Intelligence

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Outline

1. What is intelligence?2. Can it be measured?3. Differences in measured intelligence

a. Within groupa. Heritabilityb. Twin studiesc. Manipulating environments to increase IQ

b. Between groups4. Are intelligence tests culturally biased?5. Are there multiple intelligences?6. Spearman’s g7. Intelligence in daily life8. Stanford-Binet

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Outline

1. What is intelligence?2. Can it be measured?3. Differences in measured intelligence

a. Within groupa. Heritabilityb. Twin studiesc. Manipulating environments to increase IQ

b. Between groups4. Are there multiple intelligences?5. Spearman’s g6. Intelligence in daily life7. Stanford-Binet8. Wechsler tests

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What is intelligence?

“Intelligence is a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience. It is not merely book learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings – “catching on,” “making sense” of things, or “figuring out” what to do.” (Gottfredson, 1994)

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Can intelligence be measured?

Yes. And intelligence tests measure it well.

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Psychologists define intelligence as “what IQ tests measure”

This makes sense. Intelligence tests

have been carefully created, revised, and improved for 100 years precisely to do that job.

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Intelligence tests are accurate (they are reliable and valid)

Intelligence tests do not measure creativity, character, personality, or other individual differences

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IQ scores from tests such as the Stanford-Binet and the WAIS are the best predictors we have of a variety of important outcomes

This includes life outcomes relating to health, career, personal relationships, and crime

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Differences in measured intelligence

a. Within groupsa. Heritability

b. Twin studies

c. Manipulating environments to increase IQ

b. Between groups

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Within group differences

Phenotypic variation

Total variation in the appearance of members of a species

Includes things that can be “made visible” (such as blood types)

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Within-group differences

Heritability the proportion of phenotypic variation in a population that is due to genetic variation among individuals in that population

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Heritability If all environments were to become exactly equal for everyone, heritability would be 1.0 because all remaining differences in IQ would have to be biological

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Heritability If every person were a clone (so that everyone had exactly the same genetic material), all remaining differences in IQ would have to be environmental (heritability would be 0)

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Members of the same family tend to differ in IQ by about 12 points on average.

Siblings have different environments and also share only half their genes (on average)

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Adopted children resemble their birth mothers more than their adoptive mothers in intelligence, even if they have never met their birth mothers

Plomin et al. (1997): no relation at all after early childhood between the IQs of adoptive parents and of the children they adopted

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McGue et al. (1993): found an average correlation of zero for adoptive siblings tested as adults

Loehlin et al. (1997) on Texas Adoption Project: shared family environment influences IQ only for very young children; genetic effects increase with age

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Twin studies

1. Are identical twins more similar in IQ than fraternal twins?

2. Are fraternal twins more similar in IQ than pairs of non-twin siblings?

3. Are non-twin siblings more similar in IQ than unrelated children raised in the same home?

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Are identical twins more similar in IQ than fraternal twins?

Plomin, DeFries, McClearn, & McGuffin (2001) summarized results of studies of more than 10,000 pairs of twins

The average correlation of IQ scores for identical twins: .86

For the fraternal twins: .60

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Are fraternal twins more similar in IQ than pairs of non-twin siblings?

Fraternal twins are no more genetically similar to each other than any pair of non-twin siblings

But fraternal twins are more similar to each other in IQ than non-twin siblings.

Why? Jensen (1998): Prenatal factors

such as mother’s age, nutrition, health

Blood antigen incompatibilities

Obstetrical procedures

19Are non-twin siblings more similar in IQ than unrelated children raised in the same home?

Jensen (1998) analyzed 27,000 sibling pairs

average IQ for ordinary siblings reared together was .49

correlation of .25 for IQs of unrelated persons raised together when IQ was measured during childhood

this correlation dropped to 0 when the children were tested as adults

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Does this mean intelligence is hereditary?

Intelligence is partially inherited

About 50% of the variability in IQ scores can be traced to genetic influences

Differences in environment quality are more important for younger children than for adolescents

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Does this mean intelligence is hereditary?

The nature of nurture effect – part of the effect of the environment may be genetic

Differences in environments may be produced by genetic differences

E.g., an artist and an engineer may create very different environments for their children

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Can we manipulate environments to increase IQ?

