Genetic Influences on Intelligence and Personality
To what extent are your intelligence and unique personality traits pre-written into your genetic code? Decades of biometric research, twin studies, and modern molecular genomics demonstrate that both intellectual performance and emotional temperament have a substantial, polygenic biological foundation.
1. What is Heritability (h²)?
Before examining specific findings, psychologists must clarify the concept of Heritability:
Definition: Heritability is a statistical metric ranging from 0.0 to 1.0 that estimates the proportion of total phenotypic variation in a specific trait across a population that can be attributed to genetic differences among individuals.
- It does NOT mean that 70% of an individual's IQ came from genes and 30% from school.
- It does NOT mean that a trait is genetically fixed, unchangeable, or fatalistic.
- It does NOT apply to cross-cultural or racial group differences.
- It explains population-level variation: why individuals in a group differ from one another.
- Even highly heritable traits (like height, h² = 0.80) can be dramatically altered by environmental factors (e.g., childhood malnutrition).
2. Genetic Foundations of Intelligence
General cognitive ability (‘g’) is one of the most heritable behavioral traits in human psychology:
The Wilson Effect: Why Heritability of IQ Increases with Age
One of the most surprising discoveries in developmental genetics (discovered by Ronald Wilson) is that the heritability of intelligence increases over a person’s lifespan:
- In early childhood, heritability of IQ is relatively low (~20–30%), because parents completely control the child’s environment (toys, books, diet).
- By middle adulthood, heritability climbs to ~70–80%.
- Explanation (Active Gene-Environment Correlation): As adolescents and adults gain personal autonomy, they actively choose books, peers, careers, and hobbies that match their innate genetic propensities (e.g., a genetically curious person seeks libraries and intellectual debates, amplifying their cognitive phenotype).
3. Genetic Foundations of Personality (The Big Five)
Cross-cultural studies of the Five-Factor Model of Personality (OCEAN) show that genetic variation accounts for approximately 40% to 50% of the variance in every major personality domain:
4. Gottesman’s Reaction Range Model
To explain how heredity and environment collaborate, geneticist Irving Gottesman formulated the Reaction Range Model:
[!TIP] Key Exam Takeaway: Genetics establishes the boundaries of what is biologically possible (the potential range); the environment determines where within that range the individual will actually function in life.