فصل 5

5

THE ECONOMICS OF EATING

As dawn breaks over the hills of Sipunga in northern Tanzania, light filters through the branches of a baobab tree. Maduru and his wife, Esta, emerge from their grass hut, joining others by the fire who have already risen. Esta sits down and nurses their one-year-old daughter. Wearing only a pair of dusty khaki shorts, Maduru sharpens an arrow while talking with other men in the camp to finalize his plans for the day.

Maduru and Esta are Hadza, a hunter-gatherer culture in the eastern Rift Valley of Africa—an area called “the cradle of mankind.” The Rift Valley earned this name due to its incredible diversity of primate fossils, which trace a lineage from modern humans back to the oldest members of our genus Homo 2.6 million years ago. The Hadza practice a foraging lifestyle that can give us insight into how all humans lived prior to the advent of agriculture approximately twelve thousand years ago. Although the Hadza are not carbon copies of our Stone Age ancestors, they may be the closest living approximation we have. Their way of life helps us understand the challenges our ancestors may have faced, and the physical and mental adaptations they evolved to meet those challenges. In this instance, they’re going to help us understand the cost-benefit calculations that drive us to overeat in the modern affluent world.

Maduru’s friend Oya mentions that he came across the tracks of several kudu, a type of large antelope, while he was returning to camp yesterday. Energized by his friend’s tip, Maduru collects his tools and prepares for the day’s hunt. He slips on his sandals, places a small ax over his shoulder, ties a small plastic bucket onto his back, slides a knife into his belt, and grasps a bow, arrows, and fire drill in his hands. He sets off alone in the general direction of the kudu tracks.

As Maduru heads off into the hills, Esta is engaged in a lively debate with five other women in camp over where to dig tubers. Eventually, they select a location and head out together, joined by a young girl and boy. The women’s technology is extremely simple: Each woman carries a sharpened digging stick approximately three feet long, a knife for keeping it sharp, and a fabric sling on her back. One woman carries embers from her hearth to use for cooking in the field. Esta carries her infant girl in her sling.

After walking about a mile, Maduru comes across the kudu tracks. They look more than a day old to him, but he follows them anyway to see if he can find fresher signs. As he follows the tracks, he is constantly making side trips to investigate whether other valuable foods may be nearby. He stops for a moment, eats a handful of sweet undushipi berries, and continues on his way. As he walks alongside the kudu tracks, he hears a pebble fall to his left. He spots a dik-dik, a diminutive African antelope, standing on a rocky crag a hundred feet away. It hasn’t detected him yet. He crouches and slowly circles downwind, concealing himself behind plants, quietly stalking it until he’s within range. He nocks his kasama, an arrow with a sharp, laurel-leaf-shaped metal point, quietly draws his bow, and lets the arrow fly, striking the dik-dik in the heart and lungs and killing it on the spot. He collects his prize, settles under the shade of a tree, cooks and eats the dik-dik’s liver, parts of its head and neck, and one front leg, and then takes a midday nap to escape the heat.

Meanwhile, Esta and her party have walked two miles to a promising location for digging tubers. Once they’ve arrived, they scan the rocky landscape for specific types of vines growing up a bush or tree, which signal the presence of an underground tuber. Esta sees an //ekwa vine climbing a large shrub, and she approaches to investigate. She examines the vine closely and then thumps the ground with the blunt end of her digging stick, listening carefully to determine the size of the tuber buried beneath. Satisfied that the tuber is worth her while, she begins digging with the sharp end of her digging stick. She digs rhythmically for ten minutes, finally pulling out a tuber that looks a bit like a long, crooked sweet potato. Each woman in the party will harvest a number of tubers of two different species. The women then use the embers they brought to build a fire and roast some of them. After cooking, they peel them and cut them into pieces. These tubers are extremely fibrous, so they chew the pieces thoroughly to extract the carbohydrate-rich juice, and spit out the fibrous quid that remains. After eating, Esta and her party take a nap in the shade of a large bush.

