The drive to eat to satisfy one’s hunger is known as hunger motivation.

The body of an organism requires constant nourishment to perform its functions and to grow. Therefore, the primary role of hunger motivation is to provide nourishment to the body. Hunger motivation is a product of various mechanisms and factors in the body. This is the reason why different people have different eating behaviors. For instance, different people express a preference for different varieties of foods. Similarly, some people eat more while others eat less. Resultantly, some people are obese while others are slim.

Since nourishment is primarily a biological need, it is generally understood as caused by biological factors. However, the factors for hunger drive are not limited only to biological factors. However, hunger has its underlying biological, psychological and social factors, as explained below.

Humans have complex biological mechanisms for regulating hunger. For instance, empty stomach sends signals to the brain to indicate the need for eating. Similarly, when some food is eaten, expansion in the (walls of) stomach sends signals to the brain to induce the feeling of satiety (fullness) to stop eating. 

Besides the stomach stimulation, there are also other mechanisms that control hunger, as follows.

Blood carries a number of nutrients to nourish various organs of the body. The level of these nutrients determines the eating behaviour to restore the normal levels of nutrients in the blood. For instance, a specific level of glucose in the blood needs to be maintained. If the level of glucose goes down to the normal level, the body feels the urge to eat some food. This normally happens when the hormone ghrelin tells the brain to activate hunger.

Hunger activation follows a complex mechanism of certain biochemical changes in the body. For instance, when the blood glucose rises above the normal level, insulin is released which converts a major portion of glucose into fats (to be stored under the skin) and glycogen (to be reserved in the liver) for future use. When the body does not get food for some time, it may use the stored fats and glycogen. However, the body does not want to go without food for a long time and to deplete the stored fats and glycogen (which are primarily meant for unavoidable starvation), therefore, in normal situations, if the nutrients level drops below the normal, the body would indicate urge for eating.

Every individual has a unique genetic makeup. The genetic makeup determines different characteristics of the human body including eating behaviour. For instance, some people have genetically a high metabolic rate who are likely to eat more without gaining much weight. Some people have a slow metabolic rate who are likely to eat less but still gain relatively more weight.

The part of the brain, known as the hypothalamus, maintains optimal levels of body’s temperature, heart rate, and blood pressure. This function, to maintain the body’s internal balance, is known as homeostasis. One of the hypothalamus functions is also to maintain nutrients in the body by regulating hunger. It does so by monitoring the level of glucose in the blood and releasing or stopping certain hormones to indicate the urge to eat or to stop eating.

The lateral hypothalamus is the hunger center whereas the ventromedial hypothalamus is the satiety center. The hunger center activates hunger to create an urge to eat. The satiety center activates the feeling of fullness as an indication to stop eating or to refrain from eating. In an experiment where the lateral hypothalamus of the rats was damaged, they refused to eat food. If the lateral hypothalamus is damaged, the person can starve to death. In the experiment, the rats with damaged ventromedial hypothalamus displayed the urge for eating excessively.

Psychological factors such as stress, anxiety and depression also impact our eating behaviour. When we are happy, we display normal eating behaviour. However, if an individual is under stress, he would display abnormal eating behaviour. In stress, some people may eat less or stop eating at all even if their favorite food is presented to them. Some people in stress may also eat excessively. This is because every person has his own style of coping with stress. For this reason, some people may lose weight whereas others may gain weight while they under severe stress.

Another important psychological factor is how we perceive food and eating habit. If we perceive eating as a healthy habit by thinking that our body requires food for getting different nutrients, we will prefer to eat and not to skip our breakfast, lunch, and dinner. However, many people also think that eating is not a healthy habit and thus, they may eat as little as possible or may skip some of the meals.

This psychological factor also relates to the perception of specific foods. For instance, an individual may skip many fruits because he thinks that it has a high sugar content which is not good for health even if the person is not a diabetic. On the other hand, people include many things in their diet because they think it provides vital nutrients (e.g., vitamins and minerals). In some specific cases, an individual also prefers food because he wants its benefits to enhance certain function of his body. For instance, an individual, wanting to enhance his mental capabilities, would prefer eating foods having omega-3 fatty acids such as fish, walnuts and almonds.

Another psychological factor is the perception of one’s weight and shape. People assume set points for their body’s weight which they strive to maintain. If their weight goes above their set point, they will show food avoiding behaviour, but if the weight goes down the setpoint, they may experience psychological ease to include some extra food in their diet. For instance, the individual may think, I have already lost a lot of weight and it's okay now to take some extra food on some occasions.

Correct perception regarding one’s weight is important because it helps the individual develop healthy eating habits and maintain healthy body weight. However, the wrong perception about one’s weight may lead to PSYCHOLOGICAL EATING DISORDERS such as ANOREXIA and BULIMIA, as explained below.

Anorexia nervosa is an eating disorder, in which, an individual refuses to eat because they perceive themselves as obese, even if they are in good shape of the body. These people are generally slim (have less body weight but due to their distorted perception about their weight, they develop a fear of gaining weight. Due to fear of gaining weight, they avoid eating food by skipping their meals.

Many anorexic people also use different tactics to avoid gaining weight. They may use certain exercises and supplements designed for weight loss. They may use laxatives and diuretics in an attempt to lose weight. Anorexia is an extremely unhealthy eating disorder and sometimes even proves to be life-threatening. According to some studies conducted in different countries, up to 10 percent of anorexics starve themselves to death.

As noted earlier, anorexic behaviour is not adopted because the individual is obese, but it is a psychological misperception where the individual thinks of himself as obese and start desiring to get a slim body shape. Different treatments are provided to anorexics to correct their perception regarding their body weight and to reverse the complications of anorexia.

Bulimia nervosa is another eating disorder, in which, the individual develops a binge eating habit, generally followed by purging. The individual eats a large amount of food at different times of the day. The person, after eating a large amount of food, feels guilty and fears weight gain. Therefore, the individual may display purging behaviour after eating. Purging means getting rid of the extra food by self-induced vomiting. For instance, the individual may put a finger in his mouth (or throat) to force himself to vomit and to throw out what they have eaten. The individual undergoes a cycle of binge-eating and purging after eating. In an attempt to avoid weight, the bulimic person may also use laxatives and diuretics to speed up their bowel movement so that the body can find little chance to get calories from the food they have eaten.

Bulimia is an extremely unhealthy eating disorder and sometimes even proves to be life-threatening. However, proper treatment helps the individual correct his psychological orientations in relation to his eating habits.

Social factors are those factors that relates to 1) societal factors and 2) eating in presence of others. Every society has its culture which provides patterns (along with patterns of many other behaviours) of eating behavior. For instance, in some cultures people eat a very little amount of food in their breakfast, lunch and dinner whereas in other cultures, people eat a large amount food in their breakfast, lunch and dinner. Similarly, different cultures have different cuisines and preferences for foods. For instance, in some cultures, excessive use of spices in dishes is preferred whereas in other cultures excessive use of spices may not be viewed as that good.

The other important factor is whether the individual is eating alone or with others. It is generally observed that when individuals eat in gatherings (e.g., with friends), they may eat a larger amount of food than when they eat alone. This is can be called the social effect of the gathering. This is the reason, sometimes an individual may go with choice (type of juice or food such as type of pizza or burger) of his friends when they are ordering food in a group at some restaurant. Moreover, the difference in eating style can also be seen when an individual is eating in a wedding ceremony (or party) and when the individual eats alone at his home. In formal and informal social events, an individual may slightly modify his eating style to make it look suitable in that situation.