Twin, Family, and Adoption Studies

Psychologists cannot randomly assign children to genes, so they look for natural experiments instead. Identical (monozygotic) twins share 100 percent of their DNA; fraternal (dizygotic) twins share about 50 percent, like any siblings. If identical twins resemble each other on a trait more than fraternal twins do, that extra similarity has to come from somewhere, and the obvious candidate is the extra shared genetics. Comparing those two correlations is the core move: it holds the shared-environment factor roughly constant across both twin types, since both kinds of twins typically grow up in the same household at the same time, and lets the genetic difference between them explain the gap in similarity.

That logic produces Falconer's formula: heritability equals two times the difference between the MZ correlation and the DZ correlation. The doubling corrects for the fact that fraternal twins already share half their genes, so the raw gap understates the genetic contribution. The resulting number is a heritability estimate, a proportion between 0 and 1 describing how much of the variation in a trait across a population traces back to genetic differences in that population, under the conditions that happened to exist when the data were collected.

Adoption studies extend the design by adding people who share genes but not environment, or environment but not genes. Adopted children resembling their biological parents points toward heredity; resembling their adoptive family points toward environment. Together, twin and adoption data triangulate the same question from different angles, setting up later topics on how genes and environment actually interact rather than just add up.