Feeling refreshed after his nap, Maduru continues stalking the kudu. However, after walking only a few hundred yards, he hears a buzzing sound dart by his ear: a honeybee. After some investigation, he spots a hole in a baobab tree that may contain the bee’s nest. He stares at the hole until the faint glint of a single departing bee confirms that he has indeed found the nest. He uses his fire drill to make a small fire and cuts six wooden pegs from a nearby tree. He takes a smoking torch from the fire and begins hammering the pegs into the bark of the baobab tree with the back of his ax. One by one, he hammers in the pegs and uses them to climb the tree until he has reached the level of the nest. He uses the smoking torch to sedate the angry bees, enlarges the entrance to the nest with his ax, and reaches his entire arm into the hole. Although he gets stung a few times, he manages to extract a large portion of honey-filled comb. After descending the tree, he eats a pint of honey on the spot and places the rest into the bucket on his back. It’s midafternoon, and he thinks the kudu are probably long gone by now, so he heads back to camp.

Upon awakening, Esta and her party place the remaining tubers in their slings and head back to camp. On the way home, they make a short detour to a baobab tree and collect fallen fruit, adding it to their slings.

In late afternoon, the men and women gradually trickle back into camp. As dusk falls, they make a fire and prepare the day’s harvest, sharing with their families and neighbors as they sit around their family hearths to eat, nurse, play, laugh, and tell stories about the day. Maduru butchers and roasts the rest of the dik-dik. Nearly every part of the animal will be eaten, including most internal organs, and a broth made from the bones. Maduru also shares his honey, which the others drink eagerly. The women roast and peel more //ekwa tubers and distribute the pieces, and open baobab fruit by striking them against a nearby rock. They extract and eat the white, tangy, chalky baobab pulp and spit out the seeds.

This fictional account illustrates what a typical day might look like for a Hadza couple, based on the detailed work of anthropologists Frank Marlowe, Brian Wood, and others. Although it may seem like a simple story, it in fact illustrates some of the complex economic choices that face the Hadza each day—choices that have shaped the structure and function of the human brain over millions of years.

OPTIMAL FORAGING

“Life is a game of turning energy into kids,” says Herman Pontzer, an associate professor of anthropology at the City University of New York who studies energy expenditure in the Hadza. Since obtaining calories is such a key requirement for survival and reproduction, it’s one of the most important drivers of the natural selection process that shaped today’s species. In a natural environment, there are more and less effective ways to obtain food, and animals that are more effective at obtaining food are more likely to pass along their genes to the next generation. In this way, natural selection gradually crafts brains that generate efficient food-seeking behaviors. As it turns out, biologists and anthropologists have been able to mathematically model the basic principles of efficient food seeking using a discipline called optimal foraging theory (OFT). OFT assumes that animals have been crafted by natural selection to acquire food from their environment efficiently, and researchers have successfully applied it to a variety of different species, including human hunter-gatherers. Given the bewildering complexity of human behavior, the basic math of OFT is disarmingly simple: image

The value of a food item, and hence whether or not it’s worth pursuing, depends on the number of calories it contains, minus the number of calories required to obtain and process it, divided by the amount of time required to obtain and process it. In other words, a food’s value is roughly determined by its calorie return rate. This is the same basic equation economists use to maximize profits, because the principles that govern efficient profit seeking are also those that govern efficient food seeking. Even though a rat doesn’t understand economic theory, natural selection has crafted its brain so that it behaves as if it understands economic theory. Natural selection has turned rats, and humans, into unwitting economists.

The work of many anthropologists demonstrates that OFT, while imperfect, is surprisingly good at predicting the foraging behavior of human hunter-gatherers. For example, OFT predicts, and observations confirm, that hunter-gatherers rarely bother collecting foods that are low in calories. One surprising implication is that hunter-gatherers don’t often collect or eat vegetables—that is, low-calorie plant foods such as leaves. If you’re a hunter-gatherer, it doesn’t make a lot of sense to burn 200 Calories collecting 50 Calories’ worth of salad.

If we examine the account of Hadza foraging above, we can identify many instances in which Maduru and Esta made important economic choices about food. Maduru set out to pursue a kudu, which is a large game animal with one of the highest energy return rates. It made sense for him to begin the search with a kudu in mind and spend a long time pursuing it, because the benefit of killing one is so great. Yet the optimal foraging strategy also involves being alert to other opportunities that may arise. For example, once Maduru spotted the smaller dik-dik, there was a good chance he could kill it and enjoy a good energy return from a short time investment. With the information that there is an unsuspecting dik-dik nearby, the energy return rate of hunting it is suddenly higher than continuing to search for the kudu. Yet if he had set out to find a dik-dik without any indication that one was nearby, he probably would have searched all day unsuccessfully. Similarly, the sound cue of the bee alerted him to a nearby nest and sent his foraging trip on a productive tangent. Honey has one of the highest energy return rates, and the Hadza will almost always drop everything to harvest it when they locate a good nest.

