Source: The Thinking Mind, pp. 389–395 (10-4d, 10-4e)
Tags: intelligence, biological influences, brain size, neural speed, heritability, twin studies, intellectual disability, giftedness, IQ, savant syndrome, Flynn effect, nature vs nurture intelligence, Psychology 1100, Ohio State
Difficulty: Intermediate Prerequisites: Familiarity with behavioural genetics concepts (heritability, twin studies) from the Developing Mind chapter is helpful, as the intelligence-genetics material relies on the same logic.
These two sections address the biology behind individual differences in intelligence and what happens at the statistical extremes. The biological influences section asks: what does the brain actually look like in people who score higher or lower on intelligence tests? The extremes section covers intellectual disability and giftedness. Both topics were frequently missed, likely because students oversimplify the nature-nurture question or confuse heritability at the group level with genetic determinism at the individual level. The final will test nuanced understanding, not just definitions.
Intelligence has measurable biological correlates (brain volume, neural processing speed, cortical thickness) and a significant genetic component (heritability around 0.50 to 0.80 in adults), but environment and gene-environment interactions also play a substantial role. At the extremes, intellectual disability (IQ below 70 with adaptive functioning deficits) and giftedness (IQ above 130) each have biological and environmental contributors.
Intelligence
The global capacity to think rationally, act purposefully, and deal effectively with one's environment. Different theories define it differently (Spearman's g, Gardner's multiple intelligences, Sternberg's triarchic theory), but the biological section focuses on what is measurable through cognitive testing and brain imaging.
Heritability
The proportion of variation in a trait within a population that can be attributed to genetic differences. A heritability of 0.60 means 60% of the variation in intelligence scores in that population is associated with genetic variation. It does not mean 60% of any individual's intelligence is genetic.
Brain volume and intelligence
Research consistently finds a modest positive correlation (around r = 0.30 to 0.40) between total brain volume and scores on intelligence tests. Larger brains tend to score somewhat higher, but the relationship is far from perfect and does not imply that brain size determines intelligence.
Neural processing speed
The speed at which the brain processes information, often measured by reaction time tasks or the speed of neural conduction. Faster processing is modestly correlated with higher intelligence test scores.
Cortical thickness
The thickness of the cerebral cortex, particularly in prefrontal and temporal regions. Some research suggests that the pattern of cortical development (thickening in childhood, thinning in adolescence) is more related to intelligence than thickness at any one time point.
Flynn effect
The well-documented observation that average IQ scores have been rising across generations (roughly 3 points per decade in many countries throughout the 20th century). This rise is too rapid to be genetic and is attributed to environmental factors: better nutrition, more education, greater familiarity with testing, and more cognitively demanding environments.
Intellectual disability (intellectual developmental disorder)
Defined by three criteria: (1) IQ score significantly below average (typically below 70), (2) significant deficits in adaptive functioning (daily living skills, social skills, communication), and (3) onset during the developmental period (before age 18).
Giftedness
Typically defined as an IQ score of 130 or above (two standard deviations above the mean), though some definitions include exceptional talent in specific domains (art, music, leadership) that may not be captured by IQ tests alone.
Savant syndrome
A rare condition in which an individual with significant cognitive or developmental limitations demonstrates extraordinary ability in a narrow domain (music, calculation, drawing, memory for dates). Demonstrates that intelligence is not a single, uniform capacity.
Genetics and heritability:
Twin studies provide the strongest evidence for genetic influence.
Identical (monozygotic) twins raised together: IQ correlation around 0.85.
Identical twins raised apart: IQ correlation around 0.75.
Fraternal (dizygotic) twins raised together: IQ correlation around 0.60.
Unrelated individuals raised together: IQ correlation around 0.25 to 0.30, and this decreases toward zero in adulthood.
Heritability estimates for intelligence in adults range from 0.50 to 0.80. This means a substantial portion of variation in intelligence within a population is associated with genetic differences.
Heritability increases with age. In childhood, shared environment (family, schooling) has a larger effect. By adulthood, genetic influence dominates as people select environments that match their genetic predispositions (gene-environment correlation).
What heritability does not mean:
A high heritability within a group does not explain differences between groups. Even if heritability is 0.80 within two populations, the difference between the populations' average scores could be entirely environmental.
Heritability is a population statistic, not an individual one. Saying intelligence is "80% genetic" for an individual person is a misinterpretation.
A highly heritable trait can still be changed by the environment. Height is highly heritable, yet average height has increased substantially due to improved nutrition.
Brain structure and function:
Brain volume: moderate positive correlation with IQ (r ≈ 0.30 to 0.40). This is statistically reliable but explains only about 10–15% of the variance in intelligence scores.
Neural efficiency: more intelligent individuals may use their brains more efficiently, showing less overall activation (less glucose consumption) when solving problems, particularly problems within their competence.
White matter integrity: better-connected neural pathways (measured by diffusion tensor imaging) are associated with faster processing and higher IQ.
Prefrontal cortex: this region is particularly important for the executive functions (planning, working memory, abstract reasoning) that intelligence tests measure.
The Flynn effect:
Average IQ scores have risen roughly 3 points per decade throughout the 20th century in industrialised nations.
This rise is too fast to be genetic (human evolution does not work that quickly).
Probable environmental causes: improved nutrition, reduced exposure to toxins (lead), more years of formal education, greater exposure to abstract thinking (technology, media), and increased familiarity with standardised testing formats.
The Flynn effect demonstrates that intelligence, while heritable, is substantially shaped by the environment.
Intellectual disability:
Diagnosis requires all three criteria: (1) significantly below-average IQ (below approximately 70), (2) deficits in adaptive functioning, and (3) onset before age 18. An IQ score alone is not sufficient for diagnosis.
