Definitions and Background

Psychology is the science of behavior, the science of "how and why organisms do what they do" (Gleitman, 1981, p. 1). Psychology's data consist of the behaviors of animals (including human animals). Psychology's goal is to understand behavior--to be able to predict, and possibly even control, behavior. Behavior of any animal is due, at least in part, to input from the genes, and is due, at least in part, to the interaction of the animal with the environment. Behavior cannot occur without a physiological/anatomical structure that has developed partly as a result of genetic programming, and behavior can also not occur without there being some interaction between the animal and its environment.

For these reasons, predicting behaviors involves taking multiple factors into account, and a theory that facilitates the grouping of different pieces of data and the making of predictions would be quite useful. Such facilitation is provided by evolutionary theory. In accordance with modern evolutionary theory, organisms should behave so as to maximize the survival of their genes, their inclusive fitness (the probability that their biological relatives will survive; Barash, 1977; Hamilton, 1964a, b; Maynard Smith, 1978). In this way natural selection--survival and reproduction of the fittest individuals--occurs. Darwin himself realized clearly that survival is not just a matter of having strong muscles and good eyesight. Survival also depends on animals behaving in ways that maintain the health of their bodies, for example obtaining adequate nutrients. In his book, The Expression of the Emotions in Man and Animals, Darwin stated that "He who admits on general grounds that the structure and habits of all animals have been gradually evolved, will look at the whole subject of Expression in a new and interesting light" (1965, p. 12). An animal's behavior occurs at the intersection between the animal's body and the environment in which the animal needs to survive. Unless an animal's behaviors are relatively advantageous, the animal's genes will not continue to exist.

Behaving in ways that are advantageous to survival does not involve, however, being born already equipped with every advantageous behavior. To the contrary, because our environment varies in ways that we, and other animals, cannot predict, our behavior must be flexible in ways that best accommodate us to whatever circumstances may confront us. Some circumstances may be best dealt with by relatively fixed responses, and thus we are born with fixed behaviors for these circumstances (Skinner, 1966). One such behavior is the reflex involving immediate withdrawal of a body part that is in contact with a painful stimulus. However, in most cases, a fixed response will not be the most advantageous one. Thus, if behavior has been influenced by evolution, we would expect in some cases for there to be fixed behaviors for certain environmental circumstances, but in most cases for behavior to change in ways that are advantageous given the animal's current and past experiences.

Just as there are different computer programs that can all find the same answer to a single problem, just as there are different visual systems that can all result in various species having excellent visual ability, so too there are different anatomical structures that may be responsible for behaviors that appear quite similar. In terms of ultimate success and usefulness, what is important is the function of the computer program, of the visual system, and of the behavior. More specifically in terms of behavior, an animal will evolve so as to behave in a way that will increase the chances of survival of the animal's genes. What is not important is the particular physiological mechanism that is used to accomplish this behavior. Thus, in order to try to understand how animals may have evolved to exhibit certain behaviors, we need to think of behaviors in terms of their functions (see Cosmides & Tooby, 1987; Zeiler, 1991).

Several different psychological processes are involved in eating behavior. These processes have been frequently investigated by experimental psychologists and include perception, learning, and motivation. Perception involves an animal's detection of stimuli that are present in the environment. Learning can be defined as the acquisition of knowledge--the acquisition of information about which stimuli and responses tend to be associated with each other. On the other hand, motivation can be defined as the factors responsible for an animal choosing to engage in a specific behavior when no new knowledge is involved in making this choice. For example, tasting cooked crab involves perception. When someone eats crab, gets sick, and then has an aversion to eating crab, this would be described as learning. When the same person is subsequently served crab at someone's home, that person's choice to eat the crab would be an example of motivation. Note that these psychological processes cannot be observed in isolation. Evidence of learning cannot be obtained without an animal choosing to engage in a certain behavior, and virtually all behaviors involve learning to some degree. In addition, no animal can react to the environment without perception of the environmental stimuli being part of this process.

