A component part of a broad or complex phenotype. The concept was introduced in 1966 by population geneticists B. John and K. R. Lewis to refer to microscopic and internal phenotypes that are not apparent to the casual observer; since then it has been applied in medicine and psychiatry in efforts to unravel the genetic and physiological basis of multifactorial diseases, such as schizophrenia and heart disease. An endophenotype is thus a measurable and genetically determined aspect of a ‘bigger picture’. For example, cortisol secretion is an endophenotype of anxiety. Similarly, in evolutionary biology, complex traits, such as behaviours, can be regarded as the net result of the assembly of various modular endophenotypes.