Leonardo is a robot, so he does what he is programmed to do and nothing more. Because he doesn’t have wants and urges—doesn’t crave friendship or desire chocolate or hate homework—he doesn’t initiate his own behavior. He can learn but not yearn. Motivation refers to the purpose or goal of an action, and human beings have two basic kinds of motivations: the biological and psychological. Let’s explore each in turn.

Humans are animals, and all animals must survive and reproduce. It is not surprising, then, that two of our most powerful biological motivations are toward food and sex. We are driven to make lunch and driven to make love, but as you will see, neither of these motivations is as simple as it sounds.

Survival: The Motivation for Food

When a thermostat detects that a room is too cold, it sends a signal that initiates a corrective action, namely, turning up the furnace. Bodies are a bit like thermostats in this way. To survive, the body must maintain precise levels of nutrition, warmth, and so on, and when these levels depart from optimality, the body sends signals to the brain, asking it to take corrective action. That signal is called a drive, which is an internal state that signals a physiological need.

The most regular and familiar of your body’s drives is what you call hunger. At every moment, your body is sending signals to your brain about the amount of energy it has available. If your body needs energy, it sends a signal to tell your brain to switch hunger on, and if your body has sufficient energy, it sends a signal to tell your brain to switch hunger off (Gropp et al., 2005). No one knows precisely what these signals are or how they are sent and received, but research has identified a few candidates.

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For example, ghrelin is a hormone that is produced in the stomach and appears to be one of the signals that tells the brain to switch hunger on (Inui, 2001; Nakazato et al., 2001). When people are injected with ghrelin, they become intensely hungry and eat about 30% more than usual (Wren et al., 2001). Another chemical called leptin is secreted by fat cells, and it appears to be one of the signals that tells the brain to switch hunger off, which makes food less rewarding (Farooqi et al., 2007). People who are born with a leptin deficiency have trouble controlling their appetites (Montague et al., 1997). Some researchers think this story is far too simple. They argue that there is no general drive called hunger, but rather, that there are many different hungers, each of which is a response to a unique nutritional deficit and each of which is switched on by a unique chemical messenger (Rozin & Kalat, 1971). For example, rats that are deprived of proteins will gladly eat proteins but will turn down fats and carbohydrates, suggesting that they are experiencing a specific “protein hunger” and not a general hunger (Rozin, 1968).

Whether hunger is signaled by one chemical or many, the primary receiver of these signals is the hypothalamus (see FIGURE 8.10). The lateral hypothalamus receives the “hunger on” signals, and when it is destroyed, animals sitting in a cage full of food will starve themselves to death. The ventromedial hypothalamus receives the “hunger off” signals, and when it is destroyed, animals will gorge themselves to the point of illness and obesity (Miller, 1960; Steinbaum & Miller, 1965). These two structures were once thought to be the “hunger center” and “satiety center” of the brain, but it turns out to be much more complicated than that (Woods et al., 1998). These structures clearly play an important role in hunger, but no one yet knows exactly what the role is (Stellar & Stellar, 1985).

EATING DISORDERS. Feelings of hunger tell us when to start eating and when to stop. But for the 10 to 30 million Americans who have eating disorders, eating is a much more complicated affair (Hoek & van Hoeken, 2003). For instance, bulimia nervosa is an eating disorder characterized by binge eating followed by purging. People with bulimia typically ingest large quantities of food in a relatively short period and then take laxatives or induce vomiting to purge the food from their bodies. These people are caught in a cycle: They eat to ease negative emotions such as sadness and anxiety, but then concern about weight gain leads them to experience negative emotions such as guilt and self-loathing, and these emotions then lead them to purge (Sherry & Hall, 2009; cf. Haedt-Matt & Keel, 2011).

Anorexia nervosa is an eating disorder characterized by an intense fear of being fat and severe restriction of food intake. People with anorexia tend to have a distorted body image that leads them to believe they are fat when they are actually emaciated, and they tend to be high-achieving perfectionists who see their severe control of eating as a triumph of will over impulse. Remember ghrelin—the chemical that turns hunger on? Well, contrary to what you might expect, people with anorexia have extremely high levels of ghrelin in their blood, which suggests that their bodies are trying desperately to switch hunger on but that hunger’s call is somehow being suppressed, ignored, or overridden (Ariyasu et al., 2001).

