Navigation
Investigates the twin studies and environmental factors that contribute to individual differences in intelligence.
Imagine meeting a stranger who shares your laugh, your test scores, and your taste in music—only to discover they are your identical twin, raised 1,000 miles away. How much of your 'genius' was written in your DNA before you were even born?
Heritability is a mathematical estimate, denoted as , that represents the proportion of variation in a trait within a population that can be attributed to genetic differences. For intelligence, studies of identical twins raised apart suggest an between and . It is crucial to remember that heritability does not tell us what percentage of an individual's intelligence is inherited. Instead, it tells us that in a specific group, to of the differences we see between people are due to their genes. As environments become more equal (e.g., everyone gets the same high-quality schooling), heritability actually increases because the only remaining differences between people must be genetic.
Quick Check
If a population has a heritability of 0.60 for IQ, does this mean 60% of your personal IQ score comes from your parents?
Answer
No. Heritability describes the variance within a population group, not the origin of a specific individual's traits.
While genes provide the blueprint, the environment acts as the builder. Socioeconomic Status (SES) significantly impacts how genetic potential is expressed. In high-SES environments, where nutrition and education are abundant, heritability estimates for IQ are high. However, in low-SES environments, environmental deprivation often 'masks' genetic potential, leading to lower heritability estimates. This suggests that poverty can prevent a child's genetic 'ceiling' from ever being reached. Furthermore, the Flynn Effect—the observed rise in average IQ scores by about 3 points per decade globally—proves that rapid environmental changes (better nutrition, more complex technology) can boost cognitive performance far faster than evolution could change our DNA.
Consider two identical seeds (genetically identical twins): 1. Seed A is planted in nutrient-rich soil with plenty of sunlight (High SES). 2. Seed B is planted in dry, rocky soil with little light (Low SES). Even though they have the same 'genetic blueprint' for height, Seed A will grow much taller. The difference in their height is 100% environmental.
Quick Check
What does the Flynn Effect suggest about the role of the environment in intelligence?
Answer
It suggests that environmental improvements (like better health and education) can significantly raise IQ scores across an entire population over short periods of time.
Psychologists use the Reaction Range model to explain the gene-environment dance. This theory suggests that genes set a 'floor' and a 'ceiling' for an individual's potential IQ, while the environment determines where the person falls within that range. For example, a person might have a genetic range of to . In a deprived environment, they may score a ; in an enriched one, a . Additionally, as we age, we engage in niche-picking (active gene-environment correlation). We seek out environments that complement our genetic tendencies. A child with a genetic predisposition for spatial reasoning may join a robotics club, further 'training' the very brain pathways their genes initially favored.
Imagine a student, Alex, has a genetic reaction range for IQ of . 1. If Alex attends an underfunded school with no library, Alex's IQ might manifest at . 2. If Alex is moved to a school with a gifted program and mentors, Alex's IQ might climb toward . 3. The environment accounts for a potential swing of points ().
Researchers compare the correlation () of IQ scores between different pairs: 1. Identical twins (100% shared genes) raised together: 2. Identical twins raised apart: 3. Fraternal twins (50% shared genes) raised together: By comparing (1) and (2), we see that changing the environment for the same genes drops the correlation by . By comparing (1) and (3), we see that halving the genetic similarity while keeping the environment the same drops the correlation by . This indicates that while both matter, genetic similarity in this sample has a stronger influence on IQ correlation than shared environment.
If the heritability of intelligence is 0.70, what does this mean?
Which concept explains why IQ scores have increased significantly over the last century?
Heritability estimates for intelligence tend to be lower in populations living in poverty.
Review Tomorrow
In 24 hours, try to explain the 'Seed and Soil' analogy to a friend to reinforce the concept of gene-environment interaction.
Practice Activity
Look up a recent news article on the 'Flynn Effect' and identify one modern environmental factor (like internet access) that might be influencing IQ scores today.