The present paper will attempt to explain how evolutionary theory helps us to understand perception, learning, and motivation with respect to food consumption. Specific examples will be given of all of these psychological processes. These examples will be based on several different specific topics within the general study of eating behavior. These topics concern why we do and do not eat certain kinds and amounts of food. The topics are: illness-induced food aversion learning, preference for high-fat foods, preference for sweet foods, preference for salty foods, weight gain and loss, and self-control with regard to eating behavior.

Illness-Induced Food Aversion Learning

Many people have had the experience of eating something, becoming ill, and then not wanting to eat that thing again. This type of learning is known as illness-induced food aversion learning. Many years ago, farmers discovered that trying to poison rats resulted in this type of learning--what the farmers called bait shyness (Barnett, 1963). However, laboratory research on this phenomenon did not begin until the experiments of Garcia and his colleagues in the 1950s. In a landmark experiment, Garcia and Koelling (1966) found that rats learned more easily to avoid a distinctive taste that had been paired with poison than a click/light flash combination that had been paired with poison. The opposite was true when shock was used instead of poison: It was easier for the rats to learn to avoid a click/light flash combination that had been paired with shock than to learn to avoid a distinctive taste that had been paired with shock. Because of these results, illness-induced food aversion learning is often referred to as taste aversion learning (Schafe & Bernstein, 1996).

The fact that it appears to be easier to associate illness with tastes than with audiovisual stimuli at first seemed to violate one of the assumptions of traditional learning theory: that any event could be associated equally well with any other event (the equipotentiality assumption; Garcia, McGowan, & Green, 1972; Seligman & Hager, 1972). In truth, many learning theorists, such as B. F. Skinner, were always aware that some associations would be easier to form than others (see, e.g., Skinner, 1956). However, others, in their race to develop universal laws of learning, did indeed state that it should be possible to learn to associate any stimulus with any other stimulus (see Seligman, 1970, for examples). Taste aversion learning provides another example of a violation of the equipotentiality assumption. Taste aversions do not form easily with every type of illness. Taste aversions are acquired most easily with gastrointestinal illness, particularly nausea (Pelchat & Rozin, 1982).

The equipotentiality assumption was not the only assumption of traditional learning theory that appeared to be violated by taste aversion learning. For example, psychologists quickly discovered that taste aversions could be acquired in a single trial with delays of up to 24 hours between consumption of the food and illness. According to traditional learning theory, learning cannot occur if there are delays of more than a few seconds between two events (Etscorn & Stephens, 1973; Garcia et.al., 1972), and many trials are often necessary in order for learning to occur.

Another apparent difference between traditional learning and taste aversion learning concerns properties of the conditioned stimuli involved in each case. In traditional learning, for example with shock, the conditioned stimuli (lights and clicks) paired with shock (the unconditioned stimulus) subsequently appear to function as signals for the shock. The conditioned stimuli are avoided in those situations in which the shock has previously occurred, such as the experimental chamber in which the conditioning has taken place, but not in other situations, such as the rat's home cage. However, when illness is used instead of shock, the conditioned stimulus (the taste of a food) is avoided wherever it is encountered (Garcia et al., 1972). The hedonic value of the taste therefore appears to change as a result of the taste being paired with illness; the taste itself apparently becomes aversive, rather than the taste simply signaling to the subject that illness is due shortly. The animal's liking of the conditioned stimulus appears to have changed (Garcia, Hankins, & Rusiniak, 1974).

All of these unusual properties are useful in avoiding poisons. The presence of poison is more likely to be indicated by a particular taste than by a particular visual or auditory stimulus, naturally-occurring poisons are more likely to cause gastrointestinal illness than other types of illness, it may take many hours before a poison will result in illness, and a poison is a poison no matter where it is encountered or how many times it is encountered. Therefore the particular characteristics of taste aversion learning seem well designed to assist organisms in avoiding illness-causing agents in foods. For example, large grazing mammals apparently easily learn to avoid grasses with the bitter taste of an alkaloid-producing fungus that makes the animals feel ill ( Clay, 1989). Based on all of this evidence, psychologists therefore postulated that the rules governing this type of learning had been shaped by evolution ( Bolles, 1973; Rozin & Kalat, 1971; Seligman, 1970; Shettleworth, 1972).

