Module 40: Studying Genetic and Environmental Influences on Intelligence

Heritability

the proportion of variation among individuals in a group that we can attribute to genes. The heritability of a trait may vary, depending on the range of populations and environments studied

3 Genetic points of evidence

1. The intelligence test scores of identical twins raised together are nearly as similar as those of the same person taking the same test twice

2. Scans reveal that identical twins’ brains have similar gray and white matter volume, and the areas associated with verbal and spatial intelligence are virtually the same. Their brains also show similar activity while doing mental tasks

3. intelligence is polygenetic, the combination of many genes

3 environmental points of evidence

1. Where environments vary widely, as they do among children of less-educated parents, environmental differences are more predictive of intelligence scores

2. Adoption enhances the intelligence scores of mistreated or neglected children. So does adoption from poverty into middle-class homes. In one large Swedish study, children adopted into wealthier families with more educated parents had IQ scores averaging 4.4 points higher than their not-adopted biological siblings

3. The intelligence scores of “virtual twins” —same age, unrelated siblings adopted as infants and raised together—correlate +0.28. This suggests a modest influence of their shared environment.

Early environmental influences

1. Test scores of identical twins raised apart are slightly less similar (though still very highly correlated) than the scores of identical twins raised together

2. Studies of children raised in extremely impoverished environments with minimal social interaction indicate that life experiences can significantly influence intelligence test performance

3. Extreme conditions—sensory deprivation, social isolation, poverty—can slow normal brain development

Early intervention

schooling is one intervention that pays intelligence score dividends

growth mindset

A growth mindset refers to the belief that one's abilities and intelligence can be developed through effort, learning, and perseverance. It is the understanding that failure and challenges are opportunities for growth rather than fixed limitations.