Physiological Motives - 11 Motivation, Emotion, and Personality - STEP 4 Review the Knowledge You Need to Score High

5 Steps to a 5: AP Psychology - McGraw Hill 2021

Physiological Motives
11 Motivation, Emotion, and Personality
STEP 4 Review the Knowledge You Need to Score High


Why do you eat? You eat when you feel hungry because your stomach is contracting or your blood sugar is low, but you also eat because you love the taste of a particular food, and because you are with friends or family who are eating. Our eating behavior is influenced by biological, social, and cultural factors.

Early research indicated that stomach contractions caused hunger. Yet even people and other animals who have had their stomachs removed still experience hunger. Recent research has revealed receptor cells in the stomach that detect food in the stomach and send neural impulses along the vagus nerve to our brain, reducing our level of hunger.

Hunger and Hormones

Secretion of the hormone cholecystokinin by the small intestine when food enters seems to stimulate the hypothalamus to reduce our level of hunger. When the small intestine releases sugars into the blood, blood sugar concentration increases. When blood sugar levels are high, the pancreas secretes the hormone insulin. For some people, the sight and smell of appealing food can stimulate the secretion of insulin. High levels of insulin generally stimulate hunger. Insulin lowers the blood glucose level by increasing the use of glucose in the tissues, by promoting storage of glucose as glycogen in the liver and muscles, and by promoting formation of fat from glucose. When blood sugar levels are low, insulin release is inhibited and the pancreas secretes the hormone glucagon. Glucagon increases the blood glucose level by stimulating rapid conversion of glycogen into glucose, which is released by the liver and muscles into the bloodstream. Thus, insulin and glucagon work antagonistically through a negative feedback loop to help maintain homeostasis. Other secretions seem involved in hunger also, such as ghrelin secreted by an empty stomach that signals hunger and leptin secreted by fat cells that decreases hunger.

Hunger and the Hypothalmus

Neurons in the liver sensitive to glucose in the surrounding fluid send signals to the hypothalamus by way of the vagus nerve. Three parts of the hypothalamus in the brain seem to integrate information about hunger and satisfaction or satiety. The lateral hypothalamus (LH) was originally called the “on” button for hunger. When stimulated, this structure of the brain will start eating behavior, but if it is lesioned or removed, the individual will not eat at all and will even starve to death. The ventromedial hypothalamus (VMH) was called the satiety center, or “off” button, for hunger. When stimulated, it turns off the urge to eat and when removed, the organism will continue to eat excessively and gain weight rapidly. Recent research indicates that a third region of the hypothalamus called the paraventricular nucleus (PVN) also helps regulate eating behavior as a result of stimulation or inhibition by neurotransmitters. Norepinephrine, GABA, and neuropeptide Y seem to increase the desire for carbohydrates, whereas serotonin seems to decrease the desire for carbohydrates. When the hormone leptin, produced by fat cells, is released into the bloodstream, it acts on receptors in the brain to inhibit release of neuropeptide Y into the PVN, thus inhibiting eating behavior.

Eating and Environmental Factors

Although eating behavior is partially regulated by biological factors, environmental factors such as learned preferences, food-related cues, and stress also influence our desire to eat. We all seem to have some inborn taste preferences for sweet foods, salty foods, and high-fat foods, but learning also influences what we eat. People from different cultures show different patterns of food consumption. Meat and potatoes are consumed in larger quantities in the United States, while rice and fish are the staple foods in Japan. Religious values also influence eating behavior by setting specific rules for the foods we may eat and those we are not permitted to eat. Finally, we tend to learn our food habits from our parents, partly by observational learning and partly by classical conditioning, for example, by pairing foods with pleasant social interactions. What, how often, and how much we should eat are expectations we have learned since we were babies.


Obesity and the potential for health problems associated with diabetes and hypertension are growing concerns in our population. People of normal weight tend to respond to internal, long-term bodily cues, such as stomach contractions and glucose—insulin levels, while those who are obese tend to pay more attention to the short-term, external cues, such as smell, attractiveness of food, and whether it is mealtime. Stress-induced arousal also stimulates eating behavior in a large proportion of the population.

Aware that obesity often leads to health problems and that millions of people try to lose weight, scientists have studied obesity and weight loss. By studying identical twins who were raised apart, they have found that some people inherit a predisposition to be overweight, while others have a predisposition to be too thin. Most people who lose weight on diets tend to put it back on. Most people who try to gain weight have difficulty keeping their weight up. These observations led to the set-point theory, that we each have a set point, or a preset natural body weight, determined by the number of fat cells in our body. When we eat less, our weight goes down and our fat cells contract, which seems to trigger processes that result in decreased metabolism and increased hunger. When we eat more, our weight goes up and our fat cells increase in size, which seems to result in increased metabolism and decreased hunger. If we continue to eat more, we can continue to gain weight, and our set point can go up. Some scientists theorize that many chronic dieters are restrained eaters who stringently control their eating impulses and feel guilty when they fail. They become disinhibited and eat excessively if their control is disrupted, which contributes to weight gain.