Research has pointed out, however, that even if one grants that evolution has shaped learning principles, this does not necessarily mean that completely different principles of learning apply in different situations. Although the principles of taste aversion learning do appear somewhat different from traditional learning, these differences tend to be differences in amount (for example, differences in the maximum interval between the unconditioned stimulus and the conditioned stimulus that will support learning), rather than differences in kind (for example, differences in whether or not the interval between the unconditioned stimulus and the conditioned stimulus influences learning; Domjan & Galef, 1983; Logue, 1979; Revusky, 1977; Shettleworth, 1983). This is perhaps not surprising in that all animals and all learning has to occur in the same physical world governed by the same physical properties. For example, in this world, event A cannot have been caused by event B unless event B preceded event A. Thus, for all types of learning, it would be adaptive for animals to learn more easily an association between a conditioned stimulus and an unconditioned stimulus if the conditioned stimulus precedes the unconditioned stimulus. This is indeed true for all types of learning, including taste aversion learning (Logue, 1979).

The discoveries of Garcia and his colleagues concerning the unusual characteristics of taste aversion learning should not be downplayed, however. Their findings were critical in opening a new approach to research, awakening many traditional learning theorists to the effects of evolution on behavior. The discoveries concerning the unusual properties of taste aversion learning were largely responsible for the fact that traditional learning theorists, starting in the 1970s, began to consider learning as consisting of principles that may be adapted to the world in general and to a species' particular ecological niche. This constituted a revolution in psychologists' conception of learning theory that subsequently extended to other areas of Psychology as well.

Preference for High-Fat Foods

Just as it is adaptive for omnivores, such as humans and rats, to learn about what foods might cause illness, so too it should be adaptive for omnivores to learn about what foods contain good nutrition. One of the best examples of this is animals' ability to learn which foods are high in calories. Calories provide energy for the body and are necessary for the body to function. In the natural environments of most species, digestible calories are not freely available and frequently are not available in sufficient amounts. In such environments, animals that could quickly learn which foods contain significant amounts of calories would be more likely to survive than would other animals. Given the wide variety of food sources available to omnivores, it would not be possible for animals to be born with knowledge of each high-calorie food source. Instead, it would be most adaptive for animals to have the ability to learn to prefer, after brief exposure to them, foods that contain substantial amounts of calories.

The development of such preferences has now been repeatedly demonstrated in the laboratory using nonhuman subjects (Capaldi, 1996). Experiments with humans have also been conducted. For example, Booth has been able to show that adult humans can learn to eat smaller meals when those meals contain a disguised high-calorie starch load associated with a distinctive taste. Further, if the subjects consume these meals when food deprived, preference for this kind of meal increases as the subjects gain experience with it. However, if the subjects consume these meals when satiated, the opposite occurs; the subjects' preference for these meals decreases as the subjects gain experience with them (Booth, 1982; Booth, Mather, & Fuller, 1982). Birch and Deysher have extended these findings to preschool children (Birch & Deysher, 1985). These researchers have demonstrated that preschool subjects learn to eat smaller meals following a taste that has been previously associated with a high-calorie snack, and larger meals following a taste that has been previously associated with a low-calorie snack.

Fat contains twice as many calories per gram than do proteins or carbohydrates. Thus it is easy to see how humans (and other animals) would learn at an early age to prefer high-fat foods. Such foods were not easy to find in the environment in which humans evolved. However, these foods are now easily and cheaply available in industrialized countries such as the United States. Thus, in our current environment, our preferences for high-calorie food make it very difficult to keep fat consumption low, as recommended by the Surgeon General (U. S. Department of Health and Human Services, 1988). With regard to calorie consumption, humans are adapted to a different environment than the one in which we live. It is this mismatch that results in our behaving in seemingly unadaptive ways.

