In the United States today, 68.8 percent of people are either overweight (BMI of 25-29.9) or obese (BMI of 30+) (NIH, 2012). This means that only 31 percent of people are either of a healthy weight or underweight. poses a serious health risk as it is strongly related to serious illnesses such as heart disease, stroke, type 2 diabetes, and cancer, and incurred 147 billion dollars in medical costs in the US in 2008 (CDC, 2014).

In an attempt to combat this trend towards overweightness, many individuals diet. On any given day, an estimated 25 percent of men and 45 percent of women in America are ( Hope, 2014). And yet, 95 percent of dieters will regain this lost weight within 1-5 years (Eating Disorder Hope, 2014). Why do we have so much trouble keeping our weight down?

Harvard evolutionary biologist Daniel Lieberman explains that humans evolved to crave sugar for energy in times of scarcity and also to support our large brains (2012). However, in modern days, there has been no scarcity of sugar. The result is that the excess sugar available to us is converted into fat, leading to an obesity epidemic.

But regardless of the greater availability of sugar and food products in general, why do we continue to eat after our bodies have acquired enough energy to sustain us? Is it for pleasure? Is it out of boredom?

We argue that obesity is not caused by physiological hunger, the purely physical intake of high-calorie meals at fast food restaurants, or eating excessively when you are full. Instead, hunger has psychological causes, and we often feel hungry even with full stomachs.

Three types of cues

"People are only hungry when their stomachs are empty, and become satiated when it is full." False.

The physiological cues associated with an empty stomach that tell us to eat are referred to as internal cues. However, even when these cues are not present, external cues can induce hunger as well. These external cues may broadly be divided into two categories: normative cues, such as portion size, that indicate appropriate levels of consumption, and sensory cues, such as smell or taste, that refer to the hedonic value of food (Herman & Polivy, 2008).

Schachter (1968) originally proposed that obese individuals are more responsive to external cues and less responsive to internal cues than others, and therefore are more likely to eat even when their bodies are not sending them the physiological signals to do so. In an updated model, Herman & Polivy (2008) suggest that while normative cues affect everyone, obese individuals are more likely to be strongly affected by sensory cues than others.

External sensory cues

Sensory cues defy the simple concept that we eat when we are hungry and stop eating when we are full. Lambert et al. (1991) conducted an experiment in which they gave either hungry or satiated subjects one of three types of sensory cues or no stimulus, and compared their desire to eat chocolate before versus after the stimulus. The different sensory cues included taste (trying a piece of chocolate), sight (viewing a photograph of chocolate), and (reading a description of chocolate).

They found that the presentation of sensory cues significantly increased participants’ desire to eat chocolate, and that participants consumed more chocolate regardless of their state of hunger or the type of stimulus presented. This indicates that a variety of external sensory cues can influence people to consume food, even if they are not hungry.

External normative cues

External normative cues have also been shown to influence how much we eat. Generally speaking, people have the tendency to finish the food on their plate. It is perhaps not surprising then, given our huge portion sizes in America compared to France, that the obesity rate in America is 35 percent (CDC, 2014) while in France it is only 7 percent (Davis, 2003). Our candy bars are 41 percent larger, our soft drinks are 52 percent larger, and even our cartons of yogurt are 82 percent larger than those products in France (Davis, 2003).

Wansink et al. (2005) conducted an experiment demonstrating that the normative cue to finish your plate (or bowl in this case) can lead participants to eat more without realizing, and without even feeling more sated afterwards. Participants were either given a normal bowl of soup or a bowl that self-refilled imperceptibly as the contents were consumed. Participants with the refilling bowls ate 73 percent more soup than those with the normal bowl, but did not believe they had eaten more and did not indicate that they felt fuller than the other group. This shows how satiety is not necessarily defined by how much we eat and how full our stomachs are, but rather by consumption norms and expectations.

for recent eating is another normative cue that influences whether or not we eat. For example, Rozin et al. (1998) conducted an experiment with amnesic patients in which they measured whether or not they would consume multiple meals in succession. The patients had no explicit memory for events that occurred more than a minute before, and specifically could not remember if they had just eaten a meal.

Rozin et al. (1998) proposed that the primary reason for determining when to begin a meal is based on when an individual had eaten their last culturally defined complete meal. Therefore, if participants could not remember eating, they would readily consume another meal if presented to them.

In fact, their results demonstrated this phenomenon: the amnesic participants consumed a second lunch offered 10-30 minutes after the first, and began to consume a third lunch offered 10-30 minutes after the second. These results are despite the fact that, presumably, the participants’ stomachs were physically full after each meal. This demonstrates that the urge to eat is not just related to physiological urges, but also to normative cues.

