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Chapter 11: Intelligence


11.1 History and Development of Intelligence Tests

  • While there is no universally accepted definition of human intelligence, we can define it as the ability to acquire, retain, and apply knowledge in many different spheres of life.
  • The first intelligence test were the Binet–Simon scale developed in 1904 to identify French schoolchildren who might need special assistance in their education.
  • The first intelligence tests for adults were the U.S. Army’s Army Alpha test, to screen draftees for potential officers, and the Army Beta test, designed for illiterate men, to determine if a man was too unintelligent to serve.
  • The Stanford–Binet test provided an intelligence quotient (IQ) by dividing a child’s mental age by his or her chronological age and multiplying by 100. Hence the average IQ, when mental age and chronological age are equal, is 100.
  • The most common IQ tests today are the Wechsler Intelligence Scale for Children (WISC) and the Wechsler Adult Intelligence Scale (WAIS). Like the Stanford–Binet, these are individually administered tests that have been standardized to make the examination as similar as possible for all participants.
  • The WISC and WAIS are normalized to reflect the average performance and a normal range of variability. Normalized IQ scores indicate a participant’s performance relative to that of other people of the same age.
  • The average IQ score today is still 100, with a standard deviation of 15 points.
  • Measurements and scores that fall into a bell-shaped normal distribution can be described based on their percentile rankings.
  • IQ tests and the closely related achievement tests are very reliable; in other words, they give consistent results, whether people are tested on two different occasions (test–retest reliability), tested by two different examiners (scoring reliability), or given two different versions of the test (cross-test reliability).
  • A test on which scores are consistent across the first and second halves is said to have split-half reliability.
  • A good psychological test must have validity, meaning it actually measures what it is intended to measure. While the different IQ tests display convergent validity, meaning they all give roughly the same evaluation of subjects, it is more difficult to assess their content validity, the extent to which they assess all the various facets of intelligence. This difficulty primarily stems from disagreement about what intelligence truly is.
  • IQ tests display good criterion-related validity, meaning they offer a statistical prediction of people’s performance on several criteria that we would expect to reflect intelligence. For example, IQ scores are correlated with performance in school and in the workplace.
  • IQ scores also correlate with health and longevity, which suggests that environmental and genetic factors that impair body function may also impair intelligence. People with very low IQs often have more serious health issues than their siblings with normal IQs.
  • Discounting people with very low IQs, there is little correlation between IQ and overall happiness.
  • All “predictions” based on IQ scores apply only to large groups of subjects. It is not possible to accurately predict how any one individual will fare in school, the workplace, or life.

11.2 The Many Facets of Intelligence

  • IQ tests measure the ability to solve problems using abstract reasoning. People who do well on one part of an IQ test tend to do well on the other parts of the test.
  • Debate continues concerning whether IQ tests truly measure generalized intelligence (g factor), and whether there is such a thing as generalized intelligence.
  • Psychologists distinguish between fluid intelligence (gF), the ability to reason logically to solve new problems, and crystallized intelligence (gC), specific knowledge that a person has acquired and the ability to apply that knowledge.
  • Some researchers have suggested that fluid intelligence may represent some sort of “processing speed” that can be assessed by measuring reaction time, the amount of time it takes someone to detect a signal and make an appropriate response.
  • Fluid intelligence, which is typically assessed by how quickly a person can solve a problem, peaks in young adulthood and then declines, but crystallized intelligence declines more slowly.
  • Several psychologists have suggested that there are multiple intelligences, and that a person may be significantly more intelligent in one category than another. Robert Sternberg suggests there are three intelligences: analytical intelligence, practical intelligence, and creative intelligence.
  • Howard Gardner proposes even more kinds of intelligence, including musical intelligence, bodily kinesthetic intelligence, interpersonal and intrapersonal intelligence, and naturalist intelligence.
  • Intellectual disabilities (mental disabilities) are a diverse group of lifelong conditions that impair intellectual development. By convention they are considered to reflect an IQ of less than 70.
  • Several genetic conditions, such as fragile X syndrome, Down syndrome, and phenylketonuria, impair intellectual development, as do several environmental conditions such as fetal alcohol syndrome, exposure to the environmental toxin lead, and iodine deficiency.
  • Autism is a disorder characterized by severely impaired social interaction, which may include impaired verbal behavior and low IQ scores. A small minority of people with autism also display savant syndrome, having extraordinary ability in some limited field, such as memory, mathematics, or music, accompanied by mental disability in other areas of life.
  • IQ tests must be periodically adjusted to keep the average score at 100 because of a steady increase in average test performance over time, a trend called the Flynn effect.
  • We don’t know what is causing the Flynn effect, but one factor could be technological progress, especially the widespread availability of media that exposes people to the type of abstract reasoning that is measured by IQ tests.