Esta also made important economic choices. In the morning, she consulted with the other women in her group to determine which location was most likely to yield the best energy return rate for tubers. That involved balancing the number of tubers they thought would be present at each location with the distance they would have to walk and the amount of effort they would have to exert to dig them up. When they were returning to camp, the sight of baobab fruit on the ground offered them the opportunity to collect a substantial amount of energy with little effort or time investment.

Whether or not they were fully aware of it, Maduru and Esta were carefully maximizing their calorie return rates. If this all sounds like common sense, that’s because our brains are wired to understand basic economic principles.

Of course, the OFT equation doesn’t explain everything. Human behavior is the result of many interacting motivations, so it’s not surprising that there are exceptions to the predictions of such a simple formula. “People have goals besides calories,” explains Bruce Winterhalder, professor of anthropology at the University of California–Davis. “Sometimes they’re out looking for the most luxurious bright red feather that is required for a ritual.” One example of this is that a food’s quality, in addition to its quantity, can impact its value. In particular, hunter-gatherers (and nearly all populations globally) typically value meat calories more than they value plant calories. Kim Hill, an anthropologist at Arizona State University whose work with Aché hunter-gatherers in Paraguay has been instrumental in adapting OFT to humans, has found that factoring this distinction into his calculations improves the ability of the OFT equation to predict foraging behavior. So even though calories are the primary consideration, the value of a food to a hunter-gatherer is determined by more than simply its calorie content.

Other factors can also come into play, such as risk. Even if the most efficient way to get calories is to pick fruit from a tall tree with weak branches, it may not be worth the risk of falling. Cultural influences such as taboos also impact food selection. For example, according to Wood, “No Hadza would pursue even a large, fatty savannah monitor lizard, because snakes and lizards are simply not considered food in Hadza culture.” Nor do they eat fish because they are “like snakes.” Personal preferences, hunger, and time delays can also play a role. The bottom line, according to Hill, is that “the brain is designed to take into account more than just energy acquisition.” Yet given all the possible motivations the basic OFT equation ignores, it does a surprisingly good job of predicting foraging behavior. This underscores the fundamental importance of energy to life, and its central role in the natural selection process that shaped the brains we have today.

OFT models the fundamental principles of food motivation that are wired into the brains of all humans. As it turns out, it has some interesting implications for those of us who hunt and gather, whether in the wild or in a supermarket.

PRIMAL INDULGENCES

The idea of moderation in eating is totally foreign to hunter-gatherers. In fact, Wood, Hill, and Pontzer explain that hunter-gatherer eating habits can be downright gluttonous. Hill recalls some of the enormous meals he observed among the Aché—men eating five pounds of fatty meat each in a sitting, drinking one and a half liters of pure honey, or eating thirty wild oranges similar to the fruit we buy in the grocery store. And it’s not just the Aché. Pontzer adds that the Hadza also drink honey “like a glass of milk.” In contrast to our modern eating habits, the Hadza make every effort to extract the maximum number of calories from their food. As soon as they kill an animal, they pinch it in several places to determine how much fat it carries. They know exactly which parts are the fattest, and if they kill a large animal, such as a kudu or a zebra, explains Wood, they cut off the fattest parts, boil it down, and drink the soup. They break open and boil every bone until it’s white and brittle, extracting every last gram of fat from its marrow. “They fully embrace the idea of ‘eat as much pure fat as you can possibly eat,’” explains Wood, adding, “there’s no hint of moderation whatsoever in their drive and motives with eating food.” Hill’s experience with the Aché echoes this: “Quite simply, they eat everything they can get and they don’t seem to have limits.” Yet despite the fact that they guzzle sugar and fat when available, neither the Hadza nor the Aché have obesity. In fact, the Hadza have an ideal body composition by modern Western standards: men average 11 percent body fat, women average 20 percent, and neither gender becomes fatter with age. The only Hadza with obesity Wood has ever encountered was a rich man who didn’t maintain a traditional diet or lifestyle. While Aché hunter-gatherers can have somewhat higher adiposity, especially young women, they rarely have obesity.

