Intelligence test

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IQ tests are designed to give approximately this Gaussian distribution. Colors delineate one standard deviation. But the true frequency of low and high IQs is greater than that given by the Gaussian curve.

An intelligence quotient or IQ is a score derived from a set of standardized tests of intelligence. Intelligence tests come in many forms, and some tests use a single type of item or question. Most tests yield both an overall score and individual subtests scores. Regardless of design, all IQ tests measure the same general intelligence. Component tests are generally designed and chosen because they are found to be predictable of later intellectual development, such as educational achievement. IQ also correlates with job performance, socioeconomic advancement, and "social pathologies". Recent work has demonstrated links between IQ and health, longevity, and functional literacy. However, IQ tests do not measure all meanings of "intelligence", such as creativity. IQ scores are relative (like placement in a race), not absolute (like the measurement of a ruler).

History

In 1905, the French psychologist Alfred Binet published the first modern intelligence test, the Binet-Simon intelligence scale. His principal goal was to identify students who needed special help in coping with the school curriculum. Along with his collaborator Theodore Simon, Binet published revisions of his intelligence scale in 1908 and 1911, the last appearing just before his untimely death. In 1912, the abbreviation of "intelligence quotient" or I.Q., a translation of the German Intelligenz-quotient, was coined by the German psychologist William Stern.

A further refinement of the Binet-Simon scale was published in 1916 by Lewis M. Terman, from Stanford University, who incorporated Stern's proposal that an individual's intelligence level be measured as an intelligence quotient (I.Q.). Terman's test, which he named the Stanford-Binet Intelligence Scale formed the basis for one of the modern intelligence tests still commonly used today. They are all colloquially known as IQ tests.

IQ and general intelligence factor

Modern IQ tests produce scores for different areas (e.g., language fluency, three-dimensional thinking, etc.), wherein a summary score is calculated from the scores on each subsection. Scores on the individual subsections tend to correlate with one another, even when seemingly disparate in content.

Analyses of individuals' scores on the subtests of a single IQ test or the scores from a variety of different IQ tests (e.g., Stanford-Binet, WISC-R, Raven's Progressive Matrices Universal Nonverbal Intellience Test, and others) reveal that they all measure a single common factor and various factors that are specific to each test. This kind of factor analysis has led to the theory that underlying these disparate cognitive tasks is a single factor, termed the general intelligence factor (or g), that corresponds with the common-sense concept of intelligence. In the normal population, g and IQ are roughly 90% correlated and are often used interchangeably.

Practical validity

Evidence for the practical validity of IQ comes has been suggested through the examination of the correlation between IQ scores and economic and social indicators of valued life outcomes.

Economic and social correlates of IQ
Factors Correlation
School grades and IQ 0.5
Total years of education and IQ 0.55
IQ and parental socioeconomic status 0.33
Job performance and IQ 0.54
Negative social outcomes and IQ −0.2
IQs of identical twins 0.86
IQs of husband and wife 0.4
Heights of parent and child 0.47
Economic and social correlates of IQ in the USA
IQ <75 75–90 90–110 110–125 >125
US population distribution 5 20 50 20 5
Married by age 30 72 81 81 72 67
Out of labor force more than 1 month out of year (men) 22 19 15 14 10
Unemployed more than 1 month out of year (men) 12 10 7 7 2
Divorced in 5 years 21 22 23 15 9
% of children w/ IQ in bottom decile (mothers) 39 17 6 7 < 1
Had an illegitimate baby (mothers) 32 17 8 4 2
Lives in poverty 30 16 6 3 2
Ever incarcerated (men) 7 7 3 1 < 1
Chronic welfare recipient (mothers) 31 17 8 2 < 1
High school dropout 55 35 6 0.4 < 0.4
Values are the percentage of each IQ sub-population, among non-Hispanic whites only, fitting each descriptor. Compiled by Gottfredson (1997) from a US study by Herrnstein & Murray (1994) pp. 171, 158, 163, 174, 230, 180, 132, 194, 247–248, 194, 146 respectively.