Anorexia may have both cultural and biological causes (Klump & Culbert, 2007). For example, women with anorexia typically believe that thinness equals beauty (which isn’t a very surprising conclusion for women to draw when the average American fashion model is seven inches taller and 23 pounds lighter than the average American woman). But anorexia is not just “vanity run amok” (Striegel-Moore & Bulik, 2007, p. 193). Many researchers believe that there are as-yet-undiscovered biological and/or genetic components to the illness as well. For example, although anorexia primarily affects women, men have a sharply increased risk of becoming anorexic if they have a female twin who has the disorder (Procopio & Marriott, 2007), suggesting that anorexia may have something to do with prenatal exposure to female hormones.

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OBESITY. America’s most pervasive eating-related problem is obesity, which is defined as having a body mass index (BMI) of 30 or greater. TABLE 8.1 allows you to compute your BMI, and the odds are that you won’t like the number. Since 1999, Americans have collectively gained more than a billion pounds (Kolbert, 2009). In 2012, only one state (Colorado) had an obesity rate lower than 20% (see FIGURE 8.11).

Why should you care? Every year, obesity-related illnesses cost our nation about $147 billion (Finkelstein et al., 2009) and about 3 million lives (Allison et al., 1999). In addition to these financial and physical costs, obese people tend to be viewed negatively by others, have lower self-esteem, and have a lower quality of life (Hebl & Heatherton, 1997; Kolotkin, Meter, & Williams, 2001). The stigma of obesity is so powerful that average-weight people are viewed negatively if they even have a relationship with someone who is obese (Hebl & Mannix, 2003).

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What causes obesity? First, obesity is highly heritable (Allison et al., 1996) and may well have a genetic component. For example, obese people are often leptin-resistant, which is to say that their brains do not respond to the chemical message that shuts hunger off (Friedman & Halaas, 1998; Heymsfield et al., 1999). Second, obesity may have environmental causes. For example, some studies suggest that toxins in the environment can disrupt the functioning of the endocrine system and predispose people to obesity (Grün & Blumberg, 2006; Newbold et al., 2005), and other studies suggest that obesity can be caused by a dearth of “good bacteria” in the gut (Liou et al., 2013).

But in most cases, obesity is simply the result of eating too much. We don’t breathe ourselves sick or sleep ourselves sick, so why do we eat ourselves sick? Blame the design. Hundreds of thousands of years ago, the main food-related problem facing our ancestors was starvation, and so our brains and bodies developed two strategies to avoid it. First, our brains developed a strong attraction to foods that provide large amounts of energy per bite (in other words, high calorie foods), which is why most of us prefer hamburgers and milkshakes to celery and water. Second, our bodies developed an ability to store excess food energy in the form of fat, which enabled us to eat more than we needed when food was plentiful and then live off our reserves when food was scarce. Our brains and bodies are beautifully engineered for a world in which high calorie foods are scarce, and the problem is that we don’t live in that world anymore. Instead, we live in a world in which the fatty miracles of modern technology—from chocolate cupcakes to sausage pizzas—are inexpensive and readily available.

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We are designed to gain weight easily—and to lose it with great difficulty. The human body resists weight loss in two ways. First, when we gain weight, we experience an increase in both the size and the number of fat cells in our bodies (usually in our abdomens if we are male and in our thighs and buttocks if we are female). But when we lose weight, we experience a decrease in the size of our fat cells but no decrease in their number. Once our bodies have added a fat cell, that cell is pretty much there to stay. It may become thinner when we diet, but it is unlikely to die. Second, our bodies respond to dieting by decreasing our metabolism, which is the rate at which energy is used by the body. When our bodies sense that we are living through a famine (which is what they conclude when we refuse to feed them), they find more efficient ways to turn food into fat, which was a great trick for our ancestors but a real nuisance for us. The bottom line is that avoiding obesity is easier than overcoming it (Casazza et al., 2013).

