Source: PSYCH 1100 General Psychology, The Ohio State University | Guided Notes, Pages 392–405
Difficulty: Introductory | Prerequisites: Read the companion notes on Intelligence: Definition, Measurement, and Theories first.
Intelligence is shaped by both genetics (heritability of roughly 0.75) and environment (nutrition, education, socioeconomic status). Brain structure and function correlate with IQ, but there is no single "intelligence centre." At the extremes of the IQ distribution, intellectual disability (IQ below 70 plus impaired functioning) and giftedness (IQ above 130) each come with distinct patterns of strengths and challenges.
Heritability
The proportion of variation in a trait (here, IQ) within a population that can be attributed to genetic differences. Intelligence has a heritability of approximately 0.75.
In simple terms, about 75% of the differences in IQ between people in a population are linked to genetic variation. This does not mean 75% of your personal IQ is genetic.
Flynn Effect
The observed worldwide increase in IQ scores of about 3 points per decade over the past century. Generally attributed to environmental improvements rather than genetic change.
Think of it as: each generation scores slightly higher on older versions of IQ tests, likely because of better nutrition, education, and health.
Intellectual disability
A diagnosis requiring all three criteria: IQ below 70, cognitive deficits appearing before adulthood, and impaired adaptive functioning (the ability to manage daily life tasks).
Adaptive functioning
The practical ability to handle everyday demands: communication, self-care, social participation, independent living. A low IQ alone does not constitute intellectual disability; adaptive functioning must also be impaired.
Giftedness
Typically defined as an IQ of 130 or above (at least two standard deviations above the mean). Associated with better physical health, emotional stability, and occupational outcomes, though the relationship is correlational.
Cerebral cortex
The outer layer of the brain. Thicker cortex, particularly in the prefrontal cortex and temporal lobes, correlates with higher IQ scores.
Prefrontal cortex
The front portion of the frontal lobes, heavily involved in reasoning, planning, and problem-solving. Damage here tends to impair fluid intelligence while leaving crystallised intelligence relatively intact.
Familial causes (of intellectual disability)
Preventable, environmental causes of mild intellectual disability, such as poverty, malnutrition, and inadequate healthcare during early development. Distinguished from genetic or medical causes.
Intelligence has a high heritability estimate of approximately 0.75, making it one of the most heritable behavioural traits studied.
Key points:
There is no single "intelligence gene." Multiple candidate genes contribute small effects, and the genetic architecture is complex.
Heritability describes variation within a population, not within an individual. Saying heritability is 0.75 means that 75% of the differences between people in that population trace to genetic variation.
High heritability does not mean environment is irrelevant. Environmental factors can still shift the entire distribution (as the Flynn Effect demonstrates).
IQ correlates with brain volume, but the relationship is more nuanced than "bigger brain = smarter."
The thickness of the cerebral cortex matters, particularly in the prefrontal cortex and temporal lobes
There is no single "intelligence centre" in the brain. Intelligence involves distributed networks across multiple regions.
How efficiently the brain works appears to matter as much as its size:
On easy tasks, higher-IQ brains show less neural activity (they solve simple problems more efficiently)
On difficult tasks, higher-IQ brains show more neural activity (they recruit more resources when the challenge demands it)
Damage to the frontal lobes can impair fluid intelligence (novel reasoning, problem-solving) while leaving crystallised intelligence (stored knowledge, vocabulary) relatively intact. This is important clinical and exam-relevant evidence that fluid and crystallised intelligence are dissociable.
Several environmental factors are known to affect IQ:
Socioeconomic status (SES): higher SES is associated with higher IQ, likely through access to better nutrition, healthcare, stimulation, and education
Nutrition: adequate nutrition, especially in early development, supports cognitive growth
Prenatal exposure to toxins: substances like lead, alcohol, and certain chemicals during pregnancy can impair cognitive development
Education: more years of schooling and higher-quality education are associated with higher IQ scores
One of the most striking findings in intelligence research is the Flynn Effect: a worldwide increase in IQ scores of about 3 points per decade over the past 100 years.
Possible explanations include:
Improved nutrition and healthcare
Greater access to education
More cognitively stimulating environments (technology, media, complex work)
Reduced exposure to environmental toxins (e.g. lead in petrol/paint)
The Flynn Effect strongly suggests that environment plays a meaningful role in shaping intelligence, even though heritability within a generation is high.
All three must be present:
IQ below 70
Cognitive deficits present before adulthood
Impaired adaptive functioning (difficulty managing everyday life tasks)
Mild (IQ 55–70)
The most common level
Many individuals live independently, hold jobs, marry, and have families
Sometimes caused by familial/environmental factors: poverty, malnutrition, inadequate early healthcare
These causes are often preventable
Moderate (IQ 40–55)
Many individuals need some level of assisted living, though this varies widely
More often the result of a medical or genetic condition
Down syndrome typically falls into this category
Severe and profound (IQ below 40)
Individuals may learn a limited vocabulary and some basic skills
Requires significant ongoing support
Often the result of serious medical conditions
Individuals with intellectual disability frequently experience:
Insecure attachments with caregivers
Affected language skills
Comorbid emotional or behavioural disorders (estimated 10–40%), including anxiety, impulsiveness, mood disorders, pica, and self-injurious behaviour
Giftedness is typically defined as an IQ of 130 or above (at least two standard deviations above the mean).
