About 75–80 percent of mental retardation is familial (runs in families), and 20–25 percent is due to organic problems, such as chromosomal abnormalities or brain damage. Mild to severe mental retardation is a symptom of several hundred single-gene disorders and many chromosomal abnormalities, including small deletions. Based on twin studies, moderate to severe mental retardation does not appear to be familial, but mild mental retardation does. That is, the relatives of the moderate to severely mentally retarded have normal ranges of IQs, whereas the families of the mildly mentally retarded have IQs skewing lower.
IQ score ranges (from DSM-IV):
Mild mental retardation: IQ 50–55 to 70; children require mild support; formally called "Educable Mentally Retarded".
Moderate retardation: IQ 35–40 to 50–55; children require moderate supervision and assistance; formally called "Trainable Mentally Retarded".
Severe mental retardation: IQ 20–25 to 35–40; can be taught basic life skills and simple tasks with supervision.
Profound mental retardation: IQ below 20–25; usually caused by a neurological condition; require constant care.
The rate of mental retardation is higher among males than females, and higher among blacks than whites, according to a 1991 U.S. Centers for Disease Control and Prevention (CDC) study.
By race, the overall rate was 16.6 per 1000 for blacks and 6.8 per 1000 for whites. Rates of mental retardation for black males, the group with the highest rates, were 1.7 times higher than black females, 2.4 times higher than white males, and 3.1 times higher than white females.
Individuals with IQs below 70 have been essentially exempted from the death penalty in the U.S. since 2002.