From ArticleWorld

In the 1940's William Sheldon, an American psychologist, formulated the Somatotypes theory. Based on his research, which by today's standards seems stereotypical, the body type of a human can be directly related to his or her temperament thereby making an excuse for the behavior of some. A kind of predisposition theory, if you will. The three body types Sheldon proposed, after much research, can be broken down into visual stereotypes. Interestingly enough, a visual of each of the three chipmunks from the cartoon Alvin and the Chipmunks can be used to illustrate the body shape of each type. ==Endomorphic body type (Theodore)==

Sheldon centered his theory of the endomorphic body type on the digestive system. A person of this body type is usually overweight and extroverted. In addition, they typically enjoy luxury and excess. Think Jolly Old Saint Nick.

Mesomorphic body type (Alvin)

The character traits of this body type exemplify fitness, energy and competitiveness. In addition, a mesomorph would be the most likely to take risks. An extreme sports enthusiast or a jock would be considered a mesomorph.

Ectomorphic body type (Simon)

The physical description of an ectomorph is stereotypically geeky. An ectomorph is thin and they are sensitive, self-conscious, introverted and artistic. A wallflower or nerdy fellow comes to mind.

Health risks

Unfortunately, Sheldon's perspective on body types has caused some concern over the health risks of such stereotypes. When his theory first came out, no man wanted to be either an ectomorph or an endomorph. Many used unhealthful ways to achieve status as a mesomorph. The ectomorph would increase caloric intake while the endomorph would rely on crash dieting.