And avoiding it isn’t as difficult as you might think. People eat when they are hungry, of course, but they also eat when cues in the environment tell them to—for instance, when they see other people eating or when the clock says it is time for lunch (Herman, Roth, & Polivy, 2003; Rozin et al., 1998; see the Real World box). If environmental cues can make us eat, can they also make us stop? Research suggests the answer is yes. In one study, snacking students ate fewer Pringles when every seventh chip was colored red, presumably because the color coding allowed them to keep track of how much they were eating (Geier, Wansink, & Rozin, 2012). In another study, people ate 22% less pasta with tomato sauce when they used a white plate instead of a red plate, presumably because the white plate provided a stark contrast that allowed them to see what they were eating (van Ittersum & Wansink, 2012). These and dozens of other studies show that small changes in our environments can prevent big changes in our waistlines.

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Reproduction: The Motivation for Sex

Food motivates us because it is essential to our survival. But sex is also essential to our survival—or at least to the survival of our DNA—which is why evolution has wired a sex drive into almost every human brain.

The chemistry behind that drive is fairly well understood. The hormone dihydroepiandosterone (DHEA) seems to be involved in the initial onset of the sex drive. Both boys and girls begin producing this slow-acting hormone at about the age of 6, which may explain why boys and girls both experience their initial sexual interest at about the age of 10. Two other hormones have more gender-specific effects. Both males and females produce testosterone and estrogen, but males produce more of the former and females produce more of the latter. As you will learn in the Development chapter, these two hormones are largely responsible for the physical and psychological changes that characterize puberty.

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But are they also responsible for the waxing and waning of the sex drive in adults? The answer appears to be yes—as long as those adults are rats. Testosterone increases the sex drive of male rats, and estrogen increases the sex drive of female rats. But the story for human beings is far more interesting. The females of most mammalian species (e.g., dogs, cats, and rats) are only interested in sex when their estrogen levels are high, which happens when they are ovulating (i.e., when they are “in estrus” or “in heat”). But female humans can be interested in sex at any point in their monthly cycles. Although the level of estrogen in a woman’s body changes dramatically over the course of her monthly menstrual cycle, studies suggest that sexual desire changes little, if at all.

If estrogen is not the chemical basis of women’s sex drives, then what is? Two pieces of evidence suggest that the answer is testosterone—the same hormone that provides the chemical basis of men’s sex drives. First, when women are given testosterone, their sex drives increase. Second, men naturally have more testosterone than women do, and they generally have stronger sex drives. Men are more likely than women to think about sex, have sexual fantasies, seek sex and sexual variety (whether positions or partners), masturbate, want sex at an early point in a relationship, and complain about low sex drive in their partners (Baumeister, Cantanese, & Vohs, 2001). All of this suggests that testosterone is the chemical basis of the sex drive in both men and women.

Although men and women have on average different levels of sex drive, they have sex for similar reasons. Sex is a prerequisite for reproduction, of course, but the vast majority of sexual acts are not meant to produce babies. College students, for example, are rarely aiming to get pregnant, but they often have sex, and when they do it is because of physical attraction (“The person had beautiful eyes”), as a means to an end (“I wanted to be popular”), to increase emotional connection (“I wanted to communicate at a deeper level”), and to alleviate insecurity (“It was the only way my partner would spend time with me”; Meston & Buss, 2007). Although men are more likely than women to report having sex for purely physical reasons, TABLE 8.2 shows that men and women don’t differ dramatically in this regard. We will have much more to say about sexual attraction and relationships in the Social Psychology chapter.

Survival and reproduction are every animal’s first order of business, so it is no surprise that we are strongly motivated by food and sex. But we are motivated by other things too. Yes, we crave kisses of both the chocolate and romantic variety, but we also crave friendship and respect, security and certainty, wisdom and meaning, and a whole lot more. The psychologist Abraham Maslow (1954) argued that all human motivations (which he called needs) could be arranged in a hierarchy (see FIGURE 8.12) with biological motivations at the bottom and psychological motivations at the top. He suggested that until motivations at one level were satisfied, motivations at higher levels were ignored. In other words, when people are hungry or exhausted, they don’t worry too much about intellectual fulfillment and moral clarity (see FIGURE 8.13).