U.S. Government’s Head Start program

Milwaukee Project Carolina

Abecedarian project

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Head Start program

Modest gains Smallest gains for

children most at risk

Gains do not survive long after child leaves the program

Neisser et al. (1996): by end of elementary school, no differences between those in vs. not in program

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Milwaukee project

Modest gains Effects do not last

long after exit from program

Gains only on tests requiring skills taught in the program – effects do not generalize to new tasks

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Carolina Abecedarian project

Intensive intervention provided daily for the first 5 years of the child’s life

IQ scores for intervention group 5 points higher than for control group at age 12 (7 years after exit from program)

Positive effect on failure & drop-out rates

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Are intelligence tests culturally biased?

No. Tests of widely varying kinds (e.g., verbal abilities, spatial abilities), including those considered most “fair,” give the same results.

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Are intelligence tests culturally biased?

Group differences just as large on Ravens Progressive Matrices as on WAIS

IQ scores have same utility for prediction regardless of race or socio-economic status.

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Bias vs. Fairness

It’s important to distinguish between these two concepts:

Bias Unfairness

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Bias vs. Fairness

A test is biased if it gives a systematically wrong result when used to predict something.

So, an intelligence test would be biased if, for example, it underestimated one group’s probability of success in a given endeavor.

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Bias vs. Fairness

Use of a test is unfair if it treats people differently

E.g., if a verbal test probes for knowledge acquired from schooling, use of that test with people who have not had such schooling would be unfair

Note that the test itself is not implicitly unfair – but use of the test may be unfair

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Bias vs. Fairness

When you use an unfair test, the result need not be biased. The result may still have good predictive value.

E.g., if you test non-native speakers of English with the SAT, that use of the test is unfair, but not biased

Results will predict academic success in English-speaking countries.

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Are group differences in IQ real?

Yes. Members of all ethnic/racial groups are found at all levels of IQ.

But groups vary in where their scores cluster (that is, in the means).

It’s important to note that the group differences are in “central tendency” (mean) – there is lots of overlap, and all groups are represented at low, medium, and high levels of IQ

IQ

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Highest IQ scores are for Ashkenazi Jews

Cochran et al. (2006): medieval social environment for European Jews selected for verbal & math intelligence (but not spatial)

Some relation to disease genes?

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Curves for some Asians are somewhat higher than for Whites; curves for Blacks, Hispanics somewhat lower than for Whites

We don’t know why these effects are found, but there is much debate on this question

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Sources of between-group differences

Next two slides have statements from leading scholars in the field on what we know (and don’t know) about the sources of between-group differences in measured intelligence

Gottfredson (1997), Intelligence Neisser et al. (1996), American

Psychologist

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“There is no definitive answer to why IQ bell curves differ across racial-ethnic groups. The reasons for these IQ differences may be markedly different from the reasons for why individuals differ among themselves within any particular group… Most experts believe that environment is important in pushing the bell curves apart, but that genetics could be involved, too.” Statement of the 52 experts, Intelligence, 1997, p.15.

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“It is clear that genes make a substantial contribution to individual differences in intelligence test scores, at least in the White population. The fact is, however, that the high heritability of a trait within a given group has no necessary implications for the source of a difference between groups…Thus the issue ultimately comes down to a personal judgment: How different are the relevant life experiences of Whites and Blacks in the United States today? At present, this question has no scientific answer.” Neisser et al., (1996), p.95

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Gene-based temperamental factors?

Family size (now decreasing in N.A.)?

SES?but differences

present when SES controlled

Caste?Lacking “effort

optimism” (Ogbu, 1978)

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Priming effects Bargh, Chen, &

Burrows (1996) IV: elderly

stereotype DV: Walking speed Primed subjects

walked more slowly

Dijksterhuis & van Knippenberg (1998) IV: intelligence/

stupidity traits primed DV: performance on

a knowledge test Trait primes affected

performance

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Relevance of the priming studies

Stereotypes may influence those who are stereotyped

Direct effect on test performance?

Indirect effect on effort optimism?

Both?

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Culture (Boykin, 1996) Assessment itself

alienates? American

schooling conflicts with deep structure of African-American culture?

Members of ethnic groups might answer some items differently but still correctly – some non-standard responses given by (e.g.) minority children may be standard in their sub-culture.

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More reading (1): People who argue group differences are real:

Neisser, U. et al. (1996), Intelligence: Knowns and Unknowns. Am. Psychologist, 51(2),77-101

Buckhalt, J.A. (2002). Learning and Individual Differences, 13, 101-114.

