Sheldon developed a theory of personality types based on physical body types. He identified three main types - ectomorphs, mesomorphs, and endomorphs. Ectomorphs are thin and frail with little muscle mass or body fat. They tend to be shy and introverted. Mesomorphs have an athletic, muscular build. They are assertive, active, and competitive. Endomorphs have a wider frame and more body fat. They are sociable, relaxed, and enjoy food and comfort. Sheldon believed a person's body type correlated with their personality traits and temperament. Other theorists like Kretschmer and Hippocrates also developed systems classifying personality based on physical characteristics and temper