While it is clinically relevant to know the cause of the intellectual disability in terms of better understanding both the cognitive profile and physical and psychiatric comorbidities, information about etiology is not required to diagnose intellectual disability.
Many conditions can be associated with intellectual disability, but the intellectual disability is not always present in conditions such as fetal alcohol spectrum disorder, ASD, Down syndrome and fragile X syndrome.
In DSM-5, severity levels are based on adaptive functioning, not IQ scores, because it is adaptive functioning that determines the level of supports the person needs.
Psychologists are trained to conduct assessments of intellectual functioning, usually in the form of an IQ test. Adaptive functioning is typically assessed through a standardized interview or measure, such as the Adaptive Behavior Assessment System (Harrison & Oakland, 2003) or the Vineland Adaptive Behavior Scale (Sparrow & Cicchetti, 1989). These measures are completed with the patient, an informant (e.g., caregiver, teacher) or both. They are generally administered by a psychologist or occupational therapist.