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. This plot is artificially generated and does represent any experimental data directly.]] An intelligence quotient or '''IQ''' is a score derived from one of several different Standardized Test s attempting to measure Intelligence . IQ tests are used as predictors of Education al achievement. People with Low IQ Scores are sometimes placed in special-needs education. While one need not consider those below a certain IQ to be "subhuman" anymore than one need consider those above it to be "superhuman", such attitudes do exist, bringing discussions on IQ far outside the purely scientific scopehttp://www.gnxp.com/MT2/archives/001952.html. The supporters of such views believe that intelligence is depictable as a single number, capable of rank ordering people in a linear order, is primarily genetically based and essentially immutableWilliam J. Matthews, Ph.D. (1998) A Review of the Bell Curve: Bad Science Makes for Bad Conclusions {Link without Title} IQ scores are also used by social scientists; in particular, they study the distribution of IQ scores in populations and the relationships between IQ score and other variables. IQ Correlates with job performance and Income , also with the social status of the parents.[http://www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdf Intelligence: Knowns and Unknowns] (Report of a Task Force established by the Board of Scientific Affairs of the American Psychological Association - Released August 7, 1995 -
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. Because age-based quotients 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 or 24. Thus the modern version of the IQ is a mathematical transformation of a raw score (based on the rank of that score in a normalization sample; see Quantile , 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". While the two methodologies yield similar results near the middle of the bell curve, the older ratio IQs yielded far higher scores for the intellectually gifted— Marilyn Vos Savant appeared in the Guinness Book Of World Records for obtaining a ratio IQ of 228. While this score could make sense using Binet's formula (and even then, only for a child), on the Gaussian curve model it would be an exceptional 7.9 standard deviations above the mean and hence virtually impossible in a population with a normal IQ distribution (see Normal Distribution ). In addition, IQ tests like the Wechsler were not intended to reliably discriminate much beyond IQ 130, as they simply do not contain enough exceptionally difficult items. 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. HOW AN IQ TEST WORKS IQ tests come in many forms, and some tests use a single type of item or question, while others use several different subtests. Most tests yield both an overall score and individual subtest scores. A typical IQ test requires the test subject to solve a fair number of problems in a set time under supervision. Most IQ tests include items from various domains, such as short-term memory, verbal knowledge, spatial visualization, and perceptual speed. Some tests have a total time limit, others have a time limit for each group of problems, and there are a few untimed, unsupervised tests, typically geared to measuring high intelligence. The most widely used standardized test for determining IQ is the WAIS ( Wechsler Adult Intelligence Scale ). The WAIS III consists of fourteen subtests, seven verbal (information, comprehension, numerical reasoning, similarities, digit memory, letter-number sequencing, and vocabulary) and seven performance (digit symbol-coding, picture completion, block design, matrix reasoning, picture arrangement, symbol search, and object assembly). When standardizing an IQ test, a representative sample of the population is tested using each test question. IQ tests are calibrated in such a way as to yield a normal distribution, or "bell curve". Each IQ test, however, is designed and valid only for a certain IQ range. Because so few people score in the extreme ranges, IQ tests usually cannot accurately measure very low and very high IQs. 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. 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. REFERENCE CHARTS See Also: IQ reference chart IQ reference charts are tables, suggested by psychologists to divide intelligence ranges in various categories. HIGH IQ SOCIETIES See Also: High IQ society A high IQ society is an organization that limits membership to people who are within a certain high percentile of IQ test results. The most well-known is Mensa International , which requires members to score in the top 2% of a standardized IQ test. IQ AND GENERAL INTELLIGENCE FACTOR See Also: General intelligence factor Modern IQ tests produce scores for different areas (e.g., language fluency, three-dimensional thinking), 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. Tests differ in their ''g''-loading, which is the degree to which the test score reflects ''g'' rather than a specific skill or "group factor" (such as verbal ability, spatial visualization, or mathematical reasoning). ''g''-loading and validity have been observed to be related in the sense that most IQ tests derive their validity mostly or entirely from the degree to which they measure ''g'' . HERITABILITY See Also: Inheritance of intelligence that is, depending on the study, a little less than half to substantially more than half of the variation in IQ among the children studied was due to variation in their genes. The remainder was thus due to environmental variation and measurement error. A heritability in the range of 0.4 to 0.8 implies that IQ is "substantially" heritable. The effect of restriction of range on IQ was examined by Matt McGue and colleagues, who write that "restriction in range in parent disinhibitory psychopathology and family SES had no effect on adoptive-sibling correlations ... IQ".http://dx.doi.org/10.1007/s10519-007-9142-7 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. Some of the correlation of IQs of twins may be a result of the effect of the maternal environment before birth, shedding some light on why IQ correlation between twins reared apart is so robust. The heritability of IQ. Devlin B, Daniels M, Roeder K. Nature. 1997 Jul 31;388(6641):468-71. There are a number of points to consider when interpreting heritability:
