clinical studies had included in-patients [18, 19].
In the present study, former child psychiatric in-patients were followed up 19 to 41 years after hospitalization by linking their records to the National Register of Criminality. The combination of a long follow-up period and diagnostic evaluation, according to the ICD-10 classification system, made this study suited for exploring the association between several different childhood diagnoses and the development of criminality in adolescence and into mid-adulthood. The mean age at follow-up was 38 years, an age after which the likelihood of criminal debut is minimal. We could, therefore, provide a comprehensive picture of lifetime criminality in adults with a childhood history of severe mental disorders.
In addition, the extensive information in the hospital records made it possible to control for vulnerability factors, other than diagnoses, that could contribute to the development of later criminality.
We wanted to test the hypothesis that there was a direct association between hyperkinetic symptoms and later criminality in former child psychiatric in-patients, with ADHD increasing the risk for delinquency, independent of conduct disorder comorbidity or not.
We also wanted to test the hypothesis that former child psychiatric in-patients with mixed disorder of conduct and emotions were at increased risk for later criminality compared to those with conduct disorder only. A final issue was to explore whether vulnerability factors other than diagnoses could enhance prediction of delinquent outcome.