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An abstract concept whose definition continually evolves and often depends upon current social values as much as scientific ideas. Modern definitions refer to a variety of mental capabilities, including the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experience as well as the potential to do so.

Several theories about intelligence emerged in the 20th century and with them debate about the nature of intelligence, whether it is hereditary, environmental or both. As methods developed to assess intelligence, theorizing occurred about the measurability of intelligence, its accuracy and this field known as psychometrics. As the 20th century drew to a close, publication of The Bell Curve by Richard J. Herrnstein and Charles Murray in 1994 stirred the controversy. Their findings pointed to links between social class, race, and IQ...