Chapter 9: Intelligence – Measurement, Theories, and Influences

Measuring Intelligence

Historical and Ethical Context

Intelligence testing has a complex history, marked by both scientific advances and significant ethical controversies. Early intelligence tests were sometimes misused, leading to harmful social policies such as forced sterilization based on low IQ scores. The case of Leilani Muir, who was sterilized after scoring below a threshold on an IQ test, highlights the dangers of misinterpreting psychological measurements.

• Legal and Ethical Implications: Misuse of intelligence tests led to legal actions and compensation for victims.

• Measurement Challenges: Intelligence lacks an objective standard, making rigorous methodology essential.

• Social Forces and Biases: Historical biases have influenced the development and application of intelligence tests.

Approaches to Intelligence Testing

Defining and measuring intelligence has been a central challenge in psychology, with various approaches reflecting different theoretical perspectives.

• Galton’s Anthropometric Approach: Francis Galton believed intelligence was linked to sensory abilities. His tests, however, did not correlate with academic performance and were eventually abandoned.

• Binet and the Stanford–Binet Test: Alfred Binet defined intelligence as complex thinking processes. The Binet-Simon test measured mental age, and Lewis Terman adapted it into the Stanford-Binet Intelligence Scale, introducing the concept of IQ:

IQ Formula:

$IQ = \frac{\text{mental age}}{\text{chronological age}} \times 100$

• Deviation IQ: For adults, IQ is standardized relative to age group, with a mean of 100.

• Normal Distribution: About 68% of people score between 85 and 115; 95% between 70 and 130.

The Wechsler Adult Intelligence Scale (WAIS)

The WAIS is the most widely used intelligence test for adolescents and adults, providing a Full Scale IQ and two main indices:

• General Ability Index (GAI): Focuses on intellectual abilities (verbal comprehension, perceptual reasoning).

• Cognitive Proficiency Index (CPI): Emphasizes working memory and processing speed.

WAIS Subtests:

• Verbal Comprehension: Similarities, Vocabulary, Information

• Perceptual Reasoning: Block Design, Matrix Reasoning, Visual Puzzles

• Working Memory: Digit Span, Arithmetic

• Processing Speed: Symbol Search, Coding

Culture-Free Intelligence Testing

Traditional intelligence tests have been criticized for cultural and linguistic biases. Raven’s Progressive Matrices, a nonverbal test using patterns and shapes, was developed to minimize these biases and is less affected by language or cultural background.

Historical Misuse and Social Implications

Intelligence testing has been used to justify social policies, including eugenics and forced sterilization, particularly targeting marginalized groups. The eugenics movement, influenced by Galton and Terman, promoted the idea of hereditary intelligence and led to discriminatory practices.

• Racial and Cultural Bias: Intelligence tests have been used to support prejudiced views and social inequities.

• Stereotype Threat: Negative stereotypes can impair test performance among minority groups.

• Socioeconomic Factors: Environmental disadvantages (nutrition, stress, education) impact cognitive development and test scores.

Understanding Intelligence

Defining Intelligence

Intelligence encompasses a range of cognitive abilities, including reasoning, problem-solving, learning, and adaptation. The existence of savant abilities—extraordinary skills in specific domains despite overall low mental capacity—challenges the notion of intelligence as a single, unified ability.

• Fluid Intelligence (Gf): The capacity to solve novel problems and think logically, independent of acquired knowledge.

• Crystallized Intelligence (Gc): The ability to use learned knowledge and experience.

General Intelligence (g)

Charles Spearman proposed that intelligence is a general cognitive ability, or g, underlying performance across various domains. High g predicts academic achievement, job performance, and even lifespan.

• Positive Correlations: Abilities in different domains (verbal, numerical) tend to correlate.

• Predictive Power: g is a strong predictor of educational and occupational outcomes.

• Neuroscience: High g is associated with efficient brain processing and reduced activation during tasks.

Multiple Intelligences and Hierarchical Models

While g explains much, it does not capture all aspects of intelligence. Spearman’s two-factor theory includes both g and specific abilities (s). Louis Thurstone identified seven primary mental abilities, suggesting intelligence is multifaceted.

• Hierarchical Model: Intelligence is structured in three levels: general intelligence (g), a few broad abilities, and many specific skills.

Fluid vs. Crystallized Intelligence

• Fluid Intelligence (Gf): Declines with age, linked to prefrontal cortex efficiency.

• Crystallized Intelligence (Gc): Remains stable or declines later, relies on long-term memory.

• Interconnection: Gf and Gc are related but distinct; higher Gf can lead to higher Gc over time.

