Behavioral genetics is a multidisciplinary field that studies both the genetic and environmental contributions to variations in behavior. Because genes cannot exist without an environment to act upon, behavioral genetics must consider the unique experiences of an organism. Behavioral geneticists seek to understand how genes influence behavior. Etiology, the study of the cause of disease, is associated with behavioral genetics because many diseases such as Huntington disease have an underlying genetic basis that manifests particular behaviors in affected individuals. Other examples of inherited diseases that express behavioral traits such as mental retardation as part of their phenotype include Down syndrome (trisomy 21) and phenylketonuria (PKU).

Behavioral genetics include studies of twins, related and unrelated family members, adoptees, and individuals with genetic abnormalities. Familial studies are used to determine whether or not a particular trait is inherited by the relative risk for family members inheriting that trait. For...