The Varieties of Temperament

William Sheldon's work, particularly The Varieties of Temperament, proposed a discredited theory linking body shape to personality traits. Sheldon classified individuals into three somatotypes: endomorphic (round, fat), mesomorphic (muscular), and ectomorphic (slim, linear). He assigned personality characteristics to each type, suggesting endomorphs were extroverted and relaxed, mesomorphs were active and competitive, and ectomorphs were introverted and quiet.12

This constitutional psychology theory, which suggested body type predicted personality and even morality, has since fallen out of favor. Modern psychology recognizes that numerous factors, including diet, exercise, and culture, influence both body composition and mental traits. Studies on somatotypes have been inconclusive, and the idea of a direct link between physique and personality is now considered pseudoscience.134