twin study

A twin study uses a natural experiment that the human family quietly provides: pairs of siblings who share either all their genes or only half of them, yet typically grew up in the same home. By comparing how alike the two types of pairs are, researchers can tease apart the genetic and environmental sources of a trait's variation.

The logic rests on contrasting identical twins, who come from one fertilised egg and share essentially all their DNA, with fraternal twins, who come from two eggs and share about half of their varying genes, like ordinary siblings. If identical pairs resemble each other much more than fraternal pairs do for some trait, that gap points to genetic influence; if both kinds of pair are similar regardless of genetic sharing, shared environment looms larger.

Twin studies have produced many of the heritability figures quoted for human traits, but they rest on assumptions worth naming honestly. The classic one is that identical and fraternal pairs experience equally similar environments, which may not always hold, and twins are not perfectly representative of everyone else. Rare studies of twins reared apart, and modern molecular methods, are used to check and refine the conclusions.