How can they engage in such gluttonous behavior and yet remain lean? The energy balance equation we encountered in chapter 1 allows only one possible answer: Their long-term average energy intake must match their energy expenditure. The reality of hunter-gatherer life is that true starvation is rare, yet they often don’t get as many calories as they would like. Despite appearing healthy and well fed, both Aché and Hadza adults frequently report feeling hungry. This isn’t the kind of mild hunger pang that reminds us to saunter over to the fridge at lunchtime, but rather the powerful hunger a person feels when he hasn’t eaten much at all that day. “When they say they’re hungry,” explains Wood, “it’s a lot more meaningful.” In other words, instances of gluttony are balanced by periods during which they’re eating less than they would like. Meat is often available, but fatty meat gorges are uncommon. Honey is often available, but not usually in sufficient quantity to exceed a person’s daily calorie needs. Simply stated, there isn’t enough food to indulge their outsized appetites.

Why do hunter-gatherers report feeling hungry even when they appear to have enough body fat and muscle, and why do they wish they had just a bit more food? The answer may lie in the dynamics of hunter-gatherer reproduction. If life is a game of turning energy into kids, then more energy (up to a point) means more kids. Since reproductive success drives natural selection, we might expect natural selection to have designed a brain that wants more energy. This is exactly what Hill thinks has happened: “Their brains are designed to want more food because more food converts into higher fertility and higher survivorship, and those things lead to higher [reproductive success].” This leads us to a key conclusion about life as a hunter-gatherer: Gluttony is good for them. Eating as much sugar, fat, protein, and starch as possible, whenever it’s available, increases their ability to thrive and bear children in a wild environment. “When those opportunities do present themselves,” argues Wood, “there’s basically no downside to going for it. It’s all upside.” Unlike in the modern affluent world, in which overeating is a major cause of ill health, in a hunter-gather environment, overeating is healthy. The same would probably have been true of our own ancestors until relatively recently in human history. For most of our history, our instinctive drives to seek large amounts of fat, sugar, starch, and protein were well aligned with our interests. There was no need to count calories or to feel guilty about eating too much. Yet in today’s world of extreme food abundance, these same drives often undermine our health and even our ability to reproduce. We try to use the sophisticated tools of our cognitive mind to restrain our impulses to overeat, yet the impulses often win. The brain that drives hunter-gatherers to gorge on calorie-dense foods—because it’s good for them—is the same brain that drives us to overeat in the modern world.

STALKING THE WILD CHICKEN NUGGET

Applying OFT to the hunter-gatherer environment and our own affluent world yields a striking contrast. In a hunter-gatherer environment, the calorie content of food items varies, but most foods require a substantial amount of effort and time to obtain and prepare. Therefore, in most cases, their overall economic value according to the OFT equation is relatively low. In other words, wild foods are usually a mediocre deal because they “cost” a lot. When high-calorie foods are easy to obtain, and therefore they have a very high value, hunter-gatherers take advantage of the situation by eating stupendous amounts of food, as we saw in the previous examples. These foods are a good deal because they deliver a lot of calories and don’t cost much—and hunter-gatherers rarely pass up a good deal.

In the affluent world, we stalk Froot Loops, buffalo wings, and chicken nuggets rather than fruit, buffalo, and wild fowl. Most of our foods are rich in calories, and the time, effort, and monetary costs involved in acquiring and preparing them have drastically declined. If we apply OFT to this situation, it becomes clear that we’re surrounded by an enormous variety of extremely valuable foods—foods that are an outstanding deal because they deliver a large number of calories and cost very little. Despite the fact that we live in a radically different environment than a hunter-gatherer, our brains are still highly attuned to good deals (have you ever seen how fast free pizza disappears?). Yet while a hunter-gatherer only encounters great deals occasionally, in our world, we encounter them multiple times per day. This leads us to overeat by activating largely nonconscious brain circuits that are constantly looking for a deal.

In Eric Ravussin’s vending machine studies that we encountered in chapter 1, recall that his team provided volunteers with a variety of high-calorie foods that were free of cost, required almost no effort to prepare, and were readily available at all times of day. To eat food, the volunteers only had to wander into the next room and enter a code. Ravussin and his team unwittingly created a situation in which the value of food, according to the OFT equation, was exceptionally high—the ultimate deal. As predicted by OFT, this scenario resulted in spectacular overeating and rapid weight gain (opportunistic voracity, as Ravussin called it).

