The following has been adapted from the Wikipedia website.
For people living in the prevailing conditions of the developed world, IQ is highly heritable, and by adulthood the influence of family environment on IQ is undetectable.
That is, significant variation in IQ between adults can be attributed to genetic variation, with the remaining variation attributable to environmental sources that are not shared within
families. In the United States, marked variation in IQ occurs within families, with siblings differing on average by around 12 points. On average, IQ scores are stable over a person's
lifetime, but some individuals undergo large changes. For example, scores can be affected by the presence of learning disabilities.
The definition of IQ
Originally, IQ was calculated with the formula. A 10-year-old who scored as high as the average 13-year-old, for example, would have an IQ of 130 {100 + (13/10)}. Because
this formula only worked for children, it was replaced by a projection of the measured rank on the Gaussian bell curve with a center value (average IQ) of 100, and a standard deviation
of 15 or occasionally 16. Thus the modern version of the IQ is a mathematical transformation of the rank (see quartile, percentile, percentile rank), which is the primary result of an
IQ test. To differentiate the two scores, modern scores are sometimes referred to as "deviance IQ", while the age-specific scores are referred to as "ratio IQ".
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.
In 1939 David Wechsler published the first intelligence test explicitly designed for an adult population, the Wechsler Adult Intelligence Scale, or WAIS. Since publication
of the WAIS, Wechsler extended his scale downward to create the Wechsler Intelligence Scale for Children, or WISC, which is still in common usage. The Wechsler scales contained separate
subscores for verbal and performance IQ, thus being less dependent on overall verbal ability than early versions of the Stanford-Binet scale, and was the first intelligence scale to
base scores on a standardized normal distribution rather than an age-based quotient. Since the publication of the WAIS, almost all intelligence scales have adopted the normal distribution
method of scoring. The use of the normal distribution scoring method makes the term "intelligence quotient" an inaccurate description of the intelligence measurement, but I.Q.
still enjoys colloquial usage, and is used to describe all of the intelligence scales currently in use.
IQ and general intelligence factor
Modern IQ tests produce scores for different areas (e.g., language fluency, three-dimensional thinking, etc.), with the summary score calculated from subtest scores. The average score,
according to the bell curve, is 100. Individual subtest scores tend to correlate with one another, even when seemingly disparate in content.
Analysis 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, Cattell Culture Fair III, Universal Nonverbal Intelligence 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.
Various IQ tests measure a standard deviation with different number of points. Thus, when an IQ score is stated, the standard deviation used should also be stated.
A result of 124 in a test with a 24-point standard deviation corresponds to a score of 115 in a test with a 15-point deviation.
Where an individual has scores that do not correlate with each other, there is a good reason to look for a learning disability or other cause for the lack of correlation.
Tests have been chosen for inclusion because they display the ability to use this method to predict later difficulties in learning.
Influences of genetics and environment
The role of genes and environment (nature and nurture) in determining IQ has been debated for decades and the degree to which nature versus nurture influences the development of human
traits (especially intelligence) is one of the most intractable scholarly controversies of modern times.
As it relates to intelligence, overall, except in unusual cases, the role of genes and heredity is far greater than the role of environment. However, environmental
factors can, and do, play a 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.
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.
The heritability of IQ measures the extent to which the IQ of children appears to be influenced by the IQ of parents. Because the heritability of IQ is less than 100%,
the IQ of children tends to "regress" towards the mean IQ of the population. That is, high IQ parents tend to have children who are less bright than their parents, whereas
low IQ parents tend to have children who are brighter than their parents.
The Flynn effect
The Flynn effect is named after James R. Flynn, a New Zealand based political scientist. He discovered that IQ scores worldwide appear to be slowly rising at a rate of around three IQ
points per decade. Attempted explanations have included improved nutrition, a trend towards smaller families, better education, greater environmental complexity, and heterosis. Tests
are therefore renormalized occasionally to obtain mean scores of 100, for example WISC-R (1974), WISC-III (1991) and WISC-IV (2003). Hence it is difficult to compare IQ scores measured
years apart, unless this is compensated for. There is recent evidence that the tendency for intelligence scores to rise has ended in some first world countries.
Sex and intelligence
Most studies show that despite sometimes significant differences in subtest scores, men and women have the same average IQ. Women perform better on tests of memory and verbal proficiency
for example, while men perform better on tests of mathematical and spatial ability. Although gender-related differences in average IQ are insignificant, male scores display a higher
variance: there are more men than women with both very high and very low IQs.
Legal issues
Legal barriers, most prominently the U.S. Civil Rights Act, as interpreted in 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 or class of jobs at issue. Instead, where there is not 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,and 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.
Title VII of the Civil Rights Act generally prohibits employment practices that are unfair or discriminatory. One provision of Title VII, codified at 42 USC 2000e-2(h),
specifically provides that it is not an "unlawful employment practice for an employer to give and to act upon the results of any professionally developed ability test provided that
such test, its administration or action upon the results is not designed, intended or used to discriminate because of race, color, religion, sex or national origin." This statute
was interpreted by the Supreme Court in Griggs v. Duke Power Co., 401 US 424 (1971). In Griggs, the Court ruled that the reliance solely on a general IQ test that was not found to be
specifically relevant to the job at issue was a discriminatory practice where it had a "disparate impact" on hiring. The Court gave considerable weight in its ruling to an
Equal Employment Opportunity Commission regulation interpreting Section 2002e-2(h)'s reference to a "professionally developed ability test" to mean "a test which fairly
measures the knowledge or skills required by the particular job or class of jobs which the applicant seeks, or which fairly affords the employer a chance to measure the applicant's ability
to perform a particular job or class of jobs." In other words, the use of any particular test would need to be shown to be relevant to the particular job or class of jobs at issue.
