William H. Sheldon, an American psychologist and physician, designed the first comprehensive system of constitutional psychology. Working through the 1930s and 1940s, Sheldon developed a detailed framework linking body type to personality, publishing his foundational work, *The Varieties of Human Physique*, in 1940. His system introduced the concept of the “somatotype,” a personal score derived from physical measurements that placed every human body somewhere on a spectrum of three fundamental builds.
Sheldon’s Three Body Types
Sheldon identified three extreme body types, which he called endomorphs, mesomorphs, and ectomorphs. Rather than sorting people into rigid categories, he envisioned these as three dimensions that every person expressed to varying degrees. Each individual received a score reflecting how much of each type they displayed.
Endomorphs were described as soft and round in build. Mesomorphs were muscular, athletic, and strong. Ectomorphs were slim, fragile, with long limbs, fine bones, and very little body fat or muscle mass. Most people, Sheldon acknowledged, were blends of all three rather than pure examples of any single type.
Linking Body Type to Personality
What made Sheldon’s system “constitutional” psychology, not just physical classification, was his claim that body type predicted temperament. He matched each somatotype to a corresponding personality profile, creating three temperament categories with their own technical names.
People with endomorphic builds were assigned what Sheldon called a “viscerotonic” temperament: relaxed, sociable, tolerant, comfort-seeking, good-humored, and emotionally warm. He saw them as people who preferred companionship and good food over competition or high activity.
Mesomorphs were linked to a “somatotonic” personality: active, dynamic, assertive, energetic, and more likely to take risks or get into trouble. Ectomorphs corresponded to a “cerebrotonic” temperament: introverted, thoughtful, inhibited, quiet, anxious, and sensitive. In Sheldon’s view, the thinnest people were dominated by mental and nervous activity, while the most muscular were driven toward physical action.
How the Scoring System Worked
Sheldon’s method involved taking standardized photographs and body measurements, then assigning each person a three-digit score. Each digit, rated on a scale, represented the degree of endomorphy, mesomorphy, and ectomorphy present in that individual’s physique. A highly muscular person with little fat and average bone structure would score low on endomorphy, high on mesomorphy, and moderate on ectomorphy. This approach was more nuanced than earlier attempts to sort people into simple categories like “short and heavy” or “tall and thin,” which used crude groupings rather than continuous scales.
The 1940 publication was well received in some circles. A review published through the American Psychological Association praised it as excellent research on the morphological aspects of constitutional organization, noting that Sheldon offered a definite method for somatotyping with reasonable precision. The reviewer recommended the book for anyone interested in reducing the variability of human physique to simpler groupings that might help explain human nature and behavior.
Why Constitutional Psychology Lost Credibility
Despite its initial appeal, Sheldon’s system ran into serious problems. The core claim, that body shape reliably predicts personality, never held up under independent testing. Critics pointed out methodological flaws in how Sheldon collected and analyzed his data, including the fact that he personally rated both the physiques and the temperaments of his subjects, introducing obvious bias. His somatotype classifications have been described in academic literature as closer to fiction than science.
The broader idea that you can read someone’s character from their body has a long and troubled history, and Sheldon’s version carried the same fundamental flaw as its predecessors: correlation between physique and behavior, even when it exists, doesn’t mean one causes the other. A muscular person might behave more assertively not because of some innate constitutional link, but because society treats muscular people differently. Psychology as a whole moved away from these kinds of typological theories in the second half of the 20th century, favoring trait-based models that measure personality dimensions independently of physical characteristics.
Where Somatotypes Survive Today
While constitutional psychology is no longer taken seriously as a personality theory, Sheldon’s body-type terminology has had a surprising second life in sports science and fitness. Researchers studying athletes still use the somatotype framework, stripped of its personality claims, as a way to describe and compare body composition. In this modern usage, endomorphy simply refers to the level of body fat, mesomorphy to muscle mass, and ectomorphy to bone structure and thinness.
Studies of elite athletes consistently find that successful competitors in a given sport tend to cluster around a particular somatotype profile that fits the physical demands of their discipline. Gymnasts, for example, show different somatotype distributions than distance runners or swimmers. In fitness and bodybuilding communities, the terms endomorph, mesomorph, and ectomorph remain widely used shorthand for describing how easily someone gains fat, builds muscle, or stays lean. The language Sheldon created outlasted the theory behind it, persisting wherever people talk about body types, even if the personality predictions he attached to those types have been left behind.