The question of how much of intelligence is determined by our genes versus shaped by our environment has been debated for decades by psychologists, scientists and philosophers alike. While it is clear both nature and nurture play a role, researchers are still working to unravel the complex interplay between genetics and environment when it comes to intelligence.
What is intelligence?
Before exploring the origins of intelligence, it is important to define what we mean by intelligence. There are many different conceptualizations of intelligence, but most definitions include the ability to learn, apply knowledge, adapt to new situations, and engage in abstract thinking. Some of the key components that comprise intelligence include:
- Memory
- Executive functioning
- Processing speed
- Verbal comprehension
- Perceptual reasoning
- Logic and reasoning abilities
- Spatial abilities
When researchers investigate the genetic and environmental influences on intelligence, they are looking at these components that make up an individual’s cognitive abilities and capacity to learn.
The nature side – genetics
There is strong evidence that genetics play a significant role in shaping intelligence. Some key findings that support the impact of our DNA include:
- Twin studies – Identical twins raised apart tend to have more similar IQ scores than fraternal twins, suggesting genetic similarity promotes cognitive similarity.
- Heritability estimates – Studies estimating the heritability of intelligence, or how much variation in IQ can be attributed to genetic differences, yield heritability estimates around 50% by adulthood.
- Gene variants – Specific gene variants like COMT and BDNF have been linked to cognitive abilities.
- Shared environment – Shared family effects account for a negligible portion of the variance in IQ scores.
While genetics clearly influence intelligence, it is not as simple as linking IQ to a single “intelligence gene.” Rather, intelligence is a polygenic trait shaped by the complex interplay of thousands of genetic variants that each contribute a small amount to the outcome.
Key Genetic Findings
Twin studies
Twin studies compare the similarity of identical twins, who share 100% of their DNA, to fraternal twins, who share around 50% of their DNA. If identical twins raised apart are more similar in their intelligence than fraternal twins raised apart, that implies a strong genetic influence, as genetics are what most differ between the two types of twins.
Numerous twin studies of IQ have found higher correlations between identical twins compared to fraternal twins. For example, one meta-analysis found an average IQ correlation of .86 for identical twins and .60 for fraternal twins raised apart.
Heritability estimates
Heritability estimates use twin studies and other methods to estimate what proportion of the variance in intelligence can be attributed to genetic differences in a population. While heritability estimates vary based on age and population, meta-analyses generally find that around 50% of the variance in IQ is due to genetic factors by adulthood.
Gene variants
Research has identified certain gene variants associated with cognitive abilities, such as:
- The COMT gene – Linked to performance on IQ tests and working memory.
- The BDNF gene – Associated with memory and learning.
However, each gene variant plays just a small role, highlighting that intelligence depends on the additive influence of thousands of genes.
Shared family environment
While genetics clearly play a role, twin studies have found that shared family effects – influences from the shared environment – account for a very small portion of individual differences in intelligence. This suggests factors shared by siblings like parenting have little long-term effect on IQ.
The nurture side – environment
At the same time, there is substantial evidence that environmental influences also impact intelligence, especially in early childhood. Some key findings supporting the importance of environment include:
- Heritability increases with age – Heritability estimates are lower in childhood compared to adulthood, suggesting environment plays a bigger role early on.
- Socioeconomic status & education – Poverty, lack of education, malnutrition and similar factors can depress cognitive ability.
- Environmental enrichment – Positive early childhood interventions can boost IQ, at least in the short term.
- Non-shared environment – Non-shared experiences account for a significant portion of the variance in IQ and help explain why siblings differ.
Overall, while genetics may set the baseline for intelligence, environmental influences appear to shift cognition up or down within the boundaries of this potential.
Key Environmental Findings
Increasing heritability with age
Heritability estimates for IQ tend to increase significantly from childhood to adulthood. A meta-analysis found heritability estimates around 41% for young children, rising to about 55% in adolescence and reaching up to 86% in adulthood.
