By John S. Allen

At any run-of-the-mill Japanese restaurant in North America, the menu features such traditional items as tempura, tonkatsu, and kara-age chicken. This crispy trio has long had an important place in Japanese cuisine. But it is surprising to find out that all three are cultural borrowings, some dating back to time periods when Japan went to great lengths to isolate itself from foreign influences. The batter-frying tempura technique (used typically for vegetables and shrimp) was borrowed from Spanish and Portuguese missionaries and traders in the 15th and 16th centuries. Tonkatsu is a breaded pork cutlet, a version of the schnitzel from Germany and Central Europe, which was added to Japanese cuisine probably no later than the early part of the 20th century. Kara-age originally meant "Chinese frying" and refers to deep-frying foods that have been coated with corn starch.

In The Babbo Cookbook, the celebrity chef and restaurateur Mario Batali wrote, "The single word 'crispy' sells more food than a barrage of adjectives. ... There is something innately appealing about crispy food." If crispy food really is innately appealing, that might help explain why Japanese cuisine was so receptive to these particular "outside" foods. In turn, it is quite possible that crispy dishes such as tempura and tonkatsu were gateway foods for the worldwide acceptance of squishier Japanese delicacies, such as sushi. Tortilla chips, potato chips, French fries, fried chicken, and other crispy items may serve as the advance guard in the internationalization of eating throughout the developed (and developing) world. Crispy conquers cultural boundaries.

The hypothesis that crispy foods are innately appealing is a fascinating one. As an anthropologist interested in the evolution of cognition and the human diet, I think that maybe our attraction to crispy foods could give us insights into how people have evolved to think the food that they eat.

Eating has been as critical to human survival as sociality, language, and sex and gender roles have, but it has not received much interest from evolutionary psychologists and other scientists interested in behavioral evolution. What we eat is, of course, shaped by culture, which influences the range of foods that are deemed edible and inedible in any given environment. But eating and food choices have also been shaped by millions of years of evolution, giving us a preference for certain tastes and textures, as well as a desire to eat more than we should when some foods are readily available.

One place to start trying to understand the appeal of crispy foods is to look at their sources in the natural world. What are the crispy foods that our current primate cousins and past primate ancestors might eat or have eaten? There are two main sources of crispy in the natural environment of primates: insects and certain parts of plants (especially stalks, some leaves, pods, perhaps roots). Insects get mixed reviews among contemporary humans. In the Western world, they are generally held