Reading focus
Syllabus: Where Do Individual Differences Come From?
Keep a population’s variation separate from an individual’s development and from differences between group averages.
Why you’re reading it. Give the genetic evidence a precise interpretation before encountering large numerical summaries.
A question to consider. A heritability estimate for test scores is 60% in a particular sample. Why does it not mean that 60% of the difference between Maya and Leo was caused by their genes?
Why this reading
Heritability is often asked to explain more than it measures. This review clarifies the statistic and the assumptions behind estimates from relatives. It supplies a vocabulary for asking what a genetic explanation of differences actually establishes.
Before you read
Variance describes how spread out scores are in a population. Heritability relates genetic variance to total phenotypic variance in a specified population and setting. Narrow-sense heritability concerns additive genetic effects; broad-sense heritability includes other genetic contributions. Neither is a percentage of an individual caused by genes.
Read for
- Connect each estimate with the population, environment, measure, and assumptions that define it.
- Follow the distinction between explaining variation within a population and explaining differences between population averages.
- Ask why a highly heritable outcome can still respond to environmental change.
Consider afterwards
- Two schools have the same genetic variance in a test score but different environmental variance. Could their heritability estimates differ, and would that tell you which school could improve its average score?
Editorial context
The conceptual distinctions remain useful, but the review describes genomics in 2008. Its examples are not current universal estimates for psychological traits. The later within-sibship study by Howe and colleagues illustrates why genetic associations across unrelated people can include demographic and indirect family effects. An association with DNA is not automatically an identified biological mechanism.