Theories of Intelligence

– Intelligence as a single trait

• “g” (general intelligence)

– Intelligence as a few/several abilities

• Ex: fluid vs. crystallized intelligence

– Intelligence as many processes

• Ex: attending, planning, reasoning, problem-solving, language comprehension, etc.

Measurement of Intelligence

– Most widely used intelligence test for children 6 years and older:

• Wechsler Intelligence Test for Children (WISC)

– Verbal and Performance sections, each with multiple subtests

– Verbal Section

• Focuses on general knowledge of the world and language skill (mainly crystallized intelligence)

– Six subtests » Exs: Information; Vocabulary; Arithmetic

– Performance Section

• Focuses on spatial and perceptual abilities (mainly fluid intelligence)

– Six subtests

» Exs: Picture Completion; Picture Arrangement; Block Design; Mazes

• Intelligence Quotient (IQ)

– Summary measure of a child’s intelligence relative to other children of the same age

• Mean (average score) is always 100, regardless of child’s age

– Can compare IQ scores at different ages

Stability of IQ Scores

• Infant measures of intelligence (developmental quotient measures)

– Test mainly perceptual and motor responses

– Are not typically correlated with later IQ or school achievement

• May help to identify babies with retardation/developmental delays

• IQ scores are fairly stable from about 5 years of age

– The closer in time two tests are, the stronger the relationship between the scores

– The older the child at the time of first testing, the better the prediction of later IQ 

• IQ scores do show some change over time as well

– Ex: Average change between 4 and 17 years is 13 points (up or down)

IQ Scores as Predictors of Later Development 

• IQ scores are positively correlated with

– School grades and achievement test scores

– Years of education completed

– Occupational success and income

• Other important predictors of these outcomes include:

– Motivation to succeed– Creativity– Social skills

Genetic and Environmental Contributions to IQ Scores

• About half the variation in IQ across individuals is related to genetic differences (in the U.S.)

• About half . . . is related to environmental differences

• Texas Adoption Project

– Compared two groups of adopted children (adoptive families were above average in income and education)

– Biological mothers with IQs below 95 and biological mothers with IQs above 120

– Both groups scored above average (>100) on IQ tests in childhood

– Children of biological mothers with low IQs had lower IQ scores than children of mothers with high IQs