Gottfredson, L.S. (1997). Intelligence, 24 (1), 13-23.Gottfredson, L.S. (2000) Psychology, Public Policy,

& Law Special Issue, 6(1), 129 – 143.Jensen, A.R. (2000). Psychology, Public Policy,

and Law Special Issue, 6 (1), 121-127.

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More reading (2): People who argue group differences are artifacts:

Chan, D., et al. (1997). J. Applied Psychology, 82 (2), 300-310.

Hale, J.B., et al. (2001). School Psychologist, Fall,113-118.

Helms, J.E. (1997). In D.P. Flanagan, J.L. Genshaft, & P.L. Harrison (Eds.), Contemporary intellectual assessment: theories, tests, and issues (517-53).

Steele, C.M. (1997 & 1998). American Psychologist, 52 (6) 613-629 and 53 (6) 797-811

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Are there “multiple intelligences?”

No. Intelligence is

multi-dimensional, but all intelligence tests, whatever their form, measure the same ability.

This is true whether tests emphasize verbal or non-verbal skills, and whether they require specific cultural knowledge (such as vocabulary).

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Howard Gardner’s idea: IQ tests tap only one of many different kinds of intelligence (e.g., bodily-kinesthetic, musical, inter-personal, etc.)

No evidence supports this view. Gardner’s tests appears to measure interest, motivation, other things.

Most cannot be scored objectively.

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Sternberg’s Triarchic Theory is just as bad as Gardner’s.

No useful tests of his “practical intelligence” exist.

He is accused of misrepresenting data, using unclear definitions, obscuring his methods, not reporting results fully, using unrepresentative samples. (Gottfredson, 2003)

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All intelligence tests measure the same ability

The most important aspect of intelligence is g –general ability

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Spearman’s g

Spearman (1904): When a large group of people are given a variety of ability tests, the correlations among the test results are almost always positive.

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Spearman’s g

Spearman (1904): That correlation is called the positive manifold

It is the basis for the idea that there is a general mental ability (called g).

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Spearman’s g

g is not the same as IQ

to find g, you have to do factor analysis

to find IQ, you use an IQ test

IQ

gS

V

Test Score

FactorAnalysis

Measurement error

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Spearman’s g

g is essentially a biological variable

all tests involving cognitive ability measure g to some extent

none measure only g: any cognitive test has some variance due to causes unique to that test.

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Spearman’s g

IQ scores reflect individual differences in underlying constructs (e.g., g and group factors), unique abilities, and measurement error.

In contrast, factors, such as g, are derived from correlations and reflect individual differences in underlying constructs. Factor scores provide best estimates of g.

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Does g matter?

Yes. g is a better predictor of educational and work performance than any other measure we have.

g is the most important determiner of scores on every test of cognitive ability (in people who can be tested).

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Is intelligence more than just g?

Yes. More than 70 different “group factors” have been identified.

Hierarchical theory: g at top (most general)

At Level II, 8 broad cognitive abilities (e.g., fluid intelligence)

At Level I, narrow abilities (memory span, inductive reasoning, etc.)

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Is intelligence important in daily life?

Yes. (See Slide 58.)

IQ is more strongly related to important educational, occupational, economic, and social outcomes than any other single variable.

Relation is strong in education, military training

Moderate in social competence

Modest in law-abidingness

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IQ increases in importance as life gets more complex – in novel, ambiguous, changing, unpredictable, or multi-dimensional situations.

IQ is important in professions, management

Less important where only routine decision-making, simple problem-solving are required

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Some personality characteristics, talents, physical capabilities, etc. are also important.

But intelligence transfers across tasks and settings; those other characteristics do so less or not at all.

High Uphill Keeping Out Yours toRisk Battle Up Ahead Lose

IQ: <70 71-90 90-110 110-130 > 130% populn 5 20 50 20 5

LF (M) 22 19 15 14 10Job (M) 12 10 7 7 2Divorce 21 22 23 15 9Illegit (F) 32 17 8 4 2Poverty 30 16 6 3 2Prison (M) 7 7 3 1 0Welfare (F) 31 17 8 2 0Dropout 55 35 6 0.4 0

LF = Out of labor force > 1 month in the last year; Job = Unemployed > 1 month/year; Welfare = Chronic welfare recipient

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Individual Intelligence Tests

administration requires advanced training tests cover wide range of age and ability examiner-subject rapport is important immediate scoring of items usually requires about one hour allows opportunity for observation

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2 Important Tests

Binet asked to identify

intellectually limited children so they could be removed from the regular classroom and put in special education

Wechsler Responded to

perceived shortcomings of the Binet test thirty years later

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Binet’s 1905 test

No intelligence tests existed to guide Binet and colleague Simon

Like Spearman, thought of intelligence as a general mental ability

Wanted tasks to measure judgment, attention, and reasoning.

