Somatotype is a highly disputed taxonomy developed in the 1940s by the American psychologist William Herbert Sheldon to categorize the human physique according to the relative contribution of three fundamental elements which he termed somatotypes, classified by him as ectomorphic, mesomorphic, and endomorphic. He created these terms borrowing from the three germ layers of embryonic development: The endoderm, (which develops into the digestive tract), the mesoderm, (which becomes muscle, heart, and blood vessels) and the ectoderm (which forms the skin and nervous system). Later variations of these categories, developed by his original research assistant Barbara Heath, and later by Lindsay Carter and Rob Rempel, are still in occasional academic use.

Constitutional psychology is a theory developed by Sheldon in the 1940s, which attempted to associate his somatotype classifications with human temperament types. The foundation of these ideas originated with Francis Galton and eugenics. Sheldon and Earnest Hooton were seen as leaders of a school of thought, popular in anthropology at the time, which held that the size and shape of a person's body indicated intelligence, moral worth and future achievement.

In his 1954 book, Atlas of Men, Sheldon categorized all possible body types according to a scale ranging from 1 to 7 for each of the three somatotypes, where the pure endomorph is 7–1–1, the pure mesomorph 1–7–1 and the pure ectomorph scores 1–1–7. From type number, an individual's mental characteristics could supposedly be predicted. In a late version of a pseudoscientific thread within criminology in which criminality is claimed to be an innate characteristic that can be recognized through particular physiognomic markers (as in Cesare Lombroso's theory of phrenology), Sheldon contended that criminals tended to be 'mesomorphic'. The system of somatotyping is still in use in the field of physical education.[1]