Intelligence Quotient (IQ)

Intelligence quotient (IQ) is a measurement of intelligence based on scores derived from standardized testing.

Although intelligence quotient (IQ) tests are still widely used in the United States, there has been increasing doubt voiced about their ability to measure the mental capacities that determine success in life. IQ testing has also been criticized for being biased with regard to race and gender.

The first modern scientist to test mental ability was Alfred Binet (1857–1911), a French psychologist who devised an intelligence test for children in 1905, based on the notion that intelligence could be expressed in terms of age. Binet created the concept of mental age and believed that the test performance of a child of average intelligence would match his or her age. According to Binet, a gifted child's performance would be on par with an older child and a slow learner's abilities would be equal to those of a younger child. Binet's test was introduced to the United States in a modified form in 1916 by Lewis M. Terman (1877–1956), a professor at Stanford University. The scoring system of the new test, devised by German psychologist William Stern (1871–1938), consisted of dividing a child's mental age by his or her chronological age and multiplying the quotient by 100 to arrive at an intelligence quotient (IQ). This number would equal 100 in a person of average ability.

The Wechsler Intelligence Scales were developed in 1949 by David Wechsler (1896–1981), chief psychologist at Bellevue Psychiatric Hospital in New York. This test raised an issue that still provokes criticism of IQ tests today: different types of intelligence. The Wechsler scales replaced the single mental age score with a verbal scale and a performance scale for nonverbal skills. The scales were meant to address each test-taker's individual combination of strengths and weaknesses.

The Stanford-Binet and Wechsler tests (in updated versions) remain the most widely administered IQ tests in the United States. Average performance at each age level is still assigned a score of 100, but today's scores are calculated solely by comparison with the performance of others in the same age group rather than with test-takers of various chronological ages. Among the general population, scores cluster around 100 and gradually decrease in either direction in a pattern known as the normal distribution (or bell-shaped) curve.

Data from hundreds of family studies provides primary supporting evidence that IQ is shaped more by heredity (genes) than environment. The correlation percentage of IQ tests within family relationships is as follows:

At the lower end of the bell-shaped curve are those groups of people defined as intellectually disabled (formerly known as mentally retarded). Intellectual disability is known to be familial; some cases are thought to be due to chromosomal abnormalities or organic brain damage. The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders Fifth edition (DSM-5), defines and describes IQ score ranges among the intellectually disabled as follows:

Although IQ scores are good predictors of academic achievement in elementary and secondary school, the correspondence between IQ and academic performance is less consistent at higher levels of education, and many have questioned the ability of IQ tests to predict success later in life. The tests do not measure many of the qualities necessary for achievement in the world of work, such as persistence, self-confidence, motivation, and interpersonal skills, or the ability to set priorities and to allocate one's time and effort efficiently. In addition, the creativity and intuition responsible for great achievements in both science and the arts are not evaluated in IQ tests. For example, creativity often involves the ability to envision multiple solutions to a problem (educators call this divergent thinking); in contrast, IQ tests require the choice of a single answer or solution to a problem, a type of task that could penalize highly creative people. In spite of this well-known limitation of IQ tests, such companies as Microsoft use their own versions of intelligence tests when evaluating potential employees.

One interesting finding has shown that IQ appears to have a positive correlation with health and longevity as well as income and accomplishment in later life. One group of British researchers reported in 2004 that the results of IQ testing of adults in late middle age predicted the onset of Alzheimer's disease more accurately than the possession of a gene known to be associated with Alzheimer's. Another group in Scotland reported that childhood IQ is a significant factor among the variables that can be used to predict age at death; people with higher IQs live longer than those with average IQs.

The value of IQ tests has also been called into question by theories that define intelligence in ways that transcend the boundaries of tests chiefly designed to measure abstract reasoning and verbal comprehension. For example, Robert J. Sternberg's triarchical model addresses not only internal thought processes but also the ways in which they operate in relation to past experience and to the external environment. Howard Gardner, a psychologist at Harvard University, has posited a theory of multiple intelligences that includes a number of different types of intelligence, including linguistic and logical-mathematical (the types measured by IQ tests); spatial; interpersonal (ability to deal with other people); intrapersonal (insight into oneself); musical; and bodily-kinesthetic (athletic ability).