Gardner’s Theory of Multiple Intelligences

Howard Gardner proposed at least nine independent intelligences, inspired by cases of savant abilities and brain damage. These include:

• Verbal/linguistic

• Logical/mathematical

• Visuospatial

• Bodily/kinesthetic

• Musical/rhythmical

• Interpersonal

• Self/intrapersonal

• Naturalist

• Existential

Criticisms: Gardner’s theory is difficult to measure and lacks empirical support, but it has influenced educational practices by encouraging recognition of diverse strengths.

Learning Styles: Myth vs. Evidence

• Learning Styles Myth: There is no scientific evidence that matching teaching methods to individual learning styles (visual, auditory, kinesthetic) improves learning outcomes.

• Effective Teaching: Deep processing and meaningful engagement are more important for retention than modality.

Biological, Environmental, and Behavioural Influences on Intelligence

Biological Factors

Intelligence is influenced by both genetic and neural factors, as well as their interaction with the environment.

• Genetics: Twin and adoption studies show a significant genetic component to intelligence. Identical twins raised together have an IQ correlation of about 0.85; raised apart, about 0.80.

• Heritability: Estimated between 40% and 80%, but varies with environment and age. Heritability is higher in wealthy environments.

• Behavioural Genomics: Over 50 genes have been linked to intelligence, each contributing a small effect. Animal studies (e.g., Doogie mice) show genetic manipulation can affect learning and memory.

• Brain Structure: Brain size and complexity (e.g., cortical folding) are modestly correlated with intelligence, especially verbal intelligence.

Environmental Influences

• Enriched Environments: Animal studies show that stimulating environments increase brain size and synaptic connections, enhancing cognitive abilities.

• Birth Order: First-born children tend to have slightly higher IQs, possibly due to more parental attention and teaching younger siblings.

• Socioeconomic Status (SES): Higher SES is associated with higher IQ, likely due to access to resources, supportive parenting, and reduced stress.

• Nutrition: Diets rich in fruits, vegetables, and omega-3s support cognitive functioning; high-fat, high-sugar diets are linked to lower IQ.

• Stress: Chronic stress, especially in poverty, impairs cognitive functions and damages brain regions critical for memory and attention.

• Education: School attendance and quality education significantly impact IQ. IQ scores can drop during summer breaks, except among the wealthiest children.

The Flynn Effect

The Flynn effect refers to the observed rise in average IQ scores over the past century, likely due to improved education, nutrition, and environmental complexity. Recent trends suggest this increase may be plateauing or reversing in some regions.

Nootropic Drugs

• Nootropics: Substances like methylphenidate and modafinil may enhance cognition but carry risks of abuse and unknown long-term effects. Their use is not recommended for healthy individuals.

• Potential Benefits: For clinical populations (e.g., Alzheimer's), even small cognitive improvements can be significant.

Gender and Intelligence

Sex Differences in Intelligence

• Overall IQ: No significant average difference between males and females.

• Variability: Males show greater variability, with more at both extremes.

• Specific Abilities: Females tend to excel in verbal and emotional perception tasks; males in visuospatial tasks.

• Sociocultural Factors: Gender gaps in math and science have narrowed due to increased female participation and changing social attitudes.

• Systemic Barriers: Stereotypes and lack of support contribute to underrepresentation of women in STEM fields.

Summary Tables

Correlations of IQ Scores by Genetic Relatedness

Relationship	IQ Correlation
Identical twins (together)	0.85
Identical twins (apart)	0.80
Fraternal twins (together)	0.59
Full siblings (together)	0.45
Adoptive siblings (together)	0.32
Adopted child & adoptive parent	0.22
Biological child & parent (shared env.)	0.40

Gardner’s Multiple Intelligences

Intelligence Type	Description
Verbal/linguistic	Reading, writing, speaking effectively
Logical/mathematical	Abstract thought, numerical reasoning
Visuospatial	Mental imagery, spatial problem-solving
Bodily/kinesthetic	Control of body movements, balance
Musical/rhythmical	Producing and understanding music
Interpersonal	Understanding others' emotions and motives
Self/intrapersonal	Self-awareness, understanding own emotions
Naturalist	Recognizing patterns in nature
Existential	Contemplating life's meaning and existence

Key Takeaways

• Intelligence is a multifaceted construct influenced by genetic, biological, environmental, and social factors.

• Measurement of intelligence must account for cultural, linguistic, and socioeconomic biases.

• Both general intelligence (g) and specific abilities contribute to cognitive performance.

• Environmental enrichment, nutrition, and education can enhance cognitive abilities, while chronic stress and deprivation can impair them.

• Gender differences in intelligence are minimal overall, but specific abilities and social factors play a role in observed disparities.

Additional info: Where the original notes were brief, academic context and definitions have been expanded for clarity and completeness.