Severity levels:
Mild (IQ 50–70): the largest group (about 85% of cases). Can often live independently with some support, hold jobs, and manage daily life.
Moderate (IQ 35–50): can learn basic self-care and simple occupational skills with training and support.
Severe (IQ 20–35): requires significant daily support. Limited speech and self-care abilities.
Profound (IQ below 20): requires constant supervision and care.
Causes:
Biological: chromosomal abnormalities (Down syndrome, trisomy 21), single-gene disorders (phenylketonuria/PKU, fragile X syndrome), prenatal infections (rubella), prenatal substance exposure (foetal alcohol syndrome).
Environmental: severe deprivation, malnutrition, lead exposure, lack of stimulation in early childhood.
Down syndrome is the most common genetic cause of intellectual disability, resulting from an extra copy of chromosome 21.
Foetal alcohol syndrome (FAS) is the most common preventable cause of intellectual disability.
Giftedness:
Typically defined by an IQ of 130+ (top 2% of the population).
Terman's longitudinal study (begun in 1921) followed gifted children into adulthood and found they were generally well-adjusted, healthy, and successful, countering the stereotype of the socially awkward genius.
Giftedness is not purely cognitive. Some definitions include exceptional creativity, leadership, or domain-specific talent.
Gifted individuals still face challenges: boredom in under-stimulating environments, social isolation from age-peers, and perfectionism.
Savant syndrome:
Occurs in individuals (often with autism spectrum disorder) who have extraordinary skill in a specific area despite significant overall cognitive limitations.
Common savant abilities: calendar calculation, musical performance, artistic drawing, mechanical or spatial skills.
Demonstrates that "intelligence" is not a single dimension and that specific cognitive abilities can develop independently of general intellectual functioning.
The heritability-environment distinction matters for education policy. If intelligence is partly environmental, then improving early childhood nutrition, reducing toxin exposure, and providing enriched educational settings can raise cognitive outcomes, and the Flynn effect confirms this at a population level. The intellectual disability criteria matter in clinical and legal settings: courts and schools use both IQ scores and adaptive functioning assessments to determine eligibility for services and accommodations. Understanding giftedness helps educators design appropriate programming rather than assuming gifted children will thrive on their own.
"High heritability means intelligence is fixed and cannot be changed." Heritability describes how much of the variation in a trait is associated with genetic variation in a particular environment. Change the environment, and you can change the outcome. The Flynn effect proves this.
"Brain size determines intelligence." The correlation is real but modest. Many people with smaller brains score higher than people with larger brains. The relationship explains only a small fraction of the variance.
"Intellectual disability is diagnosed by IQ alone." Diagnosis requires significant deficits in adaptive functioning as well as low IQ. A person with an IQ of 68 who manages daily life independently would not meet diagnostic criteria.
"Gifted people are socially maladjusted." Terman's longitudinal study found the opposite: gifted individuals were generally healthier and better adjusted than the general population on average.
⚠️ Understand heritability as a population statistic and be able to explain why high heritability within a group does not explain differences between groups.
⚠️ Know the twin study correlations and what they tell us about genetic and environmental contributions.
⚠️ The Flynn effect is a common exam topic because it illustrates the role of environment on a trait with high heritability. Be ready to explain what it is and what it demonstrates.
⚠️ Know the three criteria for diagnosing intellectual disability (below-average IQ, adaptive functioning deficits, developmental onset). Expect a scenario asking whether a person meets criteria.
⚠️ Be able to identify common causes of intellectual disability (Down syndrome, FAS, PKU) and whether each is genetic or environmental.
True or False: A heritability estimate of 0.70 means that 70% of a single person's intelligence is determined by their genes.
A: False. Heritability describes variation within a population, not within an individual. It means 70% of the variation in intelligence scores in that population is associated with genetic differences.
Fill in the blank: The observation that average IQ scores have been rising across generations is known as the ________ effect.
A: Flynn effect.
True or False: Intellectual disability can be diagnosed solely on the basis of an IQ score below 70.
A: False. Diagnosis also requires deficits in adaptive functioning and onset during the developmental period.
Q: Identical twins raised apart have an IQ correlation of about 0.75. What does this tell us about the contributions of genetics and environment to intelligence?
A: The high correlation despite different environments points to a strong genetic component. However, the fact that the correlation is not 1.0 (and is lower than the ~0.85 for identical twins raised together) shows that environment also contributes.
Q: A politician argues that because intelligence is highly heritable, educational interventions are a waste of money. How would you respond using evidence from the chapter?
A: Heritability is a population statistic that applies within a given environment; it does not mean intelligence is fixed. The Flynn effect shows that environmental improvements (better nutrition, education, reduced toxins) have raised average IQ scores by roughly 3 points per decade. Additionally, a trait can be highly heritable and still respond to environmental change, just as height is highly heritable yet has increased with better nutrition.
Q: What are the three criteria required for a diagnosis of intellectual disability?
A: (1) Significantly below-average intellectual functioning (IQ approximately 70 or below), (2) significant deficits in adaptive functioning (communication, self-care, social skills, daily living), and (3) onset during the developmental period (before age 18).
Q: What is savant syndrome, and what does it tell us about the nature of intelligence?
A: Savant syndrome involves extraordinary ability in a specific domain (such as music, calculation, or drawing) alongside significant overall cognitive limitations. It suggests that intelligence is not a single, unitary capacity but involves multiple, somewhat independent abilities.
The biological-influences material connects directly to the behavioural genetics section in the Developing Mind chapter (twin studies, heritability). It also connects to the neuroscience material in the Biological Mind chapter (brain structure, neural processing). The intellectual disability section overlaps with developmental psychology (prenatal development, teratogen exposure). The nature-nurture debate that runs through the intelligence material is one of the broadest themes in the entire course.
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