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Figure 9.6 Distribution of IQ Scores in the General Population
The normal distribution of IQ scores in the general population shows that most people have about average
intelligence, while very few have extremely high or extremely low intelligence.
Extremely Low Intelligence
One end of the distribution of intelligence scores is defined by people with very low
IQ. Mental retardation is a generalized disorder ascribed to people who have an IQ below 70,
who have experienced deficits since childhood, and who have trouble with basic life skills, such
as dressing and feeding oneself and communicating with others (Switzky & Greenspan,
2006). [4] About 1% of the United States population, most of them males, fulfill the criteria for
mental retardation, but some children who are diagnosed as mentally retarded lose the
classification as they get older and better learn to function in society. A particular vulnerability
of people with low IQ is that they may be taken advantage of by others, and this is an important
aspect of the definition of mental retardation (Greenspan, Loughlin, & Black, 2001). [5] Mental
retardation is divided into four categories: mild, moderate, severe, and profound. Severe and
profound mental retardation is usually caused by genetic mutations or accidents during birth,
whereas mild forms have both genetic and environmental influences.
One cause of mental retardation is Down syndrome, a chromosomal disorder leading to mental
retardation caused by the presence of all or part of an extra 21st chromosome. The incidence of