Individuals with mental retardation have co-existing mental disorders at a rate that is estimated to be three to four times greater than in the general population. In some cases this may be due to a shared etiology (e.g., head trauma may result in mental retardation and in personality change). All types of mental disorders can occur in people with mental retardation, and there is no evidence that the nature of a given mental disorder differs in individuals with or without mental retardation. A psychiatric diagnosis, however, is often complicated by the fact that the clinical presentation may be modified by the severity of the retardation. The most common associated disorders are attention-deficit/hyperactivity disorder, mood disorders, pervasive developmental disorders, stereotypic movement disorder, and mental disorders due to a general medical condition. Individuals who have Down Syndrome may be at higher risk for developing dementia of the Alzheimer's type later in life.
The diagnosis of mental retardation must be differentiated from learning or communication disorders. In these disorders there is no generalized impairment in intellectual development and adaptive functioning. However a learning or communication disorder may exist in an individual with mental retardation when the delay in these areas is below what is expected with the individual's level of retardation.