Two major concepts: Age differentiation General mental

ability.

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Binet’s principles of test construction:

Age differentiation: Binet searched for tasks that could be completed by 2/3 to ¾ of the children in a particular age group & were completed by fewer younger children and more older children.

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Binet’s principles of test construction:

General mental ability:

Measured only the total output on the various tasks.

Judged value of task in terms of its correlation with the combined result of all other tasks.

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Binet-Simon (1905):

First formal intelligence test

30 items ordered by difficulty

Drawbacks: Output: labeled

children idiot, imbecile, and moron (these were technical terms at that time)

Norms: produced using only 50 children

Validity: no evidence offered

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Binet-Simon (1908):

Grouped items according to age level rather than simply according to increasing difficulty. Introduced concept of mental age to deal with problem of output

Increased norm group to 203 children.

Still produced only one score heavily dependent on verbal, language, and reading abilities

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1916 Stanford Binet scale:

Lewis Terman of Stanford University translated Binet test into English and introduced it to America.

Terman increased size of standardization sample, but included only white native-Californian children.

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1916 Stanford Binet scale:

Introduced intelligence quotient (IQ) concept to show subjects’ rate of mental development.

IQ = (MA/CA) x 100

Maximum mental age was 19.5. Set maximum chronological age at 16.

S-B tests used on millions of U.S. Army recruits starting in 1917, after mobilization for World War I – a rich source of data for post-war research.

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1937 Stanford-Binet Scale

Extended age range down to 2 and up to 22 years, 10 months.

Some performance items added – but 75% of items still verbal

Scoring standards and instructions were improved

Standardization sample improved to include 3184 subjects from 11 states.

Developed alternate forms (Forms L & M) to facilitate research

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Problems with 1937 Stanford-Binet

Reliability higher for older subjects than for younger ones and higher for those in the lower IQ ranges

Scores were most unstable for young children with high IQ

Each age group also had different standard deviations which made interpretation difficult

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1960 Stanford-Binet:

Used Binet’s principles to redo scale

Looked for tasks on which success becomes more likely as age increases

Looked for tasks for which scores correlated with test scores.

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1960 Stanford-Binet:

Introduced the deviation IQ concept. Set mean at 100 with SD of 16.

Could now compare scores of one age level with another.

Deviation IQ: score worked out in terms of number of SDs above or below age mean

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1960 Stanford Binet

Each boundary (---) marks 1 standard deviation

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1986 Stanford-Binet scale

Multi-dimensional, but hierarchical – with g at the top of the structure

4 main factors:

Verbal reasoning Abstract/visual

reasoning Quantitative

reasoning Short-term memory

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The individual tests (such as Pattern Analysis or Vocabulary) each give information about one of the factors

The factors in turn give information about g.

g reflects common variability across all the tasks

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Now 5 main factors, each tested in verbal & nonverbal domains

Fluid Reasoning Knowledge Quantitative

Reasoning Visual-Spatial

Processing Working Memory

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Uses routing tests to estimate subject’s level of ability

Verbal and non-verbal routing tests

Back to original age-scale approach (items with differing content grouped together on basis of difficulty).

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The S-B5 was normed on a stratified random sample of 4,800 individuals (2 – 85 years of age) that matches the 2000 U.S. Census.

Bias reviews on all items for sex, ethnic, cultural/religious, regional, and socioeconomic status issues.

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Psychometric properties of 2003 S-B

Internal consistency reliability is .98 for composite and .93-.97 for area scores.

Some individual test scores are lower: .73 for memory for objects is the lowest.

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Test-retest reliabilities for composite score were .91 and .90 for 5 and 8-year-olds.

Factor analysis supports the structure of the test.

Correlations with other IQ tests are generally in the .70s and .80s

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Convergent validity assessed with:

Stanford-Binet Intelligence Scale, 4th Edition, the Stanford-Binet Form L-M, the Woodcock-Johnson III, the WAIS-III, the WISC-III, and the WPPSI-R

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Intelligence