Preference for Sweet Foods

Although it would not be possible to have an animal with an inborn preference for each of the large variety of high-calorie foods, there might be some specific substances for which animals could have inborn preferences. In nature, the taste of sweet is often associated with a high concentration of quickly available sugar and thus with readily available calories. In the environment in which humans evolved, one concentrated and relatively quick source of sugar and therefore of calories was ripe fruit, which is characterized by a sweet taste. In addition to sugar, fruit provides many vitamins and minerals necessary for body function and growth. A preference for sweet foods and drinks that would encourage consumption of ripe fruit was probably advantageous to our early ancestors (Konner, 1988; Rozin, 1976; Rozin, 1982). Thus it would have been adaptive for humans and other omnivores to have evolved with an innate preference for the taste of sweet. In fact, there is much evidence suggesting that this is the case.

First, people of any age are likely to pick sweet foods over others (Einstein & Hornstein, 1970; Meiselman, 1977; Peryam, Polemis, Kamen, Eindhoven, & Pilgrim, 1960). This is also frequently true of many other species, such as, horses, bears, and ants (Capaldi, Bradford, Sheffer, & Pulley, 1989; Pfaffman, 1977). Common laboratory lore holds that if you are having trouble training your rat to press a lever in a Skinner box, smearing a little milk chocolate on the lever will solve the problem.

Many pieces of data suggest that early exposure to the taste of sweet is not necessary for there to be a preference for the taste of sweet. For example, 1- to 3-day-old human infants prefer sweet over nonsweet fluids (Desor, Maller, & Turner, 1973). In addition, there are several documented cases in which cultures that have lacked sweet foods and drinks (with the exception of milk, which is slightly sweet), cultures such as the Eskimos of Northern Alaska, have come into contact with cultures that regularly consume sweet foods and drinks. In none of these cases have the cultures previously without sugar rejected the sugar-containing foods and drinks of the other culture (Bell, Draper, & Bergan, 1973; Mouratoff, Carroll, & Scott, 1967). Further, newborn infants, without any prior breast or bottle feeding, show an acceptance response the first time that they taste sweet. This response involves what appears to be a slight smile, licking of the upper lip, and sucking. This acceptance response is apparently an innate, reflexive response that is designed in such a way that whatever substance elicits the response will tend to be ingested. Rats also show this response when they taste sweet (Grill & Norgren, 1978; Steiner, 1977).

Several different pieces of evidence indicate that there are receptors specific to the taste of sweet on the surface of the tongue. This is not true for most tastes, which are detected by a variety of nonspecific receptors (Logue, 1991). Further, in many species, there are more fibers maximally sensitive to the taste of sweet in the chorda tympani nerve (the nerve that relays taste sensations from the tongue to the brain) than to any other taste (Frank, 1977). Together, this evidence suggests that the taste of sweet is more important to the body than is any other taste, again suggesting that the preference for sweet has a substantial genetic component.

Finally, rats can be selectively bred to have a greater or lesser preference for sweet (Nachman, 1959). The fact that this is possible shows clearly that genes can play a role in the preference for sweet in rats. However, this does not, of course, necessarily mean that genes make a significant contribution to the preference for sweet in humans.

In summary, although it has not been conclusively proven that the preference for sweet in humans and other animals has a strong genetic as opposed to environmental component, there is a great deal of evidence that strongly suggests that this is the case. As with the tendency to learn a preference for high-calorie foods, the preference for sweet foods would have been adaptive in the environment in which humans evolved. However, it is not adaptive now, when so many sweet foods are easily and cheaply available. The result is that we tend to overeat sweet foods, as well as other high-calorie (especially high-fat) foods. The resulting obesity and related adult-onset diabetes are an increasing problem in industrialized countries such as the United States.

Preference for Salty Foods

Similar to calories, salt is essential for the body to function properly. This is true for humans as well as for other species. Many physiological functions depend on the presence of salt, and even on a particular concentration of salt (Bloch, 1978; Denton, 1982). For example, the concentration of salt in the blood must be kept at a specific level. In humans, small amounts of salt are lost continually through sweat and through the action of the kidneys. If someone ceased to ingest salt, the body would excrete water in an attempt to keep the concentration of salt in the blood at the optimal level. Eventually that person would die of dehydration (Block, 1978).