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Maslow was correct in observing that psychological motivations are usually more deferrable than biological motivations. But these two kinds of motivations differ in other ways as well. For example, although we share our biological motivations with most other animals, our psychological motivations are uniquely human. Chimps and rabbits and robins and turtles are all motivated to have sex, but only human beings are motivated to imbue the act with meaning. In addition, although our biological motivations are few—food, sex, oxygen, sleep, and a small handful of other things—our psychological motivations are so numerous and varied that no psychologist has ever been able to make a complete list (Hofmann, Vohs, & Baumeister, 2012). Nonetheless, even if you looked at an incomplete list, you’d quickly notice that psychological motivations vary on three key dimensions: extrinsic versus intrinsic, conscious versus unconscious, and approach versus avoidance. Let’s examine each of these.

Intrinsic vs. Extrinsic

Taking a psychology exam and eating a French fry are different in many ways. One makes you tired and the other makes you chubby, one requires that you move your lips and the other requires that you don’t, and so on. But the key difference between these activities is that one is a means to an end and one is an end in itself. An intrinsic motivation is a motivation to take actions that are themselves rewarding. When we eat a French fry because it tastes good, dance because it feels good, or listen to music because it sounds good, we are intrinsically motivated. These activities don’t have a payoff: They are a payoff. Conversely, an extrinsic motivation is a motivation to take actions that lead to reward. When we floss our teeth so we can avoid gum disease (and get dates), when we work hard for money so we can pay our rent (and get dates), and when we take an exam so we can get a college degree (and get money to get dates), we are extrinsically motivated. None of these things directly brings pleasure, but all may lead to pleasure in the long run.

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Extrinsic motivation gets a bad rap. Americans tend to believe that people should “follow their hearts” and “do what they love,” and we feel sorry for students who choose courses just to please their parents and for parents who choose jobs just to make a living. But the fact is that our ability to engage in behaviors that are unrewarding in the present because we believe they will bring greater rewards in the future is one of our species’ most significant talents, and no other species can do it quite as well as we can (Gilbert, 2006). In research on the ability to delay gratification (Ayduk et al., 2007; Mischel et al., 2004), people are typically faced with a choice between getting something they want right now (e.g., a scoop of ice cream) or waiting and getting more of what they want later (e.g., two scoops of ice cream). Waiting for ice cream is a lot like taking an exam or flossing: It isn’t much fun, but you do it because you know you will reap greater rewards in the end. Studies show that 4-year-old children who can delay gratification are judged to be more intelligent and socially competent 10 years later and have higher SAT scores when they enter college (Mischel, Shoda, & Rodriguez, 1989). In fact, the ability to delay gratification is a better predictor of a child’s grades in school than is the child’s IQ (Duckworth & Seligman, 2005). Apparently there is something to be said for extrinsic motivation.

There is a lot to be said for intrinsic motivation too (Patall, Cooper, & Robinson, 2008). People work harder when they are intrinsically motivated, they enjoy what they do more, and they do it more creatively. Both kinds of motivation have advantages, which is why many of us try to build lives in which we are both intrinsically and extrinsically motivated by the same activity—lives in which we are paid for doing exactly what we like to do best. Who hasn’t fantasized about becoming an artist or an athlete or a wealthy celebrity’s personal party planner? Alas, research suggests that it is difficult to get paid for doing what you love and still end up loving what you do because extrinsic rewards can undermine intrinsic interest (Deci, Koestner, & Ryan, 1999; Henderlong & Lepper, 2002). For example, in one study, college students who were intrinsically interested in a puzzle either were paid to complete it or completed it for free, and those who were paid were less likely to play with the puzzle later on (Deci, 1971). It appears that under some circumstances, people take rewards to indicate that an activity isn’t inherently pleasurable (“If they had to pay me to do that puzzle, it couldn’t have been very much fun”); thus rewards can cause people to lose their intrinsic motivation.

Just as the promise of rewards can undermine intrinsic motivation, the threat of punishment can create it. In one study, children who had no intrinsic interest in playing with a toy suddenly gained an interest when the experimenter threatened to punish them if they touched it (Aronson, 1963). And when a group of day care centers got fed up with parents who arrived late to pick up their children, some of them instituted a financial penalty for tardiness. As FIGURE 8.14 shows, the financial penalty caused an increase in late arrivals (Gneezy & Rustichini, 2000). Why? Because parents are intrinsically motivated to fetch their kids and they generally do their best to be on time. But when the day care centers imposed a fine for late arrival, the parents became extrinsically motivated to fetch their children—and because the fine wasn’t particularly large, they decided to pay a small financial penalty in order to leave their children in day care for an extra hour. When punishments and rewards change intrinsic motivation into extrinsic motivation, unexpected consequences can follow.

