The somatotypes are basic classifications of animal body types according to the prominence of different basic tissue types (roughly, digestive, muscular, and nervous tissues). They form the core of a theory, developed in the 1940s by American psychologist William Sheldon, associating body types with human temperament types. This linkage is fairly simplistic and is seen as outdated in physiological science, but the account of somatotypes is still probably a valid if limited way to sort basic body types.
Using anthropometric methods Sheldon studied the photographed bodies of some 4,000 men from front view, side view, and back view. He concluded that the physique of men can be divided into the contribution of three fundamental elements: the somatotypes. He named his somatotypes after the three germ layers of embryonic development: the endoderm, that develops into the digestive tract, the mesoderm, that is to become muscle, heart and blood vessels, and the ectoderm that is to form the nervous system. Sheldon's 'somatotypes' and their associated psychological traits can be summarised as follows:
- The endomorphic body type is centered around the digestive system and is easily overweight. The endomorphic person also has a visceral temperament, which means that they are tolerant, love comfort and luxury, and are extroverted - in short he or she loves food and people.
- The mesomorphic body type is centered around muscle and the circulatory system and has well developed muscles. The mesomorphic person has a somatotonic temperament, and is courageous, energetic, active, dynamic, assertive, aggressive, competitive, and often a risk taker.
- The ectomorphic body type is centered around the brain and nerves. These people are slim and possibly underweight. The ectomorphic person has a cerebrotonic temperament, and is artistic, sensitive, apprehensive and highly self-aware. A more negative way to put it is that he or she is introverted and socially restrained.
In his book Atlas of Men Sheldon categorised all possible body types according to a scale ranging from 1 to 7 for each of the somatotypes, where the true endomorph is 7-1-1, the pure mesomorph 1-7-1 and the 100% ectomorph scores 1-1-7. Setting an order of endomorph, mesomorph, and ectomorph, Sheldon used a scale of 1 to 7 (the closer the number to 1, the less relation to the somatotype; the closer to 7, the greater) to assign values to each. So, for example, a true endomorph would be assigned a number of 711, a mesomorph 171, and an ectomorph 117. Of course, since few people fit into one specific mold, numbers such as 541 (a combination endomorph and mesomorph), 246 (a combination ectomorph and mesomorph), and 153 (a combination mesomorph and ectomorph) are much more common. From type number, an individual's mental characteristics could supposedly be predicted. Sheldon held that a predisposposal for criminality might be influenced by the mix of an individual's body type formula.
Some of this is useful general and human biology, if weak behavioral science. Advanced 'triploplastic' animals, such as mammals, or modern humans in particular, do have these three basic tissue layers. If you disect a cadaver, and weigh the tissue broken down into the three layers of origin, you obtain results that support Sheldon's thesis, but only to a degree. The physical traits are still a useful way to defining body types, and filtered of these conclusions, may provide a base-line for future research.