Brian Wansink, director of the Food and Brand Lab at Cornell University, has conducted clever experiments that illustrate the outsized influence of effort cost on our eating behavior. In one study, he recruited administrative assistants and placed candy dishes containing Hershey’s Kisses in one of three different locations in each of their offices: on the desk, in the top drawer of the desk, or in a filing cabinet six feet away. Eating a Kiss on the desk required only a small arm motion, while Kisses in the drawer required a larger arm motion, and those in the filing cabinet required getting up and walking across the room. Each additional effort barrier, although small, made the Kisses a less attractive deal.

STEP AWAY FROM THE SNACKS

The practical implications for avoiding overeating are clear: Don’t make it too easy for yourself to eat food throughout the day. Even effort barriers as small as having to open a cabinet, twist off a lid, peel an orange, or shell nuts can make the difference between eating the right amount and overeating. Keeping easy, tempting foods in plain sight, such as an open bag of chips or bowl of candy, creates a situation that is simply too tempting for the parts of our brains that are constantly on the lookout for a good deal.

Remarkably, these seemingly trivial differences in effort cost resulted in large differences in candy intake. Participants with candy bowls on their desks ate an average of nine Kisses per day. Those with candy bowls in their desk drawers ate six Kisses per day, and those who had to hike all the way across the room only ate four Kisses. As Wansink puts it, “It’s not worth the effort for an Eskimo to locate and overeat mangos.” Would we be leaner as a nation if we all had to walk three miles and climb a tree each time we wanted a hamburger and fries (or ice cream, or pizza)? Almost certainly. Yet our food today is more convenient than it has ever been in human history.

YOUR BRAIN ON POP-TARTS

As I described in chapter 4, our food system has changed dramatically over the last century. Yet those changes don’t just apply to the rewarding properties of foods but also to the costs of food, such as time, effort, and money, that determine their economic value to our brains. For example, between 1929 and 2012, US food spending dropped from 23 percent of disposable income to 10 percent. This alone makes food today a much better deal than it was when our grandparents were our age. The old adage says, “Hunger is the best sauce,” but I would argue that “cheap” is a pretty good sauce too.

Food has become more convenient as the time and effort costs of obtaining and preparing it have dwindled. Grocery stores became common in the 1920s, providing us with the convenience of a single location for all our food shopping needs, and they have continued to expand in size since then. As each decade has passed, we’ve outsourced more of our food preparation effort to professionals in the restaurant and processed-food industry. And over the last fifty years, we’ve increasingly gravitated toward the ultimate convenience meal: fast food. Most of these restaurants focus on foods you can eat with your hands, dispensing with the dreadful inconvenience of silverware. We don’t even have to leave our cars (or stop driving!) to eat anymore.

Even the workload in our own kitchens has declined. The food industry has responded to consumer demand for more convenient foods by creating zero-effort meals we can buy at the grocery store and eat at home. In his wonderful book Salt Sugar Fat, Michael Moss describes how the food industry giants carefully designed these products for maximum consumer appeal. Lunchables are premade lunch packs that allow busy parents to save the time and effort they might otherwise have to spend packing lunches for their kids. Pop-Tarts aim to replace breakfast with a thin, glazed pastry that heats up in the toaster. TV dinners and related meals only have to be heated in the microwave. The list of time- and effort-saving innovations developed by the food industry is endless. Innate human economic preferences created the demand, and the food industry was happy to meet this demand by inventing ultraconvenient food options.

Just as we engineered our food to satisfy our innate food preferences, humans have shaped our food environment to satisfy our innate economic preferences. Together, huge decreases in the time, effort, and monetary costs of food make many of our modern foods an exceptionally good deal. In the same manner that the economic preferences of the Hadza brain drive them to gorge when they encounter food that’s a great deal, our own brains compel us to overeat in the same situation. The difference is that the Hadza only encounter great deals occasionally, whereas we encounter them multiple times per day. While many of these economic changes have improved our lives by freeing up time and money for other things, they have also contributed to our expanding waistlines by appealing to the innate economic logic of the human brain.

We’re wired to seize a good deal when we see it, even at the expense of our waistlines. How does the brain recognize a good deal and motivate us to act on it?

THE VALUE CALCULATOR

In the laboratory of Camillo Padoa-Schioppa, associate professor of neuroscience and economics at Washington University in St. Louis, a rhesus monkey stares intently at a small black dot on a computer monitor. Suddenly, more shapes appear on the screen: a yellow square on the left and a blue square on the right. Then a black dot near each colored square. The monkey shifts its gaze to the black dot near the yellow box, and less than a second later, it receives a drop of grape juice through a tube in its mouth.

