The ectomorph, mesomorph, and endomorph are the three basic body types as identified by psychologist William Sheldon. In his system, the ectomorph accumulates fat most easily, and has a cheerful or jolly disposition. The mesomorph gains muscle easily, has an athletic to powerful build, and bears himself confidently. The ectomporph is skinny, has trouble gaining weight, and has a generally nervous or uneasy disposition. Sheldon thought that by measuring and determining a particular body type, he could make various predictions about a given individual's psychological traits.

To arrive at his conclusions, Sheldon measured the proportions of thousands of students during the early 1950s. To arrive at a more descriptive score rather than mere type, he rated bodies on their components, including body fat, muscle size, and nervousness. An extreme ectomorph would be a 7-1-1, an extreme mesomorph 1-7-1, and an extreme endomorph 1-1-7. While extreme scores would be little more than stereotypes, most would have mixed scores with perhaps one component being dominant. A score of 4-4-4 would be completely average in all respects.

Sheldon called these body types somatotypes, and associated each with a variety of mental and physical characteristics. The ectomorph is easy-going, affable, possibly lazy, holds personal relationships in high esteem, and has a low sex drive. The mesomorph is ruggedly confident, energetic, and free of worry but not careless. The mesomorph's sex drive is higher than the ectomorph, but lower than that of the endomorph. The endomorph is described as nervous, even worrying, with a shy posture and an inhibited disposition, although he has the highest sex drive.

It was also thought that the response of the different types to alcohol confirmed the findings in some respects. The ectomorph enjoys a few drinks with friends, naturally attracting listeners to his stories or songs. The mesomorph, by contrast, was thought to go from confident to overbearing or aggressive when under the effects of alcohol. The endomorph, assumed to be inhibited, would become more nervous and strained in such a setting, and might prefer to remain at home.

Although the types are no longer considered valid by psychologists, later studies confirmed they may contain a bit of truth. The correlations are not very significant, though, and may encourage the practice of stereotyping. The terms are sometimes still used by people like weight lifters, trainers, and other athletes. For instance, since an endomorph supposedly has great difficulty adding muscle, a targeted training program may be recommended for such a person.