Environment See Also: Health and intelligence environment and intelligence Environmental factors play a role in determining IQ. Proper childhood Nutrition appears critical for Cognitive Development ; Malnutrition can lower IQ. Other research indicates environmental factors such as prenatal exposure to Toxin s, duration of Breastfeeding , and Micronutrient deficiency can affect IQ. It is well known that it is possible to increase one's IQ score by training, for example by regularly playing puzzle games, or strategy games like may increase IQ. (Klingberg ''et al.'', 2002) Family environment In the developed world, nearly all raised separately are highly similar in IQ (0.86), more so than Dizygotic (fraternal) Twins raised together (0.6) and much more than adoptive siblings (~0.0). The American Psychological Association 's report ''Intelligence: Knowns and Unknowns'' (1995) states that there is no doubt that normal child development requires a certain minimum level of responsible care. Severely deprived, neglectful, or abusive environments must have negative effects on a great many aspects of development, including intellectual aspects. Beyond that minimum, however, the role of family experience is in serious dispute. Do differences between children's family environments (within the normal range) produce differences in their intelligence test performance? The problem here is to disentangle causation from correlation. There is no doubt that such variables as resources of the home and parents' use of language are correlated with children's IQ scores, but such correlations may be mediated by genetic as well as (or instead of) environmental factors. But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family? Recent twin and adoption studies suggest that while the effect of the family environment is substantial in early childhood, it becomes quite small by late adolescence. These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children's lives make little long-term difference for the skills measured by intelligence tests. It also stated "We should note, however, that low-income and non-white families are poorly represented in existing adoption studies as well as in most twin samples. Thus it is not yet clear whether these studies apply to the population as a whole. It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence." On the other hand, the degree to which these increases persisted into adulthood are not clear from the study. Biased older studies? Stoolmiller (1999)Stoolmiller, M. (1999). Implications of the restricted range of family environments for estimates of heritability and nonshared environment in behavior-genetic adoption studies. Psychological Bulletin, 125, 392-409. found that the range restriction of family environments that goes with adoption, that adopting families tend to be more similar on for example socio-economic status than the general population, means that role of the shared family environment have been underestimated in previous studies. Corrections for range correction applied to adoption studies indicate that socio-economic status could account for as much as 50% of the variance in IQ. However, the effect of restriction of range on IQ for adoption studies was examined by Matt McGue and colleagues, who wrote that "restriction in range in parent disinhibitory psychopathology and family socio-economic status had no effect on adoptive-sibling correlations [in] IQ".http://dx.doi.org/10.1007/s10519-007-9142-7 Eric Turkheimer and colleagues (2003),Eric Turkheimer and colleagues (2003) not using an adoption study, included impoverished US families. Results demonstrated that the proportions of IQ variance attributable to genes and environment vary nonlinearly with socio-economic status. The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared family environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse. Socioeconomic status modifies heritability of iq in young children Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, Irving I. Gottesman. Psychological Science 14 (6), 623–628. 2003 They suggest that the role of shared environmental factors may have been underestimated in older studies which often only studied affluent middle class families. New Thinking on Children, Poverty & IQ November 10, 2003 Connect for Kids Maternal (fetal) environment A meta-analysis, by Devlin and colleagues in '' Nature '' (1997), The heritability of IQ. Devlin B, Daniels M, Roeder K. Nature. 1997 Jul 31;388(6641):417-8. of 212 previous studies evaluated an alternative model for environmental influence and found that it fits the data better than the 'family-environments' model commonly used. The shared maternal (fetal) environment effects, often assumed to be negligible, account for 20% of covariance between twins and 5% between siblings, and the effects of genes are correspondingly reduced, with two measures of heritability being less than 50%. They argue that the shared maternal environment may explain the striking correlation between the IQs of twins, especially those of adult twins that were reared apart . Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different than previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports. They write that: Chipuer et al. and Loehlin conclude that the postnatal rather than the prenatal environment is most important. The Devlin et al conclusion that the prenatal environment contributes to twin IQ similarity is especially remarkable given the existence of an extensive empirical literature on prenatal effects. Price (1950), in a comprehensive review published over 50 years ago, argued that almost all MZ twin prenatal effects produced differences rather than similarities. As of 1950 the literature on the topic was so large that the entire bibliography was not published. It was finally published in 1978 with an additional 260 references. At that time Price reiterated his earlier conclusion . Research subsequent to the 1978 review largely reinforces Price’s hypothesis ( The Dickens and Flynn model Dickens and Flynn (2001) Dickens and Flynn (2001) postulate that the arguments regarding the disappearance of the shared family environment should apply equally well to groups separated in time. This is contradicted by the Flynn Effect . Changes here have happened too quickly to be explained by genetics. This paradox can be explained by observing that the measure "heritability" includes both a direct effect of the Genotype on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues (the model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects). The Flynn effect can be explained by a generally more stimulating environment for all people. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they taught children how to replicate outside the program the kinds of cognitively demanding experiences that produce IQ gains while they are in the program and motivate them to persist in that replication long after they have left the program.William T. Dickens and James R. Flynn, Heritability Estimates Versus Large Environmental Effects:The IQ Paradox Resolved , ''Psychological Review'' 2001. Vol. 108, No. 2. 346-369.William T. Dickens and James R. Flynn, " The IQ Paradox: Still Resolved ," ''Psychological Review'' 109, no. 4 (2002). MENTAL HANDICAPS See Also: Mental retardation Individuals with an unusually low IQ score, varying from about 70 ("Educable Mentally Retarded") to as low as 20 (usually caused by a neurological condition), are considered mentally retarded. However, there is not any IQ-based classification for Developmental Disabilities as that condition is considered to be different from Mental Retardation . IQ AND THE BRAIN See Also: Neuroscience and intelligence Significant brain injuries, especially those occurring at a young age, may not significantly affect IQ.Bava S, Ballantyne AO, Trauner DA. Disparity of verbal and performance IQ following early bilateral brain damage. J Int Neuropsychol Soc. 2004 Jan;10(1):82-90. Pubmed link The idea that head size is related to intelligence has attracted people for years Galton, F. 1888 Head Growth in Students at the University of Cambridge, Nature, 38; 14-15; Pearl, R. 1906 On the Correlation between Intelligence and the Size of the Head, Jour. Comp. Neurol. and Psychol., 16: 189-199., and children frequently draw pictures of people with larger heads than is realistic, possibly because the human mind is what makes us ourselves.Gellert E Children's constructions of their self-images. Percept Mot Skills 1975 Feb;40(1):307-24 The results of the scientific research are highly controversary. Many works conclude that in the typical (non pathologic) case there is no direct correlation between the brain size and IQReed, T.E., & Jensen, A.R. 1993. Cranial capacity: new Caucasian data and comments on Rushton's claimed Mongoloid-Caucasoid brain-size differences. Intelligence, 17, 423-431, but it is also possible to find recent works that support this old hypothesis.McDaniel, M.A. (2005) Big-brained people are smarter: A meta-analysis of the relationship between ''in vivo'' brain volume and intelligence. ''Intelligence, 33'', 337-346. PDF An alternative approach has sought to link differences in neural plasticity with intelligence (Garlick, 2002 Garlick, D. (2002). Understanding the nature of the general factor of intelligence: The role of individual differences in neural plasticity as an explanatory mechanism. Psychological Review, 109, 116-136. {Link without Title} ), and this view has recently received some empirical support (Shaw et al., 2006 Shaw, P., Greenstein, D., Lerch, J., Clasen, L., Lenroot, R., Gogtay, N., Evans, A., Rapoport, J., & Giedd, J. (2006). Intellectual ability and cortical development in children and adolescents. Nature, 440, 676-679.). THE FLYNN EFFECT See Also: 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 (Flynn, 1999). 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. GROUP DIFFERENCES Among the most controversial issues related to the study of intelligence is the observation that intelligence measures such as IQ scores vary between populations. While there is little scholarly debate about the ''existence'' of some of these differences, the ''reasons'' remain highly controversial both within academia and in the public sphere. Health and IQ See Also: Health and intelligence Several factors can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the Blood-brain Barrier is less effective. Such impairment may sometimes be permanent, sometimes be partially or wholly compensated for by later growth. Several harmful factors may also combine, possibly causing greater impairment. Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring Fortification of certain food products and laws establishing safe levels of pollutants (e.g. lead, mercury, and organochlorides). Comprehensive policy recommendations targeting reduction of cognitive impairment in children have been proposed.Olness, K. " Effects on brain development leading to cognitive impairment: a worldwide epidemic ," ''Journal of Developmental and Behavioral Pediatrics'' 24, no. 2 (2003): 120–30. |
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