Although salt is necessary for the body to function properly, it is not easily available in the wild. Prior to industrialization, humans sometimes had great difficulty obtaining enough salt. Many species must constantly consume salt in order to have sufficient amounts. Therefore it would not be surprising that natural selection would result in an innate preference for salt and that this preference would be present in most species (Denton, 1982).

The evidence strongly supports this hypothesis, although the situation is somewhat different from that of the preference for sweet. Unlike the preference for sweet, human infants are not born preferring salt. This is because humans cannot taste salt well until approximately 4 months of age, at which point they demonstrate a preference for salty over nonsalty solutions. Then, by around 24 months of age, children have learned which foods are supposed to be salty, and they reject foods that do not contain the customary degree of saltiness. As people get older, it is possible to decrease their need-free preference for salt to some extent by giving them weeks of experience only with foods with relatively low salt content (Beauchamp, 1987; Beauchamp, Cowart, & Moran, 1986).

Thus, as for the preference for sweet, the preference for salt appears to be virtually universal and to have a substantial genetic component. However, it is possible to influence the preference for salt by experience. Also similar to the preference for sweet, although the preference for salt was probably adaptive in the environment in which we evolved, it is no longer adaptive. Foods that are very salty, as well as very sweet and high in fat, are available in every vending machine, in every food store, and in every cafeteria. The New York Times recently reported (Drucker, 1996) that, on any given day, 7% of the United States population visits McDonald's, many of whose foods are quite salty, sweet, and/or high in fat. We overconsume these foods, much to the detriment of our future health. Food manufacturers take advantage of our evolutionary heritage, resulting in their making a great deal of money and in us spending money and becoming overweight and unhealthy.

Weight Gain and Loss

In an environment in which there is a limited or erratic food supply, it would be adaptive for animals to take in as much food as they can, whenever it is available. Then, if possible, these same animals should retain (as opposed to use) the calories thus consumed, as insurance against future periods of food scarcity.

From this perspective, and given that humans evolved in an environment in which there was indeed limited and erratic access to food, it can be seen that a number of facts about human weight regulation are all directed towards the maximization of stored energy. First, the number of adipose cells (the cells that store fat) in the body can increase (following a body weight gain), but can never decrease. These cells are related to the set point, the long-term weight that the body tends to maintain. When the adipose cells are not full of stored fat, the person will be hungry; when they are full of stored fat, the person will not be hungry (Le Magnen, 1985; Sj”str”m, 1978). Therefore, a person's hunger will make it extremely difficult for that person ever to maintain a weight lower than the person's highest attained weight. Second, if someone tries to lose weight by restricting caloric intake, that person's metabolic rate (the rate at which energy is burned in the body) will decrease, making it increasingly difficult to lose weight. Further, that person's metabolic rate will stay low even when calories are no longer restricted, making weight gain almost inevitable when the diet is over (Elliot, Goldberg, Kuehl, & Bennett, 1989; Keesey & Corbett, 1984; Leibel, Rosenbaum, & Hirsch, 1995; Steen, Oppliger, & Brownell, 1988). Third, in our current environment, in which we are surrounded by foods for which we have a high preference, foods that were scarce in the environment in which we evolved, we tend to overeat and gain weight. (Exercise might mitigate some of this weight gain. However, despite ensuing health problems, most adults prefer to conserve energy and not to exercise, and they use labor saving devices such as elevators and washing machines whenever possible.) Experiments with both rats and humans have shown that when large amounts of very tasty foods are available, subjects will overeat and become obese (Bobroff & Kissileff, 1986; Jordan & Spiegel, 1977; Sclafani & Springer, 1976).

