13.3 Factors Influencing Intelligence

Genetic and Environmental Influences on Intelligence

Intelligence isn't determined by a single factor. It emerges from a constant back-and-forth between your genes and your environment, with each shaping the other across your entire lifespan. Understanding how these factors interact is central to the intelligence debate in cognitive psychology.

Genetic vs. Environmental Factors in Intelligence

Genetic contributions to intelligence involve the combined effects of many genes, each with a small influence. There's no single "intelligence gene." Instead, hundreds or thousands of genetic variants contribute to cognitive ability.

The strongest evidence for genetic influence comes from twin and adoption studies:

• Twin studies compare identical (monozygotic) twins, who share 100% of their DNA, with fraternal (dizygotic) twins, who share about 50%. When identical twins show more similar IQ scores than fraternal twins, that gap points to genetic influence.
• Adoption studies look at whether adopted children's IQ scores correlate more strongly with their biological parents (suggesting genetic influence) or their adoptive parents (suggesting environmental influence). Results typically show correlations with both, but biological parent correlations tend to strengthen over time.
• Heritability estimates for IQ range from 40–80% depending on the study and population. This means that in a given population, 40–80% of the variation in IQ scores can be attributed to genetic differences.

Environmental contributions are equally important and operate at every stage of development:

• Prenatal environment: Maternal nutrition, stress levels, and toxin exposure (e.g., lead, alcohol) all affect fetal brain development before a child is even born.
• Early childhood experiences: The amount of language exposure, cognitive stimulation, and responsive caregiving a child receives shapes neural connections during critical developmental windows.
• Nutrition and health: Deficiencies like iron deficiency or chronic illness can impair brain function and learning capacity.
• Education: Access to quality schooling and enrichment activities provides the cognitive stimulation needed for knowledge acquisition and skill development.

Gene-environment interactions make the picture more complex than "nature vs. nurture":

• Epigenetics shows that environmental factors can alter how genes are expressed without changing the DNA sequence itself. Stress or nutrition, for example, can turn certain genes "on" or "off."
• Reaction range theory proposes that genes set an upper and lower boundary for intelligence, and your environment determines where within that range you actually land.

Shared vs. non-shared environments affect siblings differently:

• Shared environment (family income, neighborhood, parenting style) tends to make siblings more similar to each other.
• Non-shared environment (different friend groups, different teachers, unique personal experiences) helps explain why siblings raised in the same household can still differ in cognitive ability.

One key developmental pattern: heritability of intelligence increases with age. In early childhood, environmental factors have a larger impact. By adulthood, genetic influences account for a greater proportion of IQ variation. This likely happens because as people gain more autonomy, they increasingly select environments that match their genetic predispositions.

Heritability in Intelligence Research

Heritability is the proportion of trait variation in a population that can be attributed to genetic differences. It does not tell you how much of one person's intelligence is genetic. This distinction trips up a lot of students.

• The heritability coefficient ranges from 0 to 1. A heritability of 0.60 means 60% of the variation in IQ scores within that population is associated with genetic differences.
• Narrow-sense heritability considers only additive genetic effects (genes that contribute independently). Broad-sense heritability also includes non-additive effects like dominance and gene interactions.

Important limitations of heritability estimates:

• They apply to populations, not individuals. A heritability of 0.70 does not mean 70% of your intelligence is genetic.
• They assume no systematic gene-environment correlation, which is rarely true in practice.
• They can change depending on the environment. In a population where everyone has equal access to education, heritability would appear higher because environmental variation is reduced.

Gene-environment correlations describe how genetic factors can actually shape the environments people experience. There are three types:

• Passive: Parents pass on both genes and environments. A parent with strong verbal ability likely has many books at home, giving their child both the genes and the environment for language development.
• Evocative: A child's genetically influenced traits elicit responses from others. A curious, engaged child may receive more attention and stimulation from teachers.
• Active (also called "niche-picking"): Individuals seek out environments that match their genetic tendencies. A child with a genetic aptitude for music gravitates toward music classes and practice.

These correlations are one reason heritability increases with age. As children grow into adults, they have more freedom to actively select environments that align with their genetic predispositions.

Socioeconomic and Cultural Factors in Intelligence

Socioeconomic Influences on Cognitive Development

Socioeconomic status (SES) affects cognitive development through several interconnected pathways, not just one:

• Income and resources: Higher-income families can afford books, technology, tutoring, and enrichment activities that provide cognitive stimulation. Lower-income families may lack these resources despite equal parental motivation.
• Parental education: Parents with more education tend to use more complex language with their children and set higher academic expectations, both of which support cognitive growth.
• School quality: Schools in higher-SES neighborhoods typically have better-funded programs, more experienced teachers, and deeper curricula. Teacher expectations also matter: research shows that when teachers expect more from students, students tend to perform better (the Pygmalion effect).

Cultural influences add another layer:

• Language and linguistic diversity affect verbal skills and how well someone performs on tests designed in a particular language or cultural context.
• Cultural values shape what kinds of thinking are prioritized. Some cultures emphasize collaborative problem-solving; others emphasize individual analytical reasoning. Neither approach is inherently "more intelligent," but standardized tests tend to favor certain cognitive styles.
• Test bias remains a concern. When test content reflects the knowledge and norms of one cultural group, members of other groups may score lower not because of lower ability, but because of unfamiliarity with the content or testing format.

The Flynn effect is the well-documented finding that average IQ scores have risen substantially across generations, roughly 3 points per decade in many countries throughout the 20th century. This increase is too fast to be genetic, which points to environmental causes. Proposed explanations include improved nutrition, more years of formal education, greater environmental complexity (more exposure to abstract thinking through technology and media), and smaller family sizes allowing more resources per child.

Stereotype threat occurs when a person is aware of a negative stereotype about their group's intellectual ability and that awareness impairs their performance on cognitive tasks. For example, research by Claude Steele showed that Black students performed worse on standardized tests when told the test measured intellectual ability, but performed equally to White students when the test was described as a problem-solving exercise. Interventions include reframing the testing situation, emphasizing that intelligence is malleable, and affirming the test-taker's sense of belonging.

The Malleability of Intelligence Debate

Can intelligence actually be changed, or is it mostly fixed? This is one of the most active debates in cognitive psychology.

Evidence supporting malleability:

• Neuroplasticity research confirms that the brain can form new neural connections throughout life, not just in childhood. This provides a biological basis for the idea that cognitive abilities can change.
• Early intervention programs have shown real results. The Perry Preschool Project (1960s) provided high-quality preschool to low-income children and found lasting benefits: participants had higher graduation rates, higher earnings, and lower crime rates decades later. Head Start, a large-scale federal program, improves school readiness for low-income children, though the size and duration of its effects are debated.
• Critical periods in brain development suggest that early childhood is the optimal window for certain interventions, particularly those targeting language and basic cognitive skills.

Evidence for limits on malleability:

• Cognitive training programs that target specific skills (like working memory) tend to show near transfer, meaning improvement on similar tasks. But far transfer to general intelligence is much harder to demonstrate. Brain training games, for instance, may make you better at those specific games without raising your overall IQ.
• Variability in responsiveness: Not everyone benefits equally from the same intervention, which may reflect underlying genetic differences in how much cognitive ability can be shifted.
• Fade-out effects: Some early intervention gains diminish over time if children return to low-resource environments, raising questions about whether the gains reflect lasting cognitive change or temporary boosts.

Methodological challenges make this debate hard to settle:

• Long-term follow-up studies are expensive and difficult to maintain, but they're necessary to know whether gains last.
• Proper control groups are essential. Without them, you can't tell whether improvements came from the intervention or from other factors like extra attention or motivation.
• Separating genetic from environmental influences remains inherently difficult, since the two are always interacting.

The current consensus leans toward a middle ground: intelligence is neither fully fixed nor infinitely malleable. Genes set a range of potential, and environmental factors, especially early in life, determine where within that range a person's abilities develop. Targeted interventions can produce meaningful gains, but expecting dramatic, permanent shifts in general intelligence from any single program is not well supported by the evidence.