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.
Sheldon's "somatotypes" and their supposed associated physical traits can be characterized 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 nor build muscle.
Mesomorphic: characterized by medium bones, solid torso, low fat levels and 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 shoulders and a large bone structure, usually referred to as fat, or chunky. Endomorphs are predisposed to storing fat.
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 and usually take part in long distance sports, such as marathon running . 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.
Also, certain people lean towards being an extreme somatotype, for example, someone with anorexia nervosa would be an extreme ectomorph and someone who suffers from obesity would be an extreme endomorph.
Sheldon's theories enjoyed a vogue as the "pop-psych flavor of the month" through the 1950s. Modern scientists, however, generally (with occasional exceptions) dismiss his claims as outdated, if not outright quackery.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 guise 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.
^Sheldon's research led to the strong confirmation of the constitutional psychologist's expectation that there is a noteworthy continuity between the structural/physical aspects of the person and his or her functional/behavioral qualities. Although Sheldon was successful in isolating and measuring dimensions for describing physique and temperament, he cautioned that the dimensions should not be examined in isolation one by one, but, rather, the pattern of relations between the variables should be studied.
^Sheldon maintained that the person's somatotype is genetically determined and causes people to develop and express personality traits consistent with their body builds. For example, he hypothesized that endomorphs (high in fatty tissue) would be sociable, complacent, and capable of easy communication of feelings. He thought mesomorphs (high in muscle tissue) would be adventurous, bold, competitive, aggressive, and energetic, whereas ectomorphs (low in fatty and muscle tissue) would be inhibited, introverted, hypersensitive to pain, and secretive. He tested these hypotheses by having observers rate individuals on these trait dimensions and found empirical support for his ideas (Sheldon, Hartl, & McDermott, 1949, pp. 26–27). Although this study has been strongly criticized on methodological grounds (Sheldon himself made both the physical and psychological ratings), more methodologically sound studies—in which investigator bias was minimized by having one investigator rate the somatotypes and having the study participants independently rate their own personality traits—have also produced supportive evidence for Sheldon's position (Child, 1950; Cortes & Gatti, 1965; Yates & Taylor, 1978).
^"Nude Photos Are Sealed At Smithsonian". New York Times. January 21, 1995. Retrieved December 1, 2011. "Later, other photographs were taken by W. H. Sheldon, a researcher who believed that there was a relationship between body shape and intelligence and other traits. Mr. Sheldon has since died, and his work has long been dismissed by most scientists as quackery. ..."
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Gerrig, Richard; Zimbardo, Phillip G. ♦ Psychology and Life ♦ 7th ed.