Research shows that general intelligence plays an important role in many valued life outcomes. In addition to academic success, IQ correlates with job performance (see below), socioeconomic advancement (e.g., level of education, occupation, and income), and "social pathology" (e.g., adult criminality, poverty, unemployment, dependence on welfare, children outside of marriage). Recent work has demonstrated links between general intelligence and health, longevity, and functional literacy. Correlations between g and life outcomes are pervasive, despite the fact that IQ and happiness are not correlates. IQ and g correlate highly with school performance and job performance, moderately with income, and to a small degree whether or not the individual will abide by the law.

General intelligence whhich in the literature is typically called "cognitive ability" is the best predictor of job performance by the standard measure of validity. Validity is the correlation between score (in this case cognitive ability, as measured, typically, by a paper-and-pencil test) and outcome (in this case job performance, as measured by a range of factors including supervisor ratings, promotions, training success, and tenure), and ranges between −1.0 (the score is perfectly wrong in predicting outcome) and 1.0 (the score perfectly predicts the outcome). See validity (psychometric). The validity of cognitive ability for job performance tends to increase with job complexity and varies across different studies, ranging from 0.2 for unskilled jobs to 0.8 for the most complex jobs.

A meta-analysis (Hunter and Hunter, 1984) which pooled validity results across many studies encompassing thousands of workers (32,124 for cognitive ability), reports that the validity of cognitive ability for entry-level jobs is 0.54, larger than any other measure including job tryout (0.44), experience (0.18), interview (0.14), age (−0.01), education (0.10), and biographical inventory (0.37).

Because higher test validity allows more accurate prediction of job performance, companies have a strong incentive to use cognitive ability tests to select and promote employees. IQ thus has high practical validity in economic terms. The utility of using one measure over another is proportional to the difference in their validities, all else equal. This is one economic reason why companies use job interviews (validity 0.14) rather than randomly selecting employees (validity 0.0).

However, the US Civil Rights Act, stemming from the 1971 United States Supreme Court decision Griggs v. Duke Power Co., have prevented American employers from using cognitive ability tests as a controlling factor in selecting employees where (1) the use of the test would have a disparate impact on hiring by race and (2) where the test is not shown to be directly relevant to the job at issue. Instead, where there is no direct relevance to the job or class of jobs at issue, tests have only been legally permitted to be used in conjunction with a subjective appraisal process. The U.S. military uses the Armed Forces Qualifying Test (AFQT), as higher scores correlate with significant increases in effectiveness of both individual soldiers and units.[1] [2] Microsoft is known for using non-illegal tests that correlate with IQ tests as part of the interview process, weighing the results even more than experience in many cases.[3] [4]

Some researchers have echoed the popular claim that "in economic terms it appears that the IQ score measures something with decreasing marginal value. It is important to have enough of it, but having lots and lots does not buy you that much." (Detterman and Daniel, 1989)[5]

However, some studies suggest IQ continues to confer large benefits at very high levels. Ability and performance for jobs are linearly related, such that at all IQ levels, an increase in IQ translates into a concomitant increase in performance (Coward and Sackett, 1990). In an analysis of hundreds of siblings, it was found that IQ has a substantial effect on income independently of family background (Murray, 1998).

