The Role of Intelligence in Mobility & The Bell Curve Controversy.

The controversy about how intelligence is defined has continued since the last half of the nineteenth century. By the early 1990s, these researchers could be divided into three categories (The Bell Curve):
The classicists (intelligence as a structure)
accept that g is an expression of core human mental ability
believe that the best standardized tests do a good job of measuring g.
tests not biased against socioeconomic, ethnic or racial subgroups.
The Bell Curve draws most from the classic tradition – attempts to understand the relationship of human abilities and public policy
The revisionists (intelligence as information processing)
believe that human intelligence has a structure, but it is not worth the effort to discover what that structure looks like
more interested in the ways in which people process information they receive than the structure of human intelligence
interested in answering questions like, what problem-solving mechanisms do people employ, how do they trade-off speed and accuracy, and how do they combine different problem-solving resources into a strategy.
The radicals (the theory of multiple intelligences)
Reject the notion of a general intelligence factor (g)
Lippman – the concept of intelligence is too diverse, too complex, too changeable, too dependent on cultural context, and too subjective to be measured by answers to an IQ test
Argues that there are seven distinct intelligences: linguistic, musical, logical-mathematical, spatial, bodily-kinesthetic, and two forms of “personal intelligence” – the interpersonal and the intrapersonal, each based on its unique computational capacity.
1.      Spearman
Drew on Galton’s work that proposed that intelligent individuals had keener sensory discriminations than individuals with low intelligence
Spearman found that measures of sensory functioning tended to be positively correlated with measures of intelligence
Proposed that scores on an intelligence measure had two components – a general component g and a specific component s (The two-factor theory of intelligence)
The g component represents that which is common in all measures of the common intellective function
The s component is specific to each measure
The correlation between any two measures of intelligence is determined by the ratio of g to s – the higher the ratio, the higher the correlation
Tests of intelligence should contain measures or sub-scales that have high g to s ratios
Intelligence is a construct (hypothetical entity)
Spearman’s methods a foundation for factor analysis
2.      Binet
Unlike Spearman’s theoretical work, Binet’s research was more empirically based
A test of intelligence should separately measure several intellectual functions or facilities
Interested in developing age-graded tests to establish the mental age of a person by reference to the characteristic age level of the intellectual tasks that person can complete
Can calculate IQ from his 30-item test by dividing a person’s mental age by their chronological age, and then multiplying the ratio by 100.
3.      Thomson
Thomson and Spearman engaged in a two decade long debate over the interpretation of the nature of g.
Thomson believed that there was a large set of independent bonds or units in the mind, and any test of ability samples some subset of these bonds.
Thus the expected value of the correlation between tests on this model is a function of the number of bond they share in common.
It is now known that both theories accounted for the existence of g – a metaphorical difference in the way they conceptualized g.
4.      Thurstone
Refuted Spearman’s claim of the existence of g
Believed that there were separate intellectual abilities that were unrelated to each other
Could not account for the positive correlation between factors that he had hypothesized as being separate or independent of each other

Psychometric approach (“Cognitive classes”) – these measure the g factor within the classical tradition
Six conclusions (from “The Bell Curve”):
There is such a thing as a general factor of cognitive ability on which human beings differ
All standardized tests of academic aptitude or achievement measure this general factor to some degree, but IQ tests expressly designed for this purpose measure it most accurately
IQ scores match, to a first degree, whatever it is that people mean when they use the word intelligence or smart in ordinary language
IQ scores are stable, although not perfectly so, over much of a person’s life
Properly administered IQ tests are not demonstrably biased against social, economic, ethnic or racial groups
Cognitive ability is substantially heritable, apparently no less than 40 percent and no more than 80 percent
Multiple forms of intelligence (e.g., Gardner, Sternberg)- or rather special talents?
Cultural variation in concepts of intelligence-different cognitive characteristics are emphasized from one situation to another, and from one sub-culture to another.
Developmental progressions (e.g., Vygotsky, Piaget) – believe that intellectual abilities are social in origin; and that intelligence develops in all children through assimilation of new information from their environment
Biological approaches – study of brain anatomy and physiology to understand what intelligence is and how to measure it. New area of study
Cognitive classes (The Bell Curve) based on IQ
V. Very Dull (50-75), IV. Dull (76-90), III. Normal (91-110), II. Bright (111-125), I. Very Bright (126-150)

