What Is the Connection between Mental Retardation and IQ?

Mental retardation and IQ are linked because having a below-average intelligence quotient (IQ) is required for diagnosis. Along with having a low IQ, the diagnosis of mental retardation (MR) involves a decreased ability to function in the world and an onset of symptoms before the age of 18. Patients can be grouped into different classes of MR based on their IQ scores. A number of standardized examinations can be used to determine IQ.

The diagnosis of mental retardation requires meeting three different criteria. First, the onset of the symptoms of impaired cognitive functioning must develop before the age of 18. The second requirement is that patients must have limitations in their abilities to live normal daily lives. They will have decreased adaptive behaviors, and have difficulties in taking care of themselves and functioning in society. The final criterion for diagnosing mental retardation is that patients have below-average intellectual function; often an IQ score of less than or equal to 70 is used as a cutoff point.

Patients with mental retardation and IQ scores of 50-70 are considered to have mild mental retardation. Often they are not diagnosed with this condition until they begin school and are required to perform higher-level intellectual tasks. With teaching, these patients are often able to achieve the knowledge and abilities typically associated with an elementary school student in U.S. grade levels three to six. They can often live on their own and earn money by performing simple jobs.

Having mental retardation and IQ scores from approximately 35-50 puts patients in the range of moderate severity. These patients often thrive living in group homes or with caretakers. Simple skills such as caring for themselves and watching out for their personal safety are often taught successfully.

Severe mental retardation, encompassing patients with IQ scores ranging from 20-35, often prevents patients from functioning on their own. Although some with severe MR can learn basic skills such as washing or feeding themselves, most will require long-term supportive care. They typically develop limited capabilities to communicate with the world around them either by verbal means or sign language.

Patients with mental retardation and IQ scores less than 20 fall in the category of profound MR. They cannot care for themselves, and need close supervision at all times. Often these patients are identified very early in their lives. Fortunately, only about 10% of all patients with mental retardation fall into the profound severity class.

The determination of whether an individual has below-average intelligence is often made with the help of IQ testing. Standardized examinations such as the Stanford-Binet Intelligence Scale®, the Weschler Intelligence Scales for Children®, and the Kaufman Assessment Battery for Children provide numerical values for the intelligence quotient. Typically, the IQ scale is set such that a person with average intelligence will earn a score of 100, with the standard deviation set at 15 points. A patient who scores a 70 is therefore two standard deviations below the average, and is therefore less intelligent than approximately 98% of his peers.