Other studies question the real-world importance of whatever is measured with IQ tests, especially for differences in accumulated wealth and general economic inequality in a nation. IQ correlates highly with school performance but the correlations decrease the closer one gets to real-world outcomes, like with job performance, and still lower with income. It explains less than one sixth of the income variance [6]. Even for school grades, other factors explain most the variance. One study found that, controlling for IQ across the entire population, 90 to 95 percent of economic inequality would continue to exist. [7]. Another recent study (2002) found that wealth, race, and schooling are important to the inheritance of economic status, but IQ is not a major contributor and the genetic transmission of IQ is even less important [8]. Some argue that IQ scores are used as an excuse for not trying to reduce poverty or otherwise improve living standards for all. Claims of low intelligence has historically been used to justify the feudal system and unequal treatment of women, nowadays, many studies find identical average IQs among men and women. In contrast, others claim that the refusal of high-IQ elites to take IQ seriously as a cause of inequality is itself immoral.[9]

Genetics versus environment

The role of genes and environment (nature vs. nurture) in determining IQ is reviewed in Plomin et al. (2001, 2003). The degree to which genetic variation contributes to observed variation in a trait is measured by a statistic called heritability. Heritability scores range from 0 to 1, and can be interpreted as the percentage of variation (e.g. in IQ) that is due to variation in genes. Twins studies and adoption studies are commonly used to determine the heritability of a trait. Until recently heritability was mostly studied in children. These studies find the heritability of IQ is approximately 0.5; that is, half of the variation in IQ among the children studied was due to variation in their genes. The remaining half was thus due to environmental variation and measurement error. A heritability of 0.5 implies that IQ is "substantially" heritable. Studies with adults show that they have a higher heritability of IQ than children do and that heritability could be as high as 0.8. The American Psychological Association's 1995 task force on "Intelligence: Knowns and Unknowns" concluded that within the White population the heritability of IQ is "around .75" (p. 85).[10]

Environment

Environmental factors play a large role in determining IQ in extreme situations. Proper childhood nutrition appears critical for cognitive development; malnutrition can lower IQ. Other research indicates environmental factors such as prenatal exposure to toxins, duration of breastfeeding, and micronutrient deficiency can affect IQ. In the developed world, there are some family effects on the IQ of children, accounting for up to a quarter of the variance. However, by adulthood, this correlation disappears, so that the IQ of adults living in the prevailing conditions of the developed world may be more heritable.

Nearly all personality traits show that, contrary to expectations, environmental effects actually cause adoptive siblings raised in the same family to be as different as children raised in different families (Harris, 1998; Plomin & Daniels, 1987). Put another way, shared environmental variation for personality is zero, and all environmental effects would be nonshared. Conversely, IQ is actually an exception to this, at least among children. The IQs of adoptive siblings, who share no genetic relation but do share a common family environment, are correlated at .32. Despite attempts to isolate them, the factors that cause adoptive siblings to be similar have not been identified. However, as explained below, shared family effects on IQ disappear after adolescence.

Active genotype-environment correlation, also called the "nature of nurture", is observed for IQ. This phenomenon is measured similarly to heritability; but instead of measuring variation in IQ due to genes, variation in environment due to genes is determined. One study found that 40% of variation in measures of home environment are accounted for by genetic variation. This suggests that the way human beings craft their environment is due in part to genetic influences.

A study of French children adopted between the ages of 4 and 6 shows the continuing interplay of nature and nurture. The children came from poor backgrounds with I.Q.’s that initially averaged 77, putting them near retardation. Nine years later after adoption, they retook the I.Q. tests, and all of them did better. The amount they improved was directly related to the adopting family’s status. "Children adopted by farmers and laborers had average I.Q. scores of 85.5; those placed with middle-class families had average scores of 92. The average I.Q. scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98."[11] This study suuggests that IQ is not stable over the course of ones lifetime and that, even in later childhood, a change in enviornment can have a significant effect on IQ.

Genetics

It is reasonable to expect that genetic influences on traits like IQ should become less important as one gains 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.[1]

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: monzygotic or identical twins raised separately are highly similar in IQ (0.86), more so than fraternal or dizygotic twins raised together (0.6) and much more than adopted siblings (~0.0).[2]

IQ correlations

Race and IQ

While IQ scores of individual members of different racial or ethnic groups are distributed across the IQ scale, groups vary in where their members cluster along the IQ scale. Ashkenazi Jews and East Asians cluster higher than Europeans, while Hispanics and Sub-Saharan Africans cluster lower.[3] Much research has been devoted to the extent and potential causes of racial-ethnic group differences in IQ, and the underlying purposes and validity of the tests has been examined. Most experts conclude that examination of many types of test bias and simple differences in socioeconomic status have failed to explain the IQ clustering differences.[4] For a summary of expert opinions, see Race and Intelligence.

