According to the American Association on Mental Retardation, an individual is considered to have mental retardation based on the following three criteria: intellectual functioning level (IQ) is below 70-75; significant limitations exist in two or more adaptive skill areas; and the condition is present from childhood.

An estimated 7.5 million people have mental retardation in the United States. Approximately 87% of that number are mildly affected, ten percent have moderate retardation, and three percent have severe or profound retardation. Overall, mental retardation is more prevalent in males than females; however, mental retardation can affect people regardless of their racial, ethnic, educational, social and economic backgrounds.

Mental retardation occurs when a condition impairs the development of the brain before birth. There are many factors that can cause this, but the most common causes are as follows:

• Genetic conditions -Over 500 genetic diseases have been found to be directly related to mental retardation disorders. These conditions can result from abnormal genes inherited from parents, errors that occur when genes combine, and other reasons. Three examples of genetic disorders include Down syndrome, fragile X syndrome and phenylketonuria (PKU).
• Complications during pregnancy -The use of alcohol or other drugs during pregnancy can cause mental retardation. In the case of fetal alcohol system, the brain does not develop properly before birth. In recent research, smoking has also been found to increase the risk of mental retardation. Other risks factors that can inhibit brain development include malnutrition, environmental contaminants, and illnesses contracted by the mother.
• Problems at birth -Premature infants and/or those with low birth weights are most often at great risk of mental retardation. Unusual stress on the head during birth, lack of oxygen or other problems during birth can also cause the condition.
• Childhood diseases & extreme malnutrition -The absence of adequate nutrition or diseases such as whooping cough, chicken pox, and measles can lead to meningitis and encephalitis which can damange the brain of a growing child.

The process for diagnosing and classifying a person as having mental retardation involves assessement on standardized intelligence tests and adaptive skills tests. Generally, a person is diagnosed with mental retardation if their intellectual functioning level is below 70-75; however, IQ is not the only determining factor. Adaptive behavior is also critical.