Constitutional psychology is a theory, eveloped in the 1940s by American psychologist William Sheldon, associating body types with human temperament types. Sheldon proposed that the human physique be classed according to the relative contribution of three fundamental elements, somatotypes, named after the three germ layers of embryonic development: the endoderm, (develops into the digestive tract), the mesoderm, (becomes muscle, heart and blood vessels), and the ectoderm (forms the skin and nervous system).