ratio and thus indicating no association). If the association is solely due to shared environmental characteristics, then population > DZ discordant = MZ discordant = 1.0. And if the risk factor causes the outcome (or both are attributable to unmeasured variables), then population = DZ discordant = MZ discordant > 1.0. If both causal (or unmeasured) and non-causal factors contribute to the association, we would expect to see a pattern where population > DZ > MZ > 1.0.