Padoa-Schioppa’s research uses simple choice tasks in monkeys to understand how our brains compute cost-benefit decisions. In this particular trial, the monkey was offered a choice between one drop of grape juice (yellow square) and one drop of unsweetened Kool-Aid (blue square). The monkey indicated its choice by looking at the black dot near the yellow box, which, via repetition, it has learned represents grape juice. Presumably because of its sweetness, rhesus monkeys almost always choose grape juice in this scenario.

In the next trial, the choices become a bit more complicated. The monkey is offered a choice between one drop of grape juice (one yellow box) and three drops of unsweetened Kool-Aid (three blue boxes). Now, there are two variables at play. Instead of only varying the type of juice, Padoa-Schioppa’s team has also varied the amount of juice. In this scenario, the monkey has to collect information about two variables for each option and decide which option it prefers overall. The monkey is a bit thirsty, so it looks at the three blue boxes and receives three drops of unsweetened Kool-Aid.

This second trial begins to hint at the complexity of everyday decisions. Some decisions are easy, such as choosing between one orange and two oranges. In this case, there is a clear quantitative difference. But what about a choice between an orange and an apple? What about a choice between a pastry behind a counter and three dollars in your wallet? We often choose between options that vary in many dimensions and have no objective means of comparison. Yet we’re able to compare them anyway, and we often make seemingly rational decisions. How is that possible? How can we compare a pastry with three dollars if they have scarcely anything in common? There has to be a common unit that the brain uses to compare options on the same scale, and that unit is subjective value.

Subjective value quantifies how much each particular option will benefit the organism so different options can be compared on the same scale. “Value is the only way to compare goods that are incommensurable, or qualitatively different,” explains Padoa-Schioppa. Economists and psychologists have long known that people behave as if their decisions are guided by assigning subjective value to options, but only recently have neuroscientists begun to understand how the brain computes it.

While Padoa-Schioppa’s monkeys make their choices, his team records the electrical activity of single neurons in a part of the brain called the orbitofrontal cortex (OFC). The OFC is a region of the prefrontal cortex, which is the part of the brain most often associated with reasoning and judgment. Relative to the rest of the brain, the prefrontal cortex is large in primates and even bigger in humans. Padoa-Schioppa has discovered that the firing patterns of individual OFC neurons represent the value of specific options. For example, there are neurons that fire a little bit when a monkey is offered one drop of grape juice, and fire a lot when it’s offered four drops. Then there are other neurons that fire in response to unsweetened Kool-Aid. And, interestingly, there are even neurons that represent the choice the monkey has made—before its eyes move to the target.

Padoa-Schioppa and other researchers have found that OFC neurons are able to integrate many kinds of cost-benefit information into their value computations, including the type of juice, its quantity, the probability of obtaining it, and the time and effort costs required to obtain it. Remarkably, the firing of these neurons seems to encode the complete subjective value of each option—in other words, each option’s overall value to the monkey. Studies in humans have also implicated the OFC in value computation, as well as a nearby brain region called the ventromedial prefrontal cortex. The activity of these neurons may in fact be responsible for how much we value a pastry and how much we value three dollars.

To return to concepts we discussed in chapter 2, the OFC is reciprocally connected to the basal ganglia, suggesting that it may be an option generator. As a reminder, the basal ganglia sort through competing bids from option generators and select the most valuable one. This suggests the following scenario, which has not been directly demonstrated but is consistent with the evidence we have.

First, the OFC uses incoming information from other brain regions to calculate the predicted value of each option. You independently compute the value of the pastry and the three dollars in your wallet. Second, the OFC sends those two independent bids to the basal ganglia, where the striatum compares them and selects the strongest one. You’re feeling very tempted by the seductive food cues behind the counter, and the three dollars isn’t worth much to you because you just got paid, so the pastry wins. Third, the basal ganglia return that selection to the OFC, and the decision is made. This activates a cascade of other competitions in cognitive and motor areas of the brain, which skillfully plan and generate the behaviors necessary to follow up on the decision. You reach into your wallet, pull out three dollars, trade them for the pastry, and dig in.

