Hunger has been studied more intensively than any otherdrive, yet its exact mechanism is still in doubt. Newer investigations, however, have added to our knowledge in three major areas. They have shown (1) that the drive does not merely stem from stomach contractions; (2) that it is regulated to a large extent by the brain and particularly the hypothalamus; and (3) that psychological factors are deeply involved in the strength of our appetite and the foods we choose to satisfy it.Earlier observations suggested that contractions taking place in an empty stomach produced hunger pangs, and these in turn led to awareness of hunger and attempts to find food. The theory received its strongest support from an experiment in which a subject swallowed a balloon which responded to the peristaltic movements. A close correspondence between stomach contractions and conscious sensations of hunger was found (Cannon and Washburn, 1912). Since that time this “local theory” of hunger has been contraverted by many other investigations. Tsang (1938) found that removal of the stomach did not eliminate food-seeking behavior in rats. Morgan and Morgan (1940) obtained the same result when they blocked the sensory connections between the stomach and the brain. Keys et al. (1950), put a number of subjects on a semistarvation diet for a long period, and discovered that they continued to feel hunger pangs even when they were taken off the diet and given full meals. In other words, the stomach does not seem to be the center, or at least the only center, for hunger sensations.Where, then, is the center Present theory holds that certain structures in the brain control the hunger drive by reacting to the chemical state of the blood. This theory was suggested by research conducted by Tschukitschew in 1929 (See Templeton and Quigley, 1930), and verified by Bash in 1939. Bash discovered that well-fed dogs would start looking for food when they were injected with blood from hungry dogs, and the stomach contractions of starving dogs would cease when they received blood from recently fed animals. Further investigation has revealed that the actual brain center which regulates the state of the blood is in the hypothalamus. Electrical stimulation of the lateral part of the structure causes satiated animals to eat (Andersson, Jewell, and Larsson, 1958), and destruction or removal of this area produces loss of appetite, a condition termed aphagia (Anand and Brobeck, 1951). Moreover, a “satiation center” has also been found in the hypothalamus. When this center is stimulated electrically, a hungry animal will stop eating; and when it is removed, the animal will eat ravenously and continuously, a condition known as hy- perphagia. It is believed that medications used for curbing the appetite have their effect on this brain area. See HYPOTHALAMUS.At present there are two competing theories concerning the relation between the hypothalamus and the blood. The glucostatic theory holds that this structure contains special “glucoceptors” which respond to variations in available glucose and alert the body when the blood-sugar level runs low (Mayer, 1955). Although the available evidence is ambiguous, the theory seems to be supported by the general observation that when candy is eaten before a meal it will destroy the appetite. A second explanation, known as the thermal theory, rests on the idea that the hypothalamus responds to changes in blood temperature associated with food deprivation and food intake. There is evidence for this view in the fact that animals will not feed when the heat content of the blood is rising; also, protein, which yields the greatest satiety, produces the greatest amount of body heat (Brobeck, 1957).At the moment there is no way of deciding between these two theories.Nevertheless they do indicate that both the brain and the blood are involved in the hunger mechanism. This general approach is a fruitful one, but it does not explain one important aspect of hunger, namely the wide variations in appetite and food preference that occur in different individuals. We are not just hungry, we are usually hungry for specific foods at specific times. Some of our cravings are doubtless due to the body’s need for certain food elements, and some of our aversions stem from allergies or other physical reactions. Moreover, psychological factors enter into the picture, since our appetite is also governed by social custom and personal taste. Some people are disgusted by the idea of eating snails. Others regard them as a delicacy. Grasshoppers are an accepted food in Japan, but they would turn the stomach of the average American. Such preferences or aversions are acquired, not innate, but they are extremely hard to change because they are surrounded by emotional associations. See SPECIFIC HUNGER. The time for eating is also dictated by custom. Many nutritionists hold that a five-times-a-day schedule is best, especially for children. Nevertheless we not only adapt ourselves to three meals, but are conditioned to specific hours. The Spanish are accustomed to a nine o’clock dinner, but Americans are adjusted to eating three hours earlier— and if we are so occupied that we let the six o’clock hour slip by, and then discover our error, we suddenly become extremely hungry. Eating behavior is also influenced by “social facilitation.” When hens eat in groups of three, they have been found to eat almost twice as much as when they are alone; and a satiated hen will eat an additional 50 per cent when exposed to other hens while they are eating (Bayer, 1929). Human beings are similarly affected. A finicky child will eat everything on his plate at a Scout picnic, and the person on a diet had better beware of dining with gourmets. Many older people who live alone do not maintain an adequate diet because they eat by themselves. See SOCIAL FACILITATION. Hunger also has a pronounced effect on perception and emotion. When we have not eaten for some time, we become increasingly aware of everything that has to do with food, such as odors, advertisements, and restaurants. In fact, we may become so selectively attuned to these stimuli that we are inattentive to everything else (Deutsch and Deutsch, 1963). Experiments show that when we are very hungry we have a tendency to interpret vague or ambiguous shapes as articles of food (R. N. Sanford, 1936). The half-starved men in the Keys experiment mentioned above became so preoccupied with food that they were unable to think or talk of anything else, and in some cases planned to change their occupation to dietetics or agriculture. Finally, emotional disturbances have been found to lie at the root of many cases of overweight and loss of appetite