Key findings:
High IQ is associated with better physical health, emotional stability, occupational attainment, and social satisfaction. However, these are correlational findings, not proof that high IQ causes these outcomes.
Gifted individuals benefit from targeted education that matches their level. This "ability grouping" strategy also benefits average and below-average students when implemented well.
High IQ is not necessarily correlated with adaptive behaviour. Someone can be intellectually gifted yet struggle with practical daily tasks.
High IQ is not a requirement for making great contributions to society. Many significant contributions come from people outside the gifted range.
Students often think high heritability means environment does not matter. Heritability describes variation within a population under current conditions. Change the environment (better nutrition, less poverty) and IQ scores shift, as the Flynn Effect shows.
Students confuse heritability with inheritance. Heritability of 0.75 does not mean 75% of your IQ comes from your genes. It means 75% of the differences between people in a population are linked to genetic variation.
Students sometimes assume intellectual disability is diagnosed by IQ alone. All three criteria must be met: IQ below 70, onset before adulthood, and impaired adaptive functioning.
Students often think giftedness guarantees success or good adaptive behaviour. High IQ is correlated with positive outcomes, but the relationship is not guaranteed, and correlation is not causation.
⚠️ Know all three diagnostic criteria for intellectual disability, not just IQ below 70.
⚠️ Be able to distinguish the three levels of intellectual disability (mild, moderate, severe/profound) by IQ range and typical functioning.
⚠️ Understand what heritability means and what it does not mean. This is a very common exam question.
⚠️ Know the Flynn Effect: direction (rising), magnitude (about 3 points per decade), and likely environmental explanations.
⚠️ Understand the brain findings: cortical thickness correlates with IQ, frontal lobe damage impairs fluid but not crystallised intelligence, and there is no single intelligence centre.
⚠️ Be prepared to explain why high IQ does not guarantee adaptive behaviour or societal contribution.
True or false: There is a single gene responsible for intelligence.
Fill in the blank: The heritability of intelligence is approximately ____.
True or false: Damage to the frontal lobes impairs both fluid and crystallised intelligence equally.
Fill in the blank: Intellectual disability requires three criteria: IQ below ____, cognitive deficits before _____, and impaired _____ functioning.
True or false: The Flynn Effect shows IQ scores have been declining worldwide.
Answers: 1. False (multiple genes contribute small effects). 2. 0.75. 3. False (frontal lobe damage impairs fluid intelligence but tends to leave crystallised intelligence intact). 4. 70; adulthood; adaptive. 5. False (scores have been rising, by about 3 points per decade).
Q: What does it mean to say intelligence has a heritability of 0.75?
A: It means that approximately 75% of the variation in IQ scores between individuals in a population can be attributed to genetic differences. It does not mean 75% of any one person's IQ is determined by genes, and it does not mean environment is unimportant.
Q: A patient suffers frontal lobe damage. Which type of intelligence is most likely impaired, and which is likely preserved? Explain why.
A: Fluid intelligence (novel reasoning, problem-solving without prior knowledge) is most likely impaired, because it relies heavily on the prefrontal cortex. Crystallised intelligence (applying stored knowledge and skills) is likely preserved, because it draws on widely distributed memory networks rather than frontal lobe processing alone.
Q: What are the three criteria required for a diagnosis of intellectual disability?
A: (1) IQ below 70, (2) cognitive deficits that are present before adulthood, and (3) impaired adaptive functioning (difficulty managing everyday life demands such as communication, self-care, and independent living).
Q: Explain the Flynn Effect and give two possible explanations for it.
A: The Flynn Effect is the well-documented worldwide rise in IQ scores of about 3 points per decade over the past century. Possible explanations include improved nutrition and healthcare, greater access to education, more cognitively stimulating environments, and reduced exposure to toxins such as lead.
Q: Why is giftedness (IQ above 130) not a guarantee of success or adaptive behaviour?
A: The association between high IQ and positive outcomes (health, career, satisfaction) is correlational, not causal. High IQ does not necessarily predict good adaptive behaviour, and many people who make great societal contributions fall outside the gifted IQ range.
The genetics and heritability discussion ties directly into the nature vs. nurture debate from the developmental psychology unit. Twin studies and adoption studies (covered in that chapter) are the main methods used to estimate heritability.
Intellectual disability connects to the abnormal psychology and clinical chapters, where comorbid conditions (anxiety, mood disorders) and diagnostic criteria are covered in more depth.
The Flynn Effect connects to research methods: it illustrates how cohort effects and environmental changes can shift population-level data, which matters for interpreting any longitudinal or cross-sectional study.
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