An intelligence quotient or IQ is a score derived from a set of standardized tests that were developed with the purpose of measuring a person's cognitive abilities ("intelligence") in relation to their age group. IQ tests do not measure intelligence the way a ruler measures height, but rather the way a race measures speed. Also, IQ tests measure actual performance, not innate potential. Among the middle class of industrial societies, IQ is highly heritable, and by adulthood the influence of family environment on IQ is undetectable. IQ test scores are correlated with measures of brain structure and function, as well as performance on simple tasks that anyone can complete within a few seconds. IQ is strongly correlated with academic success, but can also predict important life outcomes such as job performance, socioeconomic advancement, and "social pathologies". Recent work has demonstrated links between IQ and health, longevity, and functional literacy.


The modern field of IQ testing started with the Stanford-Binet test. Alfred Binet and his colleague Theodore Simon created the IQ test in 1905, aimed at identifying students who could benefit from extra help in school. Their assumption was that a lower IQ indicated the need for more teaching, not an inability to learn. This interpretation is still held by some modern experts. The term "intelligence quotient" comes from Binet's test, in which each student's score was the quotient of his or her tested academic age with his or her actual age.

Today, the most commonly administered IQ test is the WISC-III test, originally developed by David Wechsler in 1974. The WISC-III test comprises ten types of problems, categorized by difficulty and by skill type (verbal and performance scales). Another notable type of IQ test is the Bailey Scale of Infant Development, regarded as the 'best' means of testing cognitive development in infants.

Online Tests

Although online IQ tests have become wildly popular with the explosion of the internet in recent years, they are highly inaccurate. Comparing results among a large set of people shows a common factor: most scores are above 110. Of course, such tests automatically measure very few people in the 70 to 90 range, and hence create a strong upward distortion. Many of these websites do not show the results immediately and instead attempt to sell certificates showing the results.


It is reasonable to expect that genetic influences on traits like IQ should become less important as we gain experiences with age. Surprisingly, the opposite occurs. Heritability measures in infancy are as low as 20%, around 40% in middle childhood, and as high as 80% in adulthood.

Shared family effects also seem to disappear by adulthood. Adoption studies show that, after adolescence, adopted siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: monozygotic (identical) twins raised separately are highly similar in IQ (0.86), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adopted siblings (~0.0).

Most of the IQ studies described above were conducted in developed countries, such as the United States, Japan, and Western Europe. However, a few studies have been conducted in Moscow, East Germany, and India, and those studies produce similar results. Any such investigation is limited to describing the genetic and environmental variation found within the populations studied. This is a caveat of any heritability study.

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