All organisms must meet biological needs. They need, at minimum, to survive and to reproduce. You already are familiar with the relevant biological mechanisms. Organisms meet survival needs by transferring material between themselves and the environment, bringing in environmental nutrients (food, drink) and excreting bodily waste. Most complex species meet reproduction needs by transferring material between two organisms: a male and female engaging in sex, with sperm and egg uniting and developing into an embryo.

That is the biology. Here, we are interested in the psychology; we need to identify the psychological processes that motivate organisms to meet these biological needs. We’ll do so starting with the need for food, the behavior of eating, and the psychological experience of hunger.

What motivates an organism to eat? Part of the answer is hunger, a feeling of food deprivation that motivates organisms to seek food. The psychological experience of hunger is created by biological systems in the body and brain. When the body needs nutrients, biochemicals circulating throughout the body signal a brain structure known as the hypothalamus (see Chapter 3). The hypothalamus, in turn, sends out biochemical signals that create the experience of hunger (Goldstone, 2006). When we have eaten enough, the hypothalamus sends signals that result in satiety, a feeling of having enough food, or being “full.”

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It sounds simple: When the body needs nutrients, we experience hunger and eat; when we have enough nutrients, we experience satiety and stop. But, as often occurs with simple stories in psychological science, the story is too simple. The true story of eating and hunger is more complex, both psychologically and biologically (Berridge, 2004). One main complication is that there is not one, but two types of hunger (Lowe & Butryn, 2007): homeostatic hunger and hedonic hunger.

HOMEOSTASIS. Homeostasis is the maintenance of a stable state. Homeostatic processes, then, are processes that maintain stability. For example, a thermostat maintains stable room temperature by detecting current temperature and activating heating or air conditioning systems when that temperature deviates from a desired temperature.

Homeostatic hunger is a motivation to eat that is based on the body’s need for energy (Lowe & Butryn, 2007). If you go for a long time without food, your body, as noted above, detects the lack of energy and produces feelings of hunger that motivate you to eat. The eating maintains energy homeostasis, that is, a stable supply of bodily energy (Woods et al., 2000). According to set-point theories of hunger and eating (see Pinel, Assanand, & Lehman, 2000), homeostatic processes control food consumption. People are motivated to eat when their energy supplies fall below their set point, the level that the body naturally tries to maintain. They stop eating when energy needs are met.

However, set-point theories are only part of the story; they do not fully explain when and why people eat (Pinel et al., 2000; Stroebe, Papies, & Aarts, 2008). If you think about it, you can probably figure out their limitation. Consider two examples:

HEDONISM. There is a second motive to eat, and it’s simple: Food tastes good. Even when people do not need more calories, they are motivated to eat because eating high-quality food is pleasurable. Hedonic hunger arises from the anticipated pleasure of eating good food (Lowe & Butryn, 2007). When it looks good, you’ll order dessert—even if the last thing you need is more calories.

From an evolutionary perspective, hedonic hunger makes a lot of sense (Pinel et al., 2000). Consider the distant evolutionary past, when all humans hunted animals and foraged for edible foods. The food supply was unpredictable. Eating only when homeostatically hungry—that is, when nutrition was needed immediately—would have been risky. At the moment nutrition was needed, there might not have been any food available, or an injury might have prevented hunting and gathering. It thus made sense for organisms to eat lots of food whenever it was available. Their bodies could store extra calories as fat, which could fuel the body if food supplies ran short later on. Organisms that were motivated to eat good-tasting food even when they did not need the calories were therefore more likely to survive and reproduce. We all inherit that motivational tendency today.

Evidence that factors beyond homeostasis motivate eating comes from a study of two patients with amnesia (Rozin et al., 1998). On each of a series of days, researchers offered them lunch, engaged them in conversation, offered them lunch again, engaged them in conversation again, and then offered them a third lunch. Being amnesic, they did not remember having already eaten when the subsequent lunches were offered. If eating were controlled by homeostatic signals, signals of satiety would have caused them to refuse the second and third lunches. But, instead, they ate again! On most days, the patients consumed most of the second lunch and much of the third (Rozin et al., 1998).

Although the hedonistic “eat tasty food whenever available” strategy might have been good in the distant past, today it has drawbacks. In contemporary industrialized societies, most people can readily access food. Wherever you are right now, you probably have eaten recently (i.e., within the past few hours) and are near a food source (refrigerator, supermarket, restaurant, vending machine). The evolved tendency to stock up on food, combined with a plentiful supply of tasty foods in the contemporary world’s “food-rich environments” (Stroebe, Papies, & Aarts, 2008, p. 172), contributes to high rates of obesity in industrialized societies.

EATING DISORDERS. The two types of hunger—homeostatic and hedonic—are so motivating that many people find it difficult to control the urge to eat. A small subset of people, however, face challenges beyond the ordinary. These are people with eating disorders: disturbances in eating in which people lose the ability to control their intake of food and, as a result, experience serious health risks (National Institute of Mental Health, 2011).

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There are three types of eating disorders: anorexia nervosa, bulimia nervosa, and binge eating disorder (Heaner & Walsh, 2013; National Institute of Mental Health, 2011).

