In a random sample of pairs, the proportion of pairs where both individuals share a certain characteristic. Often used in twin studies to determine the extent to which a certain gene or group of genes is responsible for a certain trait, such as homosexuality.
Example: In a pair of identical twins, if one twin has schizophrenia, the other is about 50% likely to be diagnosed with the disease at some point. In fraternal twins, the concordance rate for schizophrenia drops to about 15%. Therefore, genetics are partially to blame for schizophrenia, though not entirely, or the concordance rate for identical twins would be 100%.