Eating Disorders

Slim models and actresses in the media are pictured as ideals in America and in some European countries. Some people are highly motivated to achieve this ideal weight, and develop eating disorders. Underweight people who weigh less than 85 percent of their normal body weight, but are still terrified of being fat, suffer from anorexia nervosa. People with this disorder are usually young women who follow starvation diets and have unrealistic body images. No matter how emaciated they become, people with anorexia still think they are fat and may continue to lose weight, which can result in death. Anorexia is associated with perfectionism, excessive exercising, and an excessive desire for self-control. Bulimia nervosa is a more common eating disorder characterized by eating binges involving the intake of thousands of calories, followed by purging either by vomiting or using laxatives. People with this disorder are also usually young women who think obsessively about food, but who are also terrified of being fat. Following the purge, people with bulimia typically feel guilty, self-critical, and depressed. Purging can cause sore throat, swollen glands, loss of tooth enamel, nutritional deficiencies, dehydration, and intestinal damage. Results of research suggest that some people suffering this disorder secrete less cholecystokinin than normal, have a low level of serotonin, have been teased for being overweight, participate in activities that require slim bodies, have been sexually abused, or are restrained eaters.


Regulation of thirst is similar to regulation of hunger. The lateral hypothalamus seems to be the “on” button for both hunger and thirst. When stimulated, this area of the hypothalamus will start drinking behavior, but if it is lesioned or removed, the individual refuses liquids, even to the point of dehydration. Different neurotransmitters are involved in hunger and thirst. Mouth dryness plays a minor role in stimulating us to drink. More important is the fluid content of cells and the volume of blood. Osmoreceptors are sensitive to dehydration of our cells. When osmoreceptors detect shrinking of our cells, we become thirsty. The hypothalamus stimulates the pituitary to release antidiuretic hormone (ADH), which promotes reabsorption of water in the kidneys, resulting in decreased urination. When we vomit, donate blood, or have diarrhea, the volume of our blood decreases, resulting in decreased blood pressure. This stimulates kidney cells to release an enzyme that causes synthesis of angiotensin, which stimulates thirst receptors in our hypothalamus and septum. Drinking behavior and reabsorption of water in the kidneys result. Not only is thirst affected by internal cues, but it is affected by external cues too. We often get thirsty when we see other people drinking in real life or advertisements. These external stimuli can act as an incentive that stimulates drinking behavior, even when we have had enough to drink. What we drink is affected by customs as well as the weather.

Pain Reduction

Whereas hunger and thirst drives promote eating and drinking behavior, pain promotes avoidance or escape behavior to eliminate causes of discomfort. (Additional information about pain is in Chapter 7.)


Like hunger and thirst, the sex drive involves the hypothalamus, but unlike hunger and thirst, the sex drive can be aroused by almost anything at any time and is not necessary for survival of an individual. The sex drive increases at puberty with an increase in male sex hormones, such as testosterone, and estrogen, as well as small amounts of testosterone, in females. Secretion of hormones by the hypothalamus stimulates the pituitary gland to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH) into the bloodstream, which stimulate gonads (testes in males and ovaries in females) to secrete testosterone and estrogen that initiate and maintain arousal. Without these two hormones, sexual desire is greatly reduced in all species, but testosterone levels in humans seem related to sexual motivation in both sexes. Humans seem less instinctively driven to have children than other animals. Many learned cues are involved in this drive. Societies attempt to regulate sexual behavior by imposing sanctions against incest and encouraging or discouraging masturbation, premarital intercourse, marital intercourse, extramarital intercourse, and homosexuality.

In the 1940s, when biology professor Alfred Kinsey coordinated preparation for a marriage course, he found little scientific information available about sex and decided to pursue that research. He began to interview people about their sex histories, trained other interviewers, and founded the Institute for Sex Research, now the Kinsey Institute, at Indiana University. His popular book Sexual Behavior in the Human Male was based on thousands of interviews, although participants were not randomly selected. A similar book about females followed.

Sexual orientation refers to the direction of an individual’s sexual interest. Homosexuality is a tendency to direct sexual desire toward another person of the same sex, and bisexuality is a tendency to direct sexual desire toward people of both sexes. Heterosexuality is a tendency to direct sexual desire toward people of the opposite sex. While most people are heterosexual, about 10 percent are estimated to be homosexual.

Although the sex drive is not necessary for survival of an individual, it is necessary for survival of the species. According to evolutionary psychologists, mammalian females lack incentive to mate with many males because they optimize reproductive success by being selective in mating, choosing a male who has resources to provide for children. On the other hand, males optimize reproductive success by mating with many females, choosing young, healthy, fertile females.

Masters and Johnson described a pattern of four stages in the biological sexual response cycle of typical men and women, including excitement characterized by sexual arousal; plateau, which involves increased breathing rate, muscle tension, heart rate, and blood pressure; orgasm, which is characterized by ejaculation in males and pleasurable sensations induced by rhythmic muscle contractions in both sexes; and resolution as blood leaves the genitals and sexual arousal lessens, followed in most males by a refractory period during which another erection or orgasm is not achieved. Females show less tendency for a refractory period and are often capable of multiple orgasms.