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Decoding the concept of human intelligence

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IQ Graph of intelligence quotient (IQ) as a normal distribution with a mean of 100 and a standard deviation of 15. The shaded region between 85 and 115 (within one standard deviation of the mean) accounts for about 68 percent of the total area, hence 68 percent of all IQ scores.

IQ

psychology

Also known as: intelligence quotient

Written and fact-checked by The Editors of Encyclopaedia Britannica

Last Updated: Jan 11, 2025 • Article History

Related Topics:: human intelligence; psychological testing

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News •

A new study about fluoridated water and lowered IQs in kids is prompting debate • Jan. 10, 2025, 7:48 AM ET (Fortune)

Study Links High Fluoride Exposure to Lower I.Q. in Children • Jan. 9, 2025, 12:14 AM ET (New York Times)

Children exposed to higher fluoride levels have lower IQs, a government study finds • Jan. 7, 2025, 12:14 AM ET (CNN)

Lead pollution linked to lower IQs in Roman Empire: Study • Jan. 6, 2025, 9:23 AM ET (The Hill)

IQ, (from “intelligence quotient”), a number used to express the relative intelligence of a person. It is one of many intelligence tests.

IQ was originally computed by taking the ratio of mental age to chronological (physical) age and multiplying by 100. Thus, if a 10-year-old child had a mental age of 12 (that is, performed on the test at the level of an average 12-year-old), the child was assigned an IQ of 12/10 × 100, or 120. If the 10-year-old had a mental age of 8, the child’s IQ would be 8/10 × 100, or 80. Based on this calculation, a score of 100—where the mental age equals the chronological age—would be average. Few tests continue to involve the computation of mental ages. See also Lewis Terman; Alfred Binet.

This article was most recently revised and updated by Jeannette L. Nolen.