Higgs (2012) showed similar effects in the typical, non-amnesic participants. When participants were asked to think about what they had eaten for lunch, they ate less than if they were asked to think about what they had eaten for lunch the day before or received no cue at all.

Social cues

In addition to sensory and normative cues, social facilitation is another type of external cue that can influence our eating behaviors. Redd & Castro (1992) found that when instructed to either eat alone, with other people, or eat as they normally would (with the choice of either eating alone or with others), undergraduate psychology students consumed more water, sodium, food, and when instructed to eat with others than when instructed to eat alone. In addition, when eating as they normally would, those participants who ate with others consumed 60 percent more than those who ate alone. Therefore, eating with others can cause individuals to consume more food than they would otherwise.

As we can see, external cues are extremely influential in determining when we feel hungry and how much we choose to eat. Despite the principal role external cues play, we are often unaware of these cues. For example, in a study in which pairs of participants were given a chance to eat, they took cues from each other, so that if one partner ate more, the other did too (Vartanian et al., 2008). However, participants did not indicate that the amount they ate was influenced by their partner’s behavior, but rather attributed their intake to taste and hunger. Thus, a lack of awareness of external factors may make it difficult to attenuate these influences on excessive food intake.

Stress

Other psychological factors may also lead us to eat unnecessarily. Stress in particular has been shown to drive us to consume more food. In a study of female participants only, Groesz et al. (2011) found that greater reported stress was associated with participants’ drive to eat, as measured by feelings of hunger, , disinhibited eating, and ineffective efforts to regulate eating. Kandiah et al. (2006) found that stress produced a change in in 81 percent of participants, and that 62 percent of these participants experienced an increase in appetite. Those with an increased appetite were also more likely to choose sweet or mixed foods such as desserts or burgers. Interestingly, while 80 percent of participants reported eating healthily normally, this number dropped to 33 percent when they were . This increased drive to eat and to eat unhealthily may explain why stress may contribute to obesity.

Ironic effects of a restricted diet

While we have explored many of the potential psychological contributions to overeating and obesity, the question still remains as to why obesity is so much more prevalent among Americans than people of other nationalities. One possibility is that there is a fundamental difference in Americans’ attitudes towards food. While Americans think about food in terms of a biological, nutritional, and health-related need, consider it a material good, and use it as a reward, French people associate food with pleasure (Werle et al., 2012; Ochs et al., 1996; Rozin et al., 1999). Werle et al. (2012) found that while Americans associate unhealthy food with tastiness, French associate healthy food with tastiness. In addition, while Americans think of eating healthy versus unhealthy food in terms of ‘right’ and ‘wrong,’ French typically associate food with pleasure, part of , sharing, and health (Werle et al., 2012).

This American classification scheme in combination with the common desirability for ‘the forbidden’ may help to explain why we associate unhealthy foods with tastiness. Unfortunately, this association may result in less healthy eating choices, especially given that we use food as a reward—we may reward behaviors with unhealthy food. Werle et al. (2012) suggest that emphasizing the pleasure of eating and decreasing the associated with consuming food may be more effective than the calorie-counting, food-shaming culture we have in America today. Ironically, allowing oneself to eat yummy foods might it easier to eat less.

This ironic effect of restriction seems to be passed on to children by their parents. One study examined non-Hispanic white girls aged 5-9 whose parents either did or did not restrict their access to food. The girls whose diets were restricted showed higher increases their tendency to eat in the absence of hunger than were girls whose parents did not restrict their eating (Birch, Fisher, & Davidson, 2003).

Summary

Overeating and obesity are not only costly problems financially but also pose a serious health risk to hundreds of millions of Americans. In evaluating the nature of the problem, it is important to understand that we do not simply overeat when our stomachs are full. Rather, the manner in which we determine whether or not we are hungry, whether or not we should eat, and what we should eat, is much more complex. Furthermore, the extent we eat is strongly influenced by psychological factors such as sensory cues, normative cues, social facilitation, memory of our past meal, stress, and our cultural attitude towards food.

Through an extensive examination of these psychological processes, perhaps we can work towards a more effective solution to combat obesity—not calorie-counting just to gain the weight back within a few years, but understanding the underlying psychological cues and using this knowledge to help us identify when our stomachs are empty and when they are full but our minds just think they are empty.

This is a guest post by Raea Rasmussen, Williams College Class of 2015.