When eating disorders strike, they usually begin in the teenage years or early adulthood. Surveys suggest that 0.5 to 1% of people experience anorexia or bulimia and that rates of binge eating disorder are about twice as high (Kessler et al., 2013; Preti et al., 2009). The disorders are much more common among women than men.

What causes eating disorders? In particular, why do some people take dramatic measures (e.g., starving themselves, purging) to reduce their intake of calories? There are a number of possible reasons. One is social. Society surrounds people—women, especially—with idealized images of thinness. Flip open a magazine, and you are sure to find ads that tell you, either explicitly or subtly, that thinness is preferred. The social pressure for thinness can become so great that it overpowers the motive to avoid hunger. The finding that eating disorders are more common in cultures that idealize thinness (Polivy & Herman, 2002) supports the idea that social and cultural factors are one important cause.

Another potential cause lies in the family environment. Parents who criticize their children’s appearance and demand that they be thin may contribute to the development of eating disorders (Polivy & Herman, 2002). However, when eating disorders occur, parents are not necessarily to blame. Researchers emphasize that parenting is only one of a large number of factors that can contribute to eating disorders and that no one parenting style is strongly linked to their occurrence (Le Grange et al., 2010).

Finally, a psychological factor that can contribute to the development of eating disorders is perfectionism, that is, a general tendency to strive to fully meet high standards of excellence. The psychotherapist Hilde Bruch (1973) argued that many girls become obsessed with meeting other people’s goals for them; they strive to be a “perfect child.” Controlling one’s eating can become a way of achieving this sense of perfection. Findings suggest that people who are more perfectionistic are more vulnerable to experiencing eating disorders (Bardone-Cone et al., 2007).

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Sex, like hunger, is biologically basic. Humans and other animals desire, pursue, and enjoy sexual activity. The reason is obvious: In the evolutionary past, organisms who were motivated for sex out-reproduced those who were not. We thus inherit sexual motivation.

Although the motivations for both food and sex are biological basics, they differ in two main ways. First, unlike our need for food, we do not literally need sex. Without food, you die. Without sex, you could get by. Many celebrated historical figures have been celibate—sexually inactive for much or all of their lives. Science’s greatest figure, Isaac Newton, was celibate, as was the Renaissance’s greatest genius, Leonardo da Vinci, a homosexual who remained celibate to avoid detection (Abbott, 2000). Today, celibacy is a requirement for clergy in a number of the world’s religions. A celibate life commonly has been prized; some cultures have made it a condition for holding a place of honor. Ancient Rome’s Vestal Virgins were charged with keeping alight the central symbol of the Roman Empire: a perpetual fire that honored Vesta, goddess of home and family, and symbolized the empire’s enduring strength. Keeping the fire burning wasn’t difficult. But to get the job, one had to agree to remain celibate for 30 years (Zoch, 1998).

DIVERSITY IN SEXUAL DESIRE. A second way that sexual motivation differs from hunger is its diversity. Hunger motivates one type of behavior, the eating of food. But sexual motivation is diverse in four ways:

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What accounts for the diversity of human sexuality? There are two “sides” to human sexual life (Tolman & Diamond, 2001). First is the biological side that we share with other species. Animals (including us) inherit biological mechanisms that impel sexual desire. Second, there is a social side. Unlike other animals, humans’ sexual activities are suffused with social meaning: worries about your partner’s enjoyment of the activity; concerns about your own attractiveness and performance; age-linked expectations for engaging in sex; social and moral prohibitions against doing so; media images that shape expectations and desires. Human sexual motivation, then, is much more than a simple expression of biological drives.

SEXUAL DESIRE AND HORMONES. Despite these social aspects of human sexuality, however, sexual motivation does have a core biological foundation: hormones, the chemical messengers that travel through the bloodstream and activate biological processes in the body (see Chapter 3). A hormone central to sexual life is testosterone, which is produced in the gonads—the testes of men and the ovaries of women.

Testosterone’s role in sexual motivation is evident from studies that manipulate testosterone levels experimentally. This is done medically to treat hypogonadism, a condition in which the body produces too little testosterone. Among adult men, hypogonadism results in low levels of sexual motivation; additional symptoms include reduced facial hair growth and muscle mass. When testosterone is administered to people with hypogonadism, these symptoms reverse. In one study, patients received testosterone on a daily basis for six months. After 30 days, their sexual motivation increased. Men reported more frequent sexual daydreams, flirting, and sexual interactions (Wang et al., 2000). Similar results have been obtained among women reporting low levels of sexual desire. Compared with women in a placebo drug condition, those receiving daily doses of testosterone reported more sexual desire, as well as more arousal during sexual activities (Buster et al., 2005).

Just as you need gasoline to fuel a car, you need testosterone to fuel sexual motivation. Once you’ve got some gas in your car, though, varying levels of gas (a quarter tank, half a tank, etc.) do not make your engine more or less powerful. Similarly, once hormone levels are in a normal range, variations in testosterone are relatively unrelated to levels of sexual activity. Research with couples finds that although women’s testosterone levels are correlated with their frequency of sexual experience, men’s levels are unrelated to the couple’s frequency and quality of sexual experiences. Sexual experiences reflect the quality of the couple’s interpersonal relationship, not variations in hormone levels (Zitzmann & Nieschlag, 2001).