In the educational context, the 9th Circuit Court of Appeals interpreted similar state and federal statutes to require that IQ Tests not be used in a manner that was
determinative of tracking students into classes designed for the mentally retarded. Larry P. v. Riles, 793 F.2d 969 (9th Cir. 1984).
The Supreme Court of the United States has utilized IQ test results during the sentencing phase of some criminal proceedings. The Supreme Court case of Atkins v. Virginia,
decided June 20, 2002, held that executions of mentally challenged criminals are "cruel and unusual punishments" prohibited by the Eighth Amendment.
The Social Security Administration also uses IQ results when deciding disability claims. In certain cases, IQ results alone are used (in those cases where the result
shows a "full scale IQ of 59 or less") and in other cases IQ results are used along with other factors (for a "full scale IQ of 60 through 70") when deciding whether
a claimant qualifies for Social Security Disability benefits.
In addition, because people with IQs below 80 (the 10th percentile, Department of Defense "Category V") are difficult to train, federal law bars their induction
into the military. As of 2005, only 4 percent of the recruits were allowed to score as low as in the 16th to 30th percentile, a grouping known as "Category IV" on the U.S.
Armed Forces' mental-aptitude exam. |
Individual versus Group Intelligence Tests (adapted from the Indiana
University website)
There are two major types of intelligence test, those administered to individuals and those administered to groups. The two main individual intelligence tests
are the Stanford-Binet Intelligence Test and the Wechsler tests, i.e. Wechsler Intelligence Test for Children (WISC) and the Wechsler Adult Intelligence Scale (WAIS) for adults.
The individual intelligence tests require one-on-one consultation. A list of some of the more commonly used intelligence measures is given below. Note that some of these are "nonverbal"
instruments. These tests rely on little or no verbal expression and are useful for a number of populations, such as non-native speakers, children with poor expressive abilities, or students
with loss.
For each test, the three part listing includes:
Test
Age Range
Description
Test = Stanford-Binet Intelligence Scale, Fifth Edition (SBIS-V)
Age range = 2 – 90+
Description = An update of the SB-IV. In addition to providing a Full Scale score, it assesses Fluid Reasoning, Knowledge, Quantitative Reasoning, Visual-Spatial Processing, and Working
Memory as well as the ability to compare verbal and nonverbal performance.
Wechsler Intelligence Scale for Children, Fourth Edition (WISC-IV)
6 – 16-11
An update of the WISC-III, this test yields a Full Scale score and scores for Verbal Comprehension, Working Memory, Perceptual Reasoning, and Processing speed.
Woodcock-Johnson III Tests of Cognitive Abilities
2 – 90+
This test gives a measure of general intellectual ability, as well as looking at working memory and executive function skills.
Cognitive Assessment System (CAS)
5 - 17
Based on the “PASS” theory, this test measures ‘Planning, ‘Attention, ‘Simultaneous, and ‘Successive cognitive processes.
Wechsler Adult Intelligence Scale (WAIS)
16 - 89
An IQ test for older children and adults, the WAIS provides a Verbal, Performance, and Full Scale score, as well as scores for verbal comprehension, perceptual organization, working
memory, and processing speed.
Comprehensive Test of Nonverbal Intelligence (CTONI)
6 – 18-11
Designed to assess children who may be disadvantaged by traditional tests that put a premium on language skills, the CTONI is made up of six subtests that measure different nonverbal
intellectual abilities.
Universal Nonverbal Intelligence Test (UNIT)
5 - 17
Designed to assess children who may be disadvantaged by traditional tests that put a premium on language skills, this test is entirely nonverbal in administration and response style.
Kaufman Assessment Battery for Children (KABC)
2-6 to 12-5
This test measures simultaneous and sequential processing skills, and has subscales that measure academic achievement as well.
Group-administered intelligence tests involve a series of different problems and are generally used in mass testing situations such as the military and schools. Examples of group tests
are:
Multidimensional Aptitude Battery
The Cognitive Abilities test
Scholastic Assessment Tests
There has been a trend towards the use of multiple choice items. Many of theses tests have separately timed sub-tests. A major distinction made between types of items
is verbal and non-verbal. In recent years there has been a trend away from verbal and mathematical items towards non-verbal represented problems in pictures.
Advantages of group tests:
*can be administered to very large numbers simultaneously
*simplified examiner role
*scoring typically more objective
*large, representative samples often used leading to better established norms
Disadvantages of group tests:
*examiner has less opportunity to obtain cooperation and maintain interest
*not readily detected if examinee tired, anxious, unwell
*evidence that emotionally disturbed children do better on individual than group tests *examinee’s responses are more restricted
*normally an individual is tested on all items in a group test and may become boredom over easy items and frustrated or anxious over difficult items
*individual tests typically provide for the examiner to choose items based on the test takers prior responses - moving onto quite difficult items or back to easier items. So individual
tests offer more flexibility. |
