Reported cases of anorexia and bulimia nervosa for Canada, U.S. and Europe including U.K. have increased severalfold in recent decades, through both actual higher incidences and more accurate diagnosis. An estimated 0.51% to 1% of young women in Western societies suffer from the full syndrome of anorexia nervosa, to 2-4% of American college women meet the criteria for bulimia nervosa, and up to 5% of young Westernized women of college age suffer from atypical or partial syndromes. Six times as many women as men are affected. A much larger percentage of Americans (more than 50%) are restrained eaters (interrupting eating behavior before normal comfortable satiety terminates a meal) in other words, the foot is always on the brake while eating or chronic dieters.

The body normally regulates nutrient intake with exquisite sensitivity around a "set point" that maintains weight within a narrow range which remains stable or changes slowly over time. When not afflicted by medical or psychological disorders or by coercive sociocultural norms, people who choose to eat primarily foods low in fats and concentrated sweets, who exercise regularly, and who deal reasonably with everyday stresses, tend to stay within a narrow and usually normal weight range. A role for genetic factors in defining this weight range is supported by twin and adoption studies. However, these built in stable patterns of "motivated behavior" are subject to many aberrations in the United States, more often from learned sociocultural norms than from medical or psychiatric diseases.

Eating disorders can be diagnosed by relatively specific symptoms and signs. Although the fundamental causes of eating disorders are unknown, these conditions can be more accurately identified than many medical disorders for which laboratory tests exist.

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