Familial aggregation refers to clustering of traits or diseases within families, which may be due to genetic, environmental, or infectious factors. Familial aggregation studies are usually the first step in determining if such clustering exists. They are designed to investigate whether the risk for a trait or disease is increased in an individual if the individual’s family members and relatives also have that trait or disease. Once a relationship is established in a familial aggregation study that a trait is clustered in families, then further studies can be conducted into the nature of the relationship and to what extent genetics, the environment, and infectious agents play a role in the disease process. This entry describes the process of studying family aggregation and examines the implication ...