The findings in this field are often thought to conflict with fundamental social philosophies, and have engendered a large controversy.[5]

Religiosity and IQ

Several studies have investigated the relationship between intelligence and the degree of religious belief (excluding humanist faith), with most showing that intelligence averages decrease significantly with the "importance of religion" as self-reporters by the testee. Many studies illustrate the same results despite when and where they were conducted. Charles Murray, author of the infamous The Bell Curve, chronicled the attitudes and beliefs of the Elites (those who have a high IQ) in regards to religion spanning from Ancient times to the Modern Day and across the globe in another book entitled Human Accomplishment. He wrote that Elites in the First World, post-1950 era were more hostile in their attitudes toward religion.

Health and IQ

Persons with a higher IQ have generally lower adult morbidity and mortality. This may be because they better avoid injury and take better care of their own health, or alternatively may be due to a slight increased propensity for material wealth (see above). Post-Traumatic Stress Disorder, severe clinical depression, and schizophrenia are less prevalent in those with higher IQ as these disorders greatly affect participant's concentration. Despite this, however, higher IQ shows a higher prevalence of those suffering from Obsessive Compulsive Disorder [12].

Research in Scotland has shown that a 15-point lower IQ meant people had a fifth less chance of seeing their 76th birthday, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long [13].

Controversy

The controversy over intelligence quotient (IQ) tests. Deciding what they measure, and what this means for society has not abated since their initial development by Alfred Binet at the beginning of the twentieth century.

IQ tests rely largely upon symbolic logic (etc.) as a means to scoring. Since symbolic logic is not inherently synonymous with intelligence, the question remains as to exactly what is being measured. For instance, it is feasible that someone could possess a high degree of emotional intelligence even though they may have difficulty comprehending the significance of sequentially arranged shapes. Additionally, someone who cannot read would be at a significant disadvantage on an IQ test and illiteracy is not indicative of limited cognitive ability. Measurements of other forms of "intelligence" have been proposed to augment the current IQ Testing Methodology, though such alternative measurements are also subject to debate.

One key issues in the debate involves the semantics of defining intelligence itself. Some proponents of IQ testing argue that lower scores by certain groups justify cutting back on welfare and other financial aid programs like Head Start and New Deal. Many claim that different IQ scores demonstrate that power and wealth will always be distributed unequally. Critics claim that IQ tests do not measure intelligence, but rather a specific skill set that is valued by those who create IQ tests. Some researchers have concluded from twin and adoption studies that IQ has high heritability. This is often interpreted by the general public as meaning that there is an immutable genetic factor affecting or determining intelligence. This hereditarian interpretation fuels much of the controversy over books such as The Bell Curve, which claimed that various racial groups have lower or higher group intelligence than other racial and ethnic groups (East Asians and Ashkenazi Jews, according to The Bell Curve, are slightly more intelligent than Whites, whereas African Americans have slightly lower IQs) and suggested changing public policy as a result of these findings. It is salient that the degree to which nature versus nurture debate on the development of human traits, especially intelligence, is one of the most intractable scholarly controversies of modern times.


Notes

  1. ↑ Plomin et al. (2001, 2003)
  2. ↑ Plomin et al. (2001, 2003)
  3. ↑ Gottfredson et al. 1994 (ctrl+f "groups")
  4. ↑ Neisser et al. 1995
  5. ↑ The controversy itself has caused some scientists to debate whether such areas of science are inappropriate, or whether group differences in traits are just another area of the science of human nature, as Steven Pinker and others argue. (See Race and intelligence#Utility of research and racism.)


References
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