In some instances, a biological need triggers a corresponding state of psychological arousal or tension. This unlearned drive is called a primary drive. Hunger, thirst, and sex are the principal primary drives.

Hunger is primarily regulated by two centers in the brain: the hunger center, which stimulates eating, and the satiety center, which reduces the feeling of hunger. Whenever the level of the simple sugar glucose in the blood falls to a certain point, neurons in the hunger center are stimulated. Receptors in the stomach and a hormone released by the small intestine also send signals to the brain. Another hunger regulator monitors long-term body weight. Both the motivation to eat and overeating are influenced by biological, psychological, cultural, and environmental factors.

Thirst parallels hunger in that both internal and external cues can trigger the thirst drive. Dehydration both inside and outside the cells prompts activation of the thirst drive; so do weather conditions as well as social, psychological, and cultural influences, and other external stimuli.

Weight loss is difficult to achieve and maintain for many obese people because the body appears to have a homeostatic mechanism, known as the set point, that regulates metabolism, fat storage, and food intake so as to maintain a preprogrammed weight. Genetic factors also play a major role in determining who is thin and who is overweight.

Anorexia nervosa is a serious eating disorder associated with an intense fear of weight gain and a distorted body image. Another eating disorder known as bulimia is characterized by binges of eating followed by self-induced vomiting. Eating disorders are notoriously difficult to treat, especially in a culture obsessed with dieting.

Sex is a primary drive that gives rise to reproductive behavior essential for the survival of the species. The sexual response cycle in humans progresses through four phases: excitement, plateau, orgasm (climax), and resolution.

Biological factors have a complex effect on sexual response. The male sex hormone testosterone influences early development, male/female differentiation, and to some extent characteristic patterns of adult sexual behavior. It is also possible that scents, called pheromones, secreted by one sex promote sexual readiness in the other sex.

Psychological influences are at least as important as biological influences in stimulating sexual arousal. People have individual preferences for certain fantasies, pictures, words, music, and so on. Men tend to be aroused by visual cues; women respond more to touch. What we find attractive is also influenced by our culture.

Sexual orientation refers to the direction of an individual’s sexual interest–heterosexuals are sexually attracted to members of the opposite sex, homosexuals to members of their own sex, and bisexuals toward members of both sexes. As with most complex behaviors, the origins of sexual orientation appear to involve both biological and environmental factors.