Many educators still maintain that IQ tests are unfair to members of minority groups because they are based on the vocabulary, customs, and values of the mainstream, or dominant, culture. Some observers have cited cultural bias in testing to explain the fact that, on average, African Americans and Hispanic Americans score lower than European Americans on IQ tests. (Asian Americans, however, score higher than European Americans.) A new round of controversy was ignited with the 1994 publication of The Bell Curve: Intelligence and Class Structure in American Life, by Richard Herrnstein and Charles Murray, who explored the relationship between IQ, race, and such pervasive social problems as unemployment, crime, and illegitimacy. Given the proliferation of theories about the nature of intelligence, many psychologists have disagreed with Herrnstein and Murray's central assumptions that intelligence is measurable by IQ tests, that it is genetically based, and that a person's IQ essentially remains unchanged over time. From a sociopolitical viewpoint, critics have taken issue with The Bell Curve and its arguments about the genetic nature of intelligence to cast doubt on the power of government to remedy many of the nation's most pressing social problems. In 1995, the American Psychological Association formed a task force to examine the implications of Herrnstein and Murray's work. The report, titled “Intelligence: Knowns and Unknowns,” was published in the APA's official journal in 1996.

Another explosive issue in the early 2000s was the significance of differences between men and women on IQ tests. While men and women have the same average IQ, women tend to score higher on tests of verbal knowledge and memory, while men tend to score higher on tests of spatial aptitude and mathematical ability. Neuroimaging studies have confirmed that men and women activate different parts of the brain when performing various tests of general intelligence. There is a general consensus that these differences in brain design result in equivalent intellectual performance; they do not indicate that one sex is “brighter” than the other. Men's IQ scores also display a wider variance than women's; that is, there are more men than women at the upper and lower extremes of the bell-shaped curve. Although these differences refer to large groups and not to individuals, the topic is sensitive enough, particularly within college and university faculties, to generate considerable controversy. The president of Harvard University, Lawrence Summers, was forced to resign in 2006 after a year-long dispute over a lecture in which he raised the possibility that women are underrepresented at the highest levels of achievement in such fields as engineering and mathematics because of innate differences in intellectual preferences between men and women. Summers was misunderstood by many as suggesting that women on average were less intelligent than men.

The U.S. government makes use of IQ testing in the selection of officer candidates in the armed forces. As part of the Armed Services Vocational Aptitude Battery (ASVAB), potential recruits take the Armed Forces Qualifying Test (AFQT). The results from the AFQT are used to determine if an individual is qualified to join the military. As of 2015, the cutoff score for Army and Marine Corps volunteers is 31; for Navy, 35; for Air Force, 36; and for Coast Guard, 45. Those with lower scores are considered disqualified for military service.

Another area in which IQ tests are of interest to the government is forensic evaluation of people convicted of capital crimes. In 2002, the Supreme Court ruled in Adkins v. Virginia that the execution of the intellectually disabled constitutes cruel and unusual punishment, which is prohibited by the Eighth Amendment to the Constitution. This was further explained in the 2014 Supreme Court decision in Hall v. Florida. Lastly, the U.S. government uses IQ scores as a basis for deciding claims for Social Security Disability benefits, typically giving benefits to those with intellectual disability.


The act or process of knowing or perceiving.
The abilityto learn or understand things or to deal with new or difficult situations.
Intelligence quotient (IQ)—
A number that represents a person's ability to reason as compared to the statistical norm or average for their age, derived from a standardized test designed to assess intelligence. An IQ score of 100 is considered average intelligence.
Stanford-Binet Intelligence Scales—
A device designed to measure a person's intelligence, obtained through a series of aptitude tests concentrating on different aspects of intellectual functioning.
Wechsler Intelligence Scales—
An intelligence test that uses a verbal scale and a performance scale for nonverbal skills to address each test-taker's individual combination of strengths and weaknesses.

A variety of environmental factors have been cited as possible explanations for the Flynn effect, including expanded opportunities for formal education that have given children throughout the world more and earlier exposure to some types of questions they are likely to encounter on an IQ test (although IQ gains in areas such as mathematics and vocabulary, which are most directly linked to formal schooling, have been more modest than those in nonverbal areas). Exposure to printed texts and electronic technology has been cited as an explanation for improved familiarity with the types of maze and puzzle questions that have generated the greatest changes in scores. Improved mastery of spatial relations has also been linked to the use of video games. Other environmental factors mentioned in connection with the Flynn effect include better health and nutrition and changes in parenting styles.

Research has shown that different types of intelligence exist and not all are reflected by IQ scores. Because of this, parents should not be overly concerned with how high a child's IQ is above the average. Children with average and above average IQs typically can learn and acquire life skills on their own. However, if a child has a low IQ (less than 70), the child is considered intellectually disabled and will need support throughout his or her life. Parents should contact their physician for resources to test their child's intelligence.

See also Intellectual disability ; Stanford-Binet Intelligence Scales ; Wechsler Intelligence Scales .



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