In 1973, professor of psychology Irving Gottesman and his colleague, James Shields, introduced the term endophenotype into human psychiatric genetic research as a result of their groundbreaking research studying schizophrenia in twins and families. Their work centered on identifying behavioral characteristics that occurred as a result of genetics. Endo refers to the concept of “within” or “internal.” Thus, endophenotype embodies a measurable, behavioral characteristic clearly [Page 548]associated with a genetic condition. This entry discusses the different natures of enophenotypes, as well as past and current research concerning them.