The report by the task force of the Board of Scientific Affairs of the American Psychological Association –
Many recent studies show that the speeds with which people perform very simple perceptual and cognitive tasks are correlated with psychometric intelligence.
In general, people with higher intelligence tests scores apprehend, scan, retrieve and respond to stimuli more quickly than those who score lower.
Tests:
Timed tests that measure speed of cognitive tasks
Choice reaction time
Inspection time – subject must judge which line is shorter
Neurological measures – studying “nerve conduction velocities”
Problems of interpretation – It is possible that high and low IQ individuals differ in other ways that affect speeded performance. Such variables could include motivation, response criteria (emphasis on speed vs. accuracy), perceptual strategies, attentional strategies, and in some cases differential familiarity with the material itself. We do not know the direction of causality that underlies many of these correlations. Do high levels of neural efficiency promote the development of intelligence, or do more intelligent people just find faster ways to carry out perceptual tasks? Or both?
Twins
MZ twins genetically identical
DZ twins no more similar to each other than siblings
The author cautions that estimates on the sources of variation in intelligence using twin data have conceptual and empirical difficulties
–         a simple additive genetic model assumes that DZ twin pairs have a genetic correlation of 0.5. This means that the DZ correlation for intelligence should be one-half the value of the MZ correlation and the difference of the correlation multiplied by two should give an estimate of the heritability of intelligence.
–         Difficulties with this procedure for estimating the heritability of intelligence
–         The genetic correlation between same-sex DZ twin pairs may be larger than 0.5
–         Estimates of genetic and environmental influences on intelligence using twin data may be distorted by special environmental experiences by twins
–         Volunteer bias – twin studies require the cooperation of both members – if twins who are more dissimilar are more likely to be discordant for the tendency to volunteer to participate; then the surviving members of any twin study are likely to be a biased sample of twin pairs who are more similar to each other than twins in the population.

Studies on twins reared together
–         Bouchard and McGue (1981) summarized the literature on kinship correlations for intelligence.
–         Reported a weighted average of 0.86 based on a sample of 4672 pairings of MZ twins reared together based on 34 correlations
–         Comparable correlation for same-sex DZ twins was 0.60 based on 41 correlations and 5546 pairings
–         On the most simplistic genetic model without considering assortative mating (the tendency of individuals who are genetically alike to mate) these data imply that 52% of the variance on IQ tests is attributable to genetic influences
–         Consideration for assortative mating would increase the estimates for the heritability of intelligence
–         Loehlin, Willerman & Horn (1988) reviewed twin studies published after Bouchard and McGue’s review
–         They note that these recent studies report larger MZ-DZ differences
–         These studies suggest a slightly higher heritability
–         It is not known if these data represent random perturbations in the obtained values of correlations, systematic changes attributable to differences in methodology, or true secular changes in the heritability of intelligence

Studies on MZ and DZ twins reared apart
–         The correlation between MZ twins reared apart provides direct estimate of the heritability of intelligence but these correlations should be dealt with cautiously – only a small number of studies with small sample sizes
–         Minnesota study of twins reared apart by Bouchard, Lykken, McGue, Segal and Tellegen (1990) – MZ twins reared apart are more similar to each other in IQ than DZ twins reared together ( the authors obtained correlations between family background variables of MZ twins reared apart and performance on IQ scores to calculate the contribution of the similarity of family background variables to the correlation of IQ scores for these twins
–         Separated MZ twins encounter more varied environments than MZ and DZ twin pairs reared together – yet they are somewhat more alike in IQ than DZ twins reared in the same family.
–         The correlation for MZ twins reared apart is a direct estimate of the heritability of IQ (which is close to 0.7)
–         Minnesota study of twins reared apart by Bouchard, Lykken, McGue, Segal and Tellegen (1990) – MZ twins reared apart are more similar to each other in IQ than DZ twins reared together ( the authors obtained correlations between family background variables of MZ twins reared apart and performance on IQ scores to calculate the contribution of the similarity of family background variables to the correlation of IQ scores for these twins
–         Separated MZ twins encounter more varied environments than MZ and DZ twin pairs reared together – yet they are somewhat more alike in IQ than DZ twins reared in the same family.
–         The correlation for MZ twins reared apart is a direct estimate of the heritability of IQ (which is close to 0.7)
Family studies
There are three contemporary studies about the relationship between IQ scores of adopted children and their biological and adopted parents
1.      Teasdale & Owen (1984) obtained data on Danish fathers whose biological sons were adopted. The correlation between biological fathers and their sons was .20. The corresponding correlation between the intelligence test scores of adopted fathers and their sons was 0.02. Thus, adopted adult children are more likely to resemble their biological parent than their adoptive parent in intelligence. Small sample….
2.      Texas Adoption Study (Horn, Loehlin & Willerman, 1979)
3.      The Colorado Adoption Project (Plomin & DeFries, 1985)
(Both longitudinal studies)
Other studies have examined the effect of being raised in upper middle class families also

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