A study of 18,415 children and adolescents in the U.K. compared the rate of mental-health problems among those with intellectual disabilities to that in other children. The researchers found that the prevalence of psychiatric disorders was 36% among children with intellectual disabilities compared to 8% among children without. Children with intellectual disabilities accounted for 14% of all British children with a diagnosable psychiatric disorder. Children with intellectual disabilities were 33.4 times more likely to suffer from autism, 8.4 times more likely to be hyperkinetic and 5.7 times more likely to have conduct disorders. The cumulative risk of exposure to social disadvantage was associated with increased prevalence of mental health problems.
Emerson, Eric and Hatton, Chris - Mental health of children and adolescents with intellectual disabilities in Britain British Journal of Psychiatry December 2007, 191, 493-499