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Conscious versus Unconscious

When prizewinning artists or scientists are asked to explain their achievements, they typically say things like, “I wanted to liberate color from form” or “I wanted to cure diabetes.” They almost never say, “I wanted to exceed my father’s accomplishments, thereby proving to my mother that I was worthy of her love.” People clearly have conscious motivations, which are motivations of which people are aware, but they also have unconscious motivations, which are motivations of which people are not aware (Aarts, Custers, & Marien, 2008; Bargh et al., 2001; Hassin, Bargh, & Zimerman, 2009).

For example, research suggests that people vary in their need for achievement, which is the motivation to solve worthwhile problems (McClelland et al., 1953). Some researchers have argued that this basic motivation is unconscious. For example, when words such as achievement are presented on a computer screen so rapidly that people cannot consciously perceive them, those people will work especially hard to solve a puzzle (Bargh et al., 2001) and will feel especially unhappy if they fail (Chartrand & Kay, 2006).

What determines whether we are conscious of our motivations? Most actions have more than one motivation, and the ease or difficulty of performing the action may determine which of these motivations we will be aware of (Vallacher & Wegner, 1985, 1987). When actions are easy (e.g., screwing in a lightbulb), we are aware of our most general motivations (e.g., to be helpful), but when actions are difficult (e.g., wrestling with a lightbulb that is stuck in its socket), we are aware of our more specific motivations (e.g., to get the threads aligned). For example, participants in an experiment drank coffee either from a normal mug or from a mug that had a heavy weight attached to the bottom, which made the mug difficult to manipulate. When asked what they were doing, those who were drinking from the normal mug explained that they were “satisfying needs,” whereas those who were drinking from the weighted mug explained that they were “swallowing” (Wegner et al., 1984).

Approach versus Avoidance

The poet James Thurber (1956) wrote, “All men should strive to learn before they die/What they are running from, and to, and why.” The motivation to “run to” pleasure is called approach motivation, which is a motivation to experience a positive outcome, and the motivation to “run from” pain is called avoidance motivation, which is a motivation not to experience a negative outcome. Pleasure is not just the lack of pain, and pain is not just the lack of pleasure. They are independent experiences that occur in different parts of the brain (Davidson et al., 1990; Gray, 1990).

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Research suggests that, all else being equal, avoidance motivations tend to be more powerful than approach motivations. Most people will turn down a chance to bet on a coin flip that would pay them $10 if it came up heads but would require them to pay $8 if it came up tails, because they believe that the pain of losing $8 will be more intense than the pleasure of winning $10 (Kahneman & Tversky, 1979). On average, avoidance motivation is stronger than approach motivation, but the relative strength of these two tendencies does differ somewhat from person to person. TABLE 8.3 shows the kind of question that has been used to measure the relative strength of a person’s approach and avoidance tendencies (Carver & White, 1994). Research shows that people who are described by the high-approach items are happier when rewarded than those who are not, and that those who are described by the high-avoidance items are more anxious when threatened than those who are not (Carver, 2006). Just as some people seem to be more responsive to rewards than to punishments (and vice versa), some people tend to think about their behaviors as attempts to get a reward rather than to avoid punishment (and vice versa; Higgins, 1997).

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Avoidance motivation is powerful, and of all the things that people want to avoid, death is pretty high on everyone’s list. All animals strive to avoid death, but only human beings realize that this striving is ultimately in vain. Some psychologists have suggested that our knowledge of death’s inevitability creates a sense of “existential terror,” and that much of our behavior is merely an attempt to manage it. Terror Management Theory is a theory about how people respond to knowledge of their own mortality, and it suggests that one of the ways that people cope with their existential terror is by developing a “cultural world-view”—a shared set of beliefs about what is good and right and true (Greenberg, Solomon, & Arndt, 2008; Solomon et al., 2004). These beliefs allow people to see themselves as more than mortal animals because they inhabit a world of meaning in which they can achieve symbolic immortality (e.g., by leaving a great legacy or having children) and perhaps even literal immortality (e.g., by being pious and earning a spot in the afterlife). According to this theory, our cultural worldview is a shield that buffers us against the anxiety that the knowledge of our own mortality creates.