FAILURE TO COMPUTE

If the OFC really plays an important role in computing value, then disrupting its function should impair decision-making in specific ways. A person with OFC damage should still be able to act out well-worn habits, because they don’t require value computations. If you always flush the toilet after using it, your brain doesn’t have to compute the value of flushing versus not flushing. The decision had already been made when you stepped into the bathroom.

Yet OFC damage should reduce a person’s ability to make decisions in response to changing conditions, because this requires the brain to compute value on the fly. And that’s exactly what happens. This time, let’s imagine you’ve just flushed the toilet. For most people, this new information would immediately reduce the value of flushing a second time, and so you wouldn’t do it. Yet someone who has trouble computing value isn’t able to incorporate that new information into her decision, so chances are, she’ll flush again out of habit. And again, and again. This phenomenon, where a person keeps performing an action even after it’s no longer useful, is called perseveration, and it often results from OFC damage. One common and striking consequence of OFC damage is overeating and weight gain. At first glance, this might seem strange. Why would damage to a brain region that computes an abstract concept like value affect eating so much? The answer may lie in the brain’s inability to update the value of food as a meal progresses. When you sit down to a meal, that first bite of food usually has the highest value, because you’re hungry. As the meal progresses, you feel increasingly full, and the value of each additional bite goes down. Once the value of the next bite is lower than the value of doing something else, like clearing the table, you stop eating. This process requires your brain to continually update the changing value of the food on your plate—precisely what people with OFC damage are unable to do. For a person with OFC damage, every bite is as compelling as the first one, so they often overeat substantially. This may explain why they continue to eat even when they report feeling extremely full: the brain registers the feeling of fullness just fine, but that feeling doesn’t get translated into behavior because it doesn’t register in the OFC.

THE LAZIEST MICE IN THE WORLD

Now we know how the brain calculates value, but once it spots a good deal, how does it motivate you to pursue it? In chapter 2, we encountered the dopamine-deficient mice created by Richard Palmiter at the University of Washington. These mice are unable to produce dopamine naturally, and as a consequence, they sit nearly motionless in their cages, unable to eat or drink until their dopamine is chemically replaced. I explained that this is because dopamine lowers the threshold for selecting behaviors, and without it, the threshold is so high that nothing gets selected.

The research of John Salamone, professor of psychology at the University of Connecticut, allows us to refine this idea: Palmiter’s mice may just be extremely lazy. So lazy, in fact, that even taking a few steps across the cage to eat and drink aren’t worth the effort. How do we know? Because Salamone’s research shows that dopamine plays a key role in motivation. When he reduces dopamine signaling in the ventral striatum of rodents, they become less willing to work for a reward. They choose easy options over hard options, even if the hard option has a much larger payoff and would normally be the better choice. In other words, reducing their dopamine signaling makes them lazy. Palmiter’s mice have no dopamine, making them quite possibly the laziest mice in the world.

Dopamine seems to play a similar role in humans. Increasing dopamine levels using amphetamine makes people more willing to work for rewards, even if the reward is small or uncertain. Dopamine turns us into go-getters.

This makes perfect sense when we think about what’s happening when the brain releases dopamine. Through repetition, your brain has learned to associate the sight, smell, sound, and other food cues with the reward of eating food. As soon as you see that bar of chocolate in the candy aisle, your dopamine level begins to spike. This burst of dopamine energizes you to grasp the chocolate and put it into your cart. If you had seen frozen green beans instead, you would have had a much smaller dopamine burst, proportional to the smaller reward, and you probably would have walked right past them. Dopamine tunes your motivation level to be proportional to the value of the reward you’re pursuing—and it does so beyond your conscious awareness.

THE VALUE OF YOUR FUTURE SELF

If the brain was always rational in the way it computes value, few people would carry credit card debt, and no one would overeat. But we often make self-destructive choices, and this is particularly true when our decisions involve the future. As opposed to decisions that pit material objects against one another, like an apple versus an orange, we often make decisions that pit our present selves against our future selves. And the evidence shows that we often shortchange our future selves, with disastrous consequences.

Let’s return to the example of a pastry versus three dollars. Most of us like the taste of pastries, yet we also recognize they’re unhealthy. The benefit of eating a pastry is its reward value: We want it; we like it. This is a benefit you experience immediately as you bite into the pastry.