11.3 The Controversy over Group Differences in IQ

  • The more closely related people are, the more likely they are to have similar IQ scores, suggesting that genes influence IQ. For example, the correlation of IQ scores is greater among monozygotic (MZ) twins, who are genetically identical, than among dizygotic (DZ) twins, who share only half of their genes.
  • The heritability of IQ is approximately 0.50, which means that about half of the variability in IQ scores in the population is due to variability in the genes.
  • Several environmental factors, such as lead, fetal exposure to alcohol, and family size, can affect IQ.
  • The interaction of genes and the environment can be seen in babies who inherit phenylketonuria (PKU); such babies can avoid becoming mentally disabled despite this genetic defect if they do not eat phenylalanine.
  • Average IQ differences among races have been consistently seen but these are population-level differences; so you cannot accurately estimate an individual’s IQ by knowing his or her race. The 0.50 heritability estimate for IQ means that those racial differences in IQ measurement may be due either to genetic differences or to environmental differences, or to both.
  • Several observations suggest that the gap in average IQ score between African Americans and white Americans may be due to environmental differences. The gap became considerably smaller in the twentieth century, as social reforms provided black Americans with better education and less discrimination. Socioeconomic factors, such as living in poverty, have been associated with differences in IQ scores among races.
  • Children adopted out of impoverished families tend to develop higher IQs than their biological parents, a reflection of a gene– environment interaction. Paradoxically, estimates of the heritability of IQ are nearly 0 among impoverished Americans, suggesting that impoverished conditions do not allow individuals to develop the full IQ that their genes would allow them if they were in middle-income conditions.
  • Early intervention programs, such as Head Start, consistently raise the IQ of impoverished children. It is not known why in some cases improved IQ “fades” after a child enters public school; it could be due to the poor quality of some public schools in impoverished communities.
  • The performance of African Americans on IQ tests is subject to stereotype threat, in which exposure to derogatory stereotypes about a particular group affects the performance of members of that group.

Think Like a Psychologist:
Principles in Action


The mind is a product of a physical machine, the brain.

There is no doubt that genes play a role in intelligence. For Edwardian England, it was obvious that the upper class members were more intelligent than the lower classes because of “breeding,” the supposedly superior hereditary endowment of the noble class.

We are consciously aware of only a small part of our mental activity.

Most of us are unaware of how our concept of intelligence is shaped by our culture. Lord Loam never considered practical knowledge of any value until his life depended on it. Our current ideas of intelligence are shaped by a society that values verbal ability and economic achievement.

We constantly modify our behavior, beliefs, and attitudes according to what we perceive about the people around us.

In the Edwardian era, the definition of intelligent depended on your social stratum. In the upper classes, a sharp wit was considered intelligent, whereas in the service class, manual dexterity and problem-solving ability was considered intelligent. The definition of intelligence will always depend upon society’s values.

Our experiences physically alter the structure and function of the brain.

In fact, any differences in abilities between Crichton and his employers were unlikely to have anything to do with heredity. Rather, Crichton had a long history of experience in manual tasks and problem solving that Lord Loam and his family had never acquired.

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