American psychologist William Sheldon developed the idea that personality traits correspond to three major body types: endomorphics (soft and round body types who are relaxed, fond of eating, and sociable), mesomorphics (muscular and rectangular body types who are filled with energy, courage, and assertion), and ectomorphics (long and fragile body types who ere brainy, artistic, and introverted). Sheldon’s body type categories exemplify a variable measured at the:.