Constitutional psychology

Constitutional psychology is a theory, developed in the 1940s by American psychologist William Herbert Sheldon, associating body types with human temperament types. Sheldon proposed that the human physique be classified 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).

In his 1954 book, Atlas of Men, Sheldon categorised 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.

The three types
Sheldon's "somatotypes" and their supposed associated physical traits can be summarized as follows:
 * Ectomorphic: characterized by long and thin muscles/limbs and low fat storage; usually referred to as slim. Ectomorphs are not predisposed to store fat or build muscle.
 * Mesomorphic: characterized by medium bones, solid torso, low fat levels, wide shoulders with a narrow waist; usually referred to as muscular. Mesomorphs are predisposed to build muscle but not store fat.
 * Endomorphic: characterized by increased fat storage, a wide waist and a large bone structure, usually referred to as fat. Endomorphs are predisposed to storing fat.

The idea that these general body-types may correlate with general psychological types did not originate with Sheldon. In general outline, it resembles ideas found, for instance, in the tridosha system of Ayurveda; The Republic, by Plato; and propounded in the twentieth century by George Gurdjieff. In addition, Friedrich Nietzsche writes that "nature ... distinguishes" three different physiological body types, which correspond to a Republic-esque hierarchy.

There is evidence that different physiques carry cultural stereotypes. For example, one study found that endomorphs are likely to be perceived as slow, sloppy, and lazy. Mesomorphs, in contrast, are typically stereotyped as popular and hardworking, whereas ectomorphs are often viewed as intelligent but fearful. Stereotypes of mesomorphs are generally much more favorable than those of endomorphs. Stereotypes of ectomorphs are somewhat mixed.

The three body type descriptions could be modulated by body composition, which can be altered by specific diets and training techniques. In a famine, a person who was once considered an endomorph may begin to instead resemble an ectomorph, while an athletic mesomorph may begin to look more like an endomorph as he or she ages and loses muscle mass.

However, some aspects of the somatotype cannot be changed: muscle and adipose mass may change but the bone structure is a fixed characteristic. In the same way, cultural conditions might mask a tendency to one or another temperament.

Modern assessments
Sheldon's theories enjoyed a vogue as the "pop-psych flavor of the month" through the 1950s. Modern scientists, however, generally (with occasional exceptions, some relying on outdated – 1978 or prior – studies) dismiss his claims as outdated, if not outright quackery.

Sheldon's photographs of naked Ivy League undergraduates, numbered in the thousands, were taken under the umbrella of a pre-existing program ostensibly evaluating student posture. The photos were in fact collected by Sheldon to provide data for his ideas about somatotypes. However, as "part of a facade or cover-up" the students and schools were told that the photos were taken to evaluate posture.