Treatments of modern measures of intelligence often begin with a discussion of the French psychologist ALFRED BINET (1857–1911). In 1905, Binet initiated the applied mental measurement movement when he introduced the first intelligence test. In response to a turn-of-the-century law in France requiring that children of subnormal mental ability be placed in special programs (rather than be expelled from school), Binet was called upon to design a test that could identify these children. Binet's first test consisted of thirty items, most of which required some degree of comprehension and reasoning. For example, one task required children to take sentences in which words were missing and supply the missing words that made sense in context (such sentence-completion tasks are still used widely). Binet grouped his test items such that the typical child of a given age group was able to answer fifty percent of the questions correctly. Individuals of similar chronological age (CA) varied widely in their scale scores, or mental age (MA). The ratio of MA to CA determined one's level of mental development; this ratio was later multiplied by 100 to calculate what is now known as the intelligence quotient (IQ).
Binet's approach was successful: children's scores on his test forecasted teacher ratings and school performance. While Binet was developing this first test of general intellectual functioning, the English psychologist Charles Spearman (1863–1945) was conducting research to identify the dominant dimension responsible for the validity of the test's predictions.
Spearman was the first to propose and offer tangible support for the idea that a psychologically cohesive dimension of general intelligence, g, underlies performance on any set of items demanding mental effort. Spearman showed that g appears to run through all heterogeneous collections of intellectual tasks and test items. He demonstrated that when heterogeneous items are all lightly positively correlated and then summed, the signal carried by each is successively amplified and the noise carried by each is successively attenuated.
Modern versions of intelligence tests index essentially the same construct that was uncovered at the turn of the twentieth century by Spearman, but with much more efficiency. For example, g is a statistical distillate that represents approximately half of what is common among the thirteen subtests comprising the Wechsler Adult Intelligence Scale. As noted by intelligence researcher Ian J. Deary, the attribute g represents the research finding that "there is something shared by all the tests in terms of people's tendencies to do well, modestly, or poorly on all of them." This "tendency" is quite stable over time. In 2001, Deary's team published a study that was the longest temporal stability assessment of general intelligence, testing subjects at the age of eleven and a second time at the age of seventy-seven. They observed a correlation of 0.62, which rose to over 0.70 when statistical artifacts were controlled.
Psychometricians have come to a consensus that mental abilities follow a hierarchical structure, with g at the top of the hierarchy and other broad groups of mental abilities offering psychological import beyond g. Specifically, mathematical, spatial-mechanical, and verbal reasoning abilities all have demonstrated incremental (additional) validity beyond g in forecasting educational and vocational outcomes.
Research on general intelligence has confirmed the validity of g for forecasting educational and occupational achievement. Empiricism also has documented general intelligence's network of relationships with other socially important outcomes, such as aggression, crime, and poverty. General intellectual ability covaries 0.70–0.80 with academic achievement measures, 0.40–0.70 with military training assignments, 0.20–0.60 with work performance (higher correlations reflect greater job complexity), 0.30–0.40 with income, and around 0.20 with obedience to the law. Measures of g also correlate positively with altruism, sense of humor, practical knowledge, social skills, and supermarket shopping ability, and correlate negatively with impulsivity, accident-proneness, delinquency, smoking, and racial prejudice. This diverse family of correlates reveals how individual differences in general intelligence influence other personal characteristics.
Experts' definitions of general intelligence fit with g's nexus of empirical relationships. Most measurement experts agree that measures of general intelligence assess individual differences pertaining to abstract thinking or reasoning, the capacity to acquire knowledge, and problem-solving ability. Traditional measures of general intelligence and standard academic achievement tests both assess these general information-processing capacities. In 1976, educational psychologist Lee Cronbach noted: "In public controversies about tests, disputants have failed to recognize that virtually every bit of evidence obtained with IQs would be approximately duplicated if the same study were carried out with a comprehensive measure of achievement" (1976, p. 211, emphasis in original).
Both genetic and environmental factors contribute to the individual differences observed in intelligence. The degree to which individual differences in intelligence are genetically influenced is represented by an estimate of heritability, the proportion of observed variation in intelligence among individuals that is attributable to genetic differences among the individuals. By pooling various family studies of g (e.g., identical and fraternal twins reared together or apart), the heritability of general intelligence in industrialized nations has been estimated to be approximately 40 percent in childhood and between 60 and 80 percent in adulthood. This pattern is thought to reflect the tendency of individuals, as they grow older and more autonomous, to increasingly self-select into environments congruent with their unique abilities and interests.
Environmental contributions to individual differences in intelligence are broadly defined as all non-genetic influences. Shared environmental factors, such as socioeconomic status and neighborhood context, are those that are shared by individuals within a given family but differ across families; non-shared environmental factors, such as the mentoring of a special teacher or one's peer group, are those that are generally unique to each individual within a family. The majority of environmental influences on intelligence can be attributable to non-shared factors for which the specifics, thus far, are not well known. Family studies of intelligence have consistently documented that the modest importance of shared environmental influences in early childhood, approximately 30 percent, decreases to essentially zero by adulthood.
The above empiricism is widely accepted among experts in the fields of measurement and individual differences. Yet research pertaining to general intelligence invariably generates controversy. Because psychological assessments are frequently used for allocating educational and vocational opportunities, and because different demographic groups (such as those based on socioeconomic status or race) differ in test scores and criterion performance, social concerns have accompanied intellectual assessment since its beginning. Because of these social concerns, alternative conceptualizations of intelligence, such as Howard Gardner's theory of multiple intelligences and Robert Sternberg's triarchic theory of intelligence, have generally been received positively by the public. Measures of these alternative formulations of intelligence, however, have not demonstrated incremental validity beyond what is already gained by conventional measures of intelligence. That is, they have not been shown to account for any more variance in important life outcomes (such as academic achievement and job performance) than that already accounted for by conventional intelligence tests.
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