Mental retardation is a significantly below average intelligence combined with difficulty adapting and functioning in daily life. All patients with mental retardation require some degree of support. To be diagnosed with mental retardation, the condition must occur before adulthood (generally recognized as an age of 18 years).

Most cases of mental retardation are mild, and many people with mental retardation can live full, healthy lives. However, other physical problems may be present depending on the cause of the mental retardation. According to the Centers for Disease Control and Prevention (CDC), mental retardation is the number one long-term condition that causes major activity limitations in America.

Mental retardation is the most common developmental disorder, according to the CDC. It affects about 12 out of every 1,000 schoolchildren in the United States. It is more common in boys than in girls. There is wide variation in frequency of mental retardation from state to state. In general, it is more common in the southeastern states. This may be at least partially due to environmental and social and economic factors that are prevalent in this region.

The severity of mental retardation is most frequently measured by the patient’s intelligence quotient (IQ). The average IQ is 100 and “normal” intelligence ranges from an IQ of 90 to 110. People with an IQ of about 70 to 89 are considered below average, but not mentally retarded. The severity of mental retardation is divided into five categories:

• Mild retardation. IQ ranging from between 50 and 55 to about 70.
  
  
• Moderate retardation. IQ ranging from between 35 and 40 to between 50 and 55.
  
  
• Severe retardation. IQ ranging from between 20 and 25 to between 35 and 40.
  
  
• Profound retardation. IQ below 20 or 25.
  
  
• Unspecified. IQ cannot be tested, but is assumed to be low.

There is some overlap in IQ between each of these categories. When a patient’s IQ falls in this overlap, his or her level of functioning is used to determine severity. For example, a patient with an IQ of 52 and a higher level of functioning would be considered to have mild retardation. However, a patient with an IQ of 53 and a lower level of functioning would be considered to have moderate retardation.

In some cases, the severity of mental retardation is determined by how much and what kind of support is required. However, this can be vague and some physicians and scientists do not find it as useful as categorizing severity by IQ ranges.

Patients with mental retardation face numerous difficulties, including problems with communication, self-care skills, social situations and school activities. They tend to develop these and other skills more slowly than average.