concordance

Concordance is the rate at which both members of a pair share the same trait — most often used for twins. If one twin has a particular condition, concordance asks how often the other twin has it too. A high concordance means the pair tends to match; a low one means they often differ. The mirror-image situation, where the pair does not match, is called discordance.

The measure earns its keep by comparison. Geneticists set the concordance of identical twins, who share essentially all their genes, against that of fraternal twins, who share about half. When identical pairs match far more often than fraternal pairs, the trait is showing a genetic signature; when both kinds of pair match at similar rates, shared environment is doing more of the work. Concordance is thus a basic readout of twin studies.

Its limits are instructive. Concordance well below 100% in identical twins — who are genetically alike — is direct proof that something beyond inherited DNA, whether environment, chance, or epigenetic and developmental noise, shapes the trait. The figure also depends on how a pair is sampled and how the trait is defined, so concordance is a clue to heritability, not a measurement of it on its own.