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Extremes of Intelligence: Retardation and Giftedness
The results of studies assessing the measurement of intelligence show that IQ is distributed in the
population in the form of anormal distribution (or bell curve), which is the pattern of scores
usually observed in a variable that clusters around its average. In a normal distribution, the bulk
of the scores fall toward the middle, with many fewer scores falling at the extremes. The normal
distribution of intelligence (Figure 9.6 "Distribution of IQ Scores in the General Population")
shows that on IQ tests, as well as on most other measures, the majority of people cluster around
the average (in this case, where IQ = 100), and fewer are either very smart or very dull. Because
the standard deviation of an IQ test is about 15, this means that about 2% of people score above
an IQ of 130 (often considered the threshold forgiftedness), and about the same percentage score
below an IQ of 70 (often being considered the threshold for mental retardation).
Although Figure 9.6 "Distribution of IQ Scores in the General Population"presents a single
distribution, the actual IQ distribution varies by sex such that the distribution for men is more
spread out than is the distribution for women. These sex differences mean that about 20% more
men than women fall in the extreme (very smart or very dull) ends of the distribution (Johnson,
Carothers, & Deary, 2009). [2] Boys are about five times more likely to be diagnosed with the
reading disability dyslexia than are girls (Halpern, 1992), [3] and are also more likely to be
classified as mentally retarded. But boys are also about 20% more highly represented in the
upper end of the IQ distribution.