Issues concerning the adaptiveness of weight status are particularly salient when the menstrual cycle and pregnancy are considered. A woman needs approximately 50,000 to 80,000 calories in order to produce a full-term baby (Frisch, 1988). In addition, during lactation, a woman requires approximately 765 to 980 calories per day above her ordinary needs (St. Jeor, Sutnick, & Scott, 1988). Therefore, ideally, from an evolutionary standpoint, women should not become pregnant, which requires the expenditure of a huge number of the woman's calories, unless there is an adequate food supply and/or adequate amounts of stored food (in other words, adequate amounts of body fat). Consistent with this hypothesis, inadequate food intake and low amounts of body fat will disrupt the menstrual cycle and inhibit ovulation, as occurs in young women with anorexia nervosa (American Psychiatric Association, DSM-IV, 1994). Also consistent with this hypothesis, women's food intake (including of high-calorie foods such as chocolate) tends to increase in the second half of the menstrual cycle, just after ovulation and around the time at which implantation of the embryo would occur (Bancroft, Cook, & Williamson, 1988; "Energy Expenditure," 1987; Leiter, Hrboticky, & Anderson, 1987; St. Jeor et al., 1988). Some of this increase in food consumption, but not all, may be attributable to women compensating for the higher metabolic rate that they have at this time ("Energy Expenditure," 1987; Leiter et al., 1987; St. Jeor et al., 1988).

If pregnancy occurs instead of menstruation, women's fat stores tend to increase, even during the first trimester, despite the higher metabolic rate that continues during pregnancy. The weight gain during the first trimester is probably due to increased release of the gut peptide cholecystokinin at that time, and is not due to excess food intake (Uvnas-Moberg, 1989). Once the baby is born, lactating women need greatly increased numbers of calories. Under these conditions, women greatly increase their caloric intake (Rosso, 1987). In addition, suckling an infant makes a woman sleepy (Uvnas-Moberg, 1989). This sleepiness may help to decrease the woman's energy usage, thus meliorating the caloric drain of the production of milk. The female body is apparently well adapted to obtaining, and retaining, the additional calories needed to sustain a successful pregnancy with subsequent lactation.

Self-Control

Self-control can be defined as choice of a more delayed outcome that is ultimately of more value over a less delayed outcome that is ultimately of less value. Impulsiveness can be defined as the opposite (Ainslie, 1974; Logue, 1988, 1998; Rachlin & Green, 1972). For example, if someone were given a choice between one bag of m&ms today and two bags of m&ms tomorrow, and that person chose the two bags, we would say that the person was showing self-control. On the other hand, if that person chose the one bag we would say that the person's behavior was impulsive.

Animals frequently encounter choices between self-control and impulsive alternatives in which at least one of the alternatives involves food consumption. For example, when someone who is unhealthily overweight chooses between having dessert and adhering to a long-term weight-loss diet, having dessert can be defined as impulsiveness and adhering to the weight-loss diet can be defined as self-control. All self-control choices, including those involving food, appear to be extremely difficult for many people to make. It is possible that this difficulty in showing self-control may be due to our evolutionary heritage.

Conclusions

Humans easily learn to avoid foods associated with gastrointestinal illness (especially nausea); prefer high-calorie, sweet, and salty foods; are more likely to gain than lose weight; and tend to choose food available relatively soon over other, ultimately more valuable, alternatives not available until later. Evolutionary theory can help us to see how humans may have evolved to exhibit these behaviors, and how all of these behaviors appear to be ones that would have helped humans to survive in nature. It helps us to see the general, and more specific, functions of these behaviors, and allows us to make predictions with regard to the eating behavior of humans in new situations (for some additional examples, see Rozin, 1996; Siegal, 1995; Winn, 1995). Evolutionary theory can be an extremely helpful theoretical framework for Psychology.

Although many aspects of human eating behavior may have helped humans to survive in a natural environment, these same aspects of human eating behavior are frequently unadaptive in our current environment, an environment in which foods that are highly preferred are also easily and cheaply available. In our current environment, these behaviors contribute significantly to our population being overweight and suffering from various health problems associated with overeating. There is a mismatch between some aspects of the environment in which we evolved and in which we currently exist.

Nevertheless, we are not doomed to engage in unhealthy behaviors. We can use various self-control techniques such as precommitment to ensure that we engage in behaviors that are of long-term great value, instead of engaging in behaviors that bring us only some immediate satisfaction. Acknowledging that humans have evolved to exhibit certain behaviors does not at all mean that behavior is fixed and immutable.

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