The percentage of cases where both twins share the same trait or disorder is known as what?

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Concordance rates describe how often both members of a twin pair express the same trait or disorder. In twin studies, researchers compare identical twins, who share nearly all their genes, with fraternal twins, who share about half. If the trait is more often shared by identical twins than fraternal twins, genetics are contributing to it; if the rates are similar, shared environment may be more influential. This measure is distinct from penetrance (how often a genotype leads to the phenotype in those who have it), heritability (the proportion of variation in a trait due to genetics in a population), and polygenic risk (risk from many genes). So the percentage of twin pairs where both share the trait is called concordance rates.