This suggests that shared environment, including factors like schooling and parenting style, has a stronger influence on intelligence in childhood compared to genetics.
Socioeconomic status
Disadvantaged environments associated with poverty, malnutrition, lack of education and similar factors can depress IQ scores. Impoverished environments provide less cognitive stimulation and fewer learning resources.
However, these environmental effects tend to fade once conditions improve – children adopted from deprived backgrounds into more enriched homes see their IQs increase.
Early interventions
High quality early childhood programs for disadvantaged children, like the famous Perry Preschool Project, have been found to temporarily raise IQ. For example, one study found participants had an average IQ of 81 at age 3, 94 at age 5 after joining the program, and 87 at age 8 a few years post-intervention.
This suggests positive environmental enrichment can enhance intelligence, but gains may fade without sustained intervention.
Non-shared environment
Non-shared environmental influences – experiences unique to each sibling – are estimated to account for around 35% of the variance in IQ. Factors like peer groups, specific parenting styles, classroom experiences, birth order and more make siblings raised together different.
Interplay between genes and environment
Rather than thinking of it as purely genetic or environmental, intelligence is believed to emerge from complex interactions between nature and nurture over the course of development. Some key insights into how genes and environment intersect include:
- Gene-environment correlation – Individuals tend to seek out environments compatible with their genetic propensities.
- Sensitive periods – Genes may make us most sensitive to environmental influences at certain stages of development.
- Moderators – Environmental factors can moderate gene expression, and vice versa.
- Multifactorial traits – Many genes combine with many environmental factors to shape intelligence.
In general, genetics appear to set a range or potential for intelligence, while our experiences determine where we end up within that range. The enriched the environment, the more opportunities to maximize inborn potential.
Gene-Environment Interplay
Gene-environment correlation
Individuals tend to seek out environments compatible with their genetic propensities – a phenomenon known as gene-environment correlation. For example, a child with strong innate cognitive abilities may enjoy reading and absorb information from books more readily, seek out intellectually stimulating activities, and generate more enriched experiences to support their abilities.
Sensitive periods
Genes may make us most sensitive to environmental influences during certain stages of development, known as sensitive periods. For example, language acquisition appears to be a sensitive period – children learn languages much more easily in early childhood when language-related genes are active.
Moderators
Genes and environment can also moderate one another. For example, socioeconomic status may act as a moderator – children with a genetic propensity for strong cognitive abilities appear to be most affected by growing up in an impoverished environment.
Multifactorial traits
Current research indicates intelligence is a highly polygenic trait influenced by thousands of genetic variants that interact with a wide variety of environmental factors over the lifespan. The interplay between experience and genetics is complex and multifactorial.
How much does intelligence change over a lifetime?
Intelligence is not necessarily fixed at birth or early childhood. Individual performance on IQ tests can change over a lifetime due to aging, health, education, social environment and other factors. However, the relative ranking of most individuals compared to their age peers remains fairly stable over time, a phenomenon known as rank order stability. Some key insights into how IQ changes over the lifespan include:
- Early childhood increases – IQ tends to increase significantly through early childhood due to brain development and environmental enrichment.
- Crystallized vs fluid intelligence – Crystallized intelligence rises with age and education, while fluid intelligence peaks in early adulthood.
- Dementia declines – IQ scores often decline later in life due to brain changes from dementia and related disorders.
- Rank order stability – Each individual tends to maintain their relative IQ ranking compared to age peers despite changes across the lifespan.
Overall, both genetic and environmental influences appear to play a role in shaping the trajectory of intelligence across the lifespan.
Changes in Intelligence Over Time
Early childhood
IQ scores tend to rise significantly through early childhood. For example, the average 6-month old scores around 85 on infant IQ tests, rising to around 100 by age 5. Brain development and increases in knowledge and reasoning abilities drive early cognitive improvements.