The costs of eating a pastry, on the other hand, are all incurred by your future self. For most of us, eating that pastry takes us one small step closer to obesity and ill health in the future. Also, what could you have done with those same three dollars next week? Would spending them take you slightly closer to defaulting on your rent or mortgage?

This is an example of how your nonconscious, intuitive brain competes with your conscious, rational brain. Your intuitive brain has no concept of the future and no understanding of abstractions like health and finances. It wants to eat things that taste good, right now. Your rational brain, on the other hand, understands the value of the future, and other abstract notions like obesity and money. It wants to protect you from the excesses of the intuitive brain; it wants your future self to be lean, healthy, and rich.

Which one wins? To a large extent, this depends on a psychological trait called delay discounting. This principle is illustrated by the famous 1970 “Stanford marshmallow experiment,” in which children were offered a choice between getting one marshmallow now or two marshmallows in fifteen minutes. The children were left alone with the marshmallow on the table during the fifteen-minute wait, so the temptation was intense. Some children talked to themselves or covered their eyes in an effort to resist. And many popped the marshmallow into their mouths as soon as the researchers left the room. The children were essentially being asked to decide between a small, immediate reward and a larger, future reward. Which one they chose depended in part on how much they implicitly discounted (undervalued) the value of a future reward versus an immediate reward. In other words, how much they valued their present selves versus their future selves. In the end, most of the children ate the marshmallow, forgoing the larger reward and shortchanging their future selves. Interestingly, a follow-up study showed that children who were better at delaying ended up slimmer thirty years later. In fact, for each additional minute they were able to delay eating the marshmallow as a child, they were slimmer by 0.2 BMI points as an adult. This means that a ten-minute difference in delay was associated with a fifteen-pound difference in adult weight. This makes sense: People who value their future selves highly will value long-term goals like leanness and health. For someone like this, two marshmallows fifteen minutes from now is worth nearly twice as much as one marshmallow right now, and a slimmer figure next summer may be worth more than a pastry right now. On the other hand, people who don’t value their future selves very much may find that one marshmallow right now is worth more than two marshmallows in fifteen minutes, and that a pastry right now is worth more than a slimmer figure next summer. Multiple studies have confirmed that people who steeply discount the value of future rewards are more likely to have obesity. They’re also more likely to be addicted to illegal drugs, alcohol, or cigarettes; more likely to gamble; and more likely to carry credit card debt. In essence, these are all examples of pursuing immediate rewards without much regard for future consequences.

Delay discounting may seem irrational at first. Why would people be willing to seriously harm their future selves for an immediate reward as trivial as a pastry or another round at the slot machine? However, from an evolutionary perspective, it makes perfect sense—for one simple reason: The future is uncertain. Our species evolved in a dangerous environment in which we had about a 50 percent chance of living to age thirty-five. If you aren’t certain you’ll be alive next year, it’s rational to value what’s happening right now more than what might happen next year. In the environment of our ancestors, it was advantageous to evolve brains that intuitively value our present selves more than our future selves.

PICTURE THIS

Is there anything we can do to fight our natural tendency to shortchange our future selves? Research from Leonard Epstein’s group and others suggests that the answer is yes—by giving the rational brain a little boost using an exercise called episodic future thinking. This phrase is a complex-sounding term for a technique that’s actually quite simple: Before making a decision, you imagine yourself in the future. When making a decision about something that pits your present self versus your future self, such as whether or not to eat a pastry, first imagine positive events in the future, such as your birthday or an upcoming vacation. Place yourself in the scene and imagine yourself enjoying it. The more vivid the imagery, the better. This process fires up the regions in your prefrontal cortex that process abstract concepts like the future and therefore causes your brain to intuitively weight the future more heavily in its decision-making process. This attenuates delay discounting. Epstein’s research shows that episodic future thinking reduces the intake of tempting, calorie-dense foods by nearly one-third in overweight women, and the same trick works for overweight children as well.

Yet in affluent countries today, the future is much more certain than ever before in human history. Mortality is far lower, and life expectancy far higher, than in the distant past. We have so much legal accountability in affluent countries that it’s rational to hand over large sums of money to investment companies that grow it at a snail’s pace, even if we won’t be able to touch the money until we’re retired! Today, it makes sense to value our future selves almost as much as our present selves—but the nonconscious brain regions that compute value and determine our motivations haven’t caught up yet. This makes it all too easy to make choices that compromise our future finances, health, and weight, even if our intentions are good. And it goes a long way toward explaining why we overeat.