Effectiveness of Multisystemic Therapy for Adolescents From Families With Intellectual Disabilities

Referral Criteria: * Known or suspected ID of the referred adolescent and/or their parent(s); * Educational level of the referred adolescent and/or highest attained diploma of their parent(s) is indicative of potential ID. Inclusion Criteria Study Sample: * Adolescent must be 10 to 19 years old at the start of treatment; * Adolescent presents with severe behavioural problems in at least two life areas; * Adolescent lives with a family or there is a family the adolescent can live with, in which parent(s) have parental custody for a longer period of time; * Parent(s) consent(s) and is/are willing to engage in treatment to prevent an out-of-home placement of the adolescent; * Adolescent and/or parent(s) have a known or suspected intellectual disability (operationalised as an intelligence quotient \[IQ\] score of between 50-85 and additional deficits in adaptive functioning) * Adolescent and/or parent(s) have sufficient knowledge of the Dutch language (as assessed by a clinician and/or researcher) in order to understand and answer the various (self-report) questionnaires. Exclusion Criteria Study Participation: * Adolescent lives independently; * Adolescent presents with severe problematic sexual behaviours, without presenting with other severe behavioural problems; * Adolescent presents suicidal, psychotic, or homicidal requiring specialised treatment (such as a crisis placement in a residential facility); * Adolescent has a severe Autism Spectrum Disorder (level 2-3 according to the Diagnostic and Statistical Manual of Mental Disorders \[DSM-V\] criteria) or a severe ID (IQ score \<50); * Adolescent has internalising psychiatric problems which are the primary reason for referral, or has serious psychiatric problems (similar to #3 as well as for example eating disorder.