Crystallized vs fluid intelligence
Some researchers distinguish between crystalized intelligence – accumulated knowledge and experience – and fluid intelligence – the capacity for memory, abstract thinking and problem solving. Crystallized intelligence steadily builds throughout life while fluid intelligence peaks around age 20.
Age-related declines
Average IQ tends to gradually decrease from middle adulthood into old age. Some cognitive functions like processing speed and memory retrieval decline with age. Dementia also depresses IQ.
Rank order stability
While IQ scores fluctuate, most individuals maintain their relative ranking compared to age-matched peers. Someone at the 75th percentile of IQ at age 12 likely stays around that percentile into adulthood, a phenomenon called rank order stability.
The nature vs nurture debate
The nature versus nurture debate over intelligence has raged for decades, with varying viewpoints:
- Nativists – Support the idea that intelligence is predominantly innate.
- Empiricists – Emphasize the importance of learning and environment on cognitive development.
- Interactionists – Embrace an interaction between biological predispositions and experiential shaping.
Modern research favors an interactionist perspective, considering both genetic potential and environmental influences crucial in the emergence of intelligence. However, some psychologists align more with particular viewpoints:
The Debate Over Time
Early empirical studies
Francis Galton, who coined the phrase “nature versus nurture”, believed intelligence was predominantly inherited based on his empirical studies. Early 20th century empiricists challenged this view, arguing nurture was more important.
Nativist resurgence
In the 1960s-70s, nativist views gained prominence again based on new hereditary evidence. Hans Eysenck was a prominent nativist.
Contemporary interactionist consensus
Today, most experts agree that complex gene-environment interplay drives cognitive development, with neither strictly more important. However, some nativists and empiricists still argue their respective sides.
Perspective | Description | Key Thinkers |
---|---|---|
Nativist | Believe genetics predominantly determine intelligence | Francis Galton, Hans Eysenck |
Empiricist | Argue environment is more impactful | Harold Skeels, James Flynn |
Interactionist | See complex gene-environment interplay | Most modern researchers |
While debates persist, modern intelligence research favors an interactionist model accounting for both genetic potential and environmental influences across development.
Can we improve intelligence?
Given the evidence that both genes and environment shape intelligence, important questions arise around how we can potentially improve IQ in society. Some possibilities include:
- Early childhood enrichment – High quality pre-school, reduced stress, proper nutrition.
- Education investments – Training in reasoning and problem solving.
- Physical exercise – Aerobic activity may boost brain health.
- Brain training – Practicing cognitive skills like memory or logic.
- Neuroenhancement – Pharmaceuticals or supplements to improve mental abilities.
However, boosting intelligence is difficult and gains are often short-lived. Some argue societies should focus less on raising IQ and more on promoting opportunity and achievement for individuals at all points on the cognitive spectrum.
Potential Methods to Improve IQ
Early childhood enrichment
High quality early childhood programs providing proper nutrition, low stress and enriching activities during sensitive developmental periods can temporarily boost IQ scores.
Education investments
Sustained educational investments to train reasoning abilities, problem solving skills and build knowledge over many years may help raise IQ and crystallized intelligence.
Physical exercise
Aerobic exercise has been linked to improved cognitive functioning on some tasks possibly by promoting brain plasticity and growth of new neurons.
Brain training
Practicing cognitive skills like memory, processing speed or logical thinking on standardized brain training exercises may improve performance on related standardized tests.
Neuroenhancement
Drugs, supplements or therapies like transcranial direct current stimulation that directly target brain function may temporarily boost specific cognitive capacities, but more research is needed.
Conclusion
Modern intelligence research indicates both our genetics and environment work together to shape intelligence over a lifetime. While the nature versus nurture debate continues in some circles, most experts agree that it is the interaction between the two that drives cognitive development. Genetics may set a range for potential IQ, but environmental factors determine where we end up in that range. Optimal intellectual growth seems to come through environmental enrichment – education, training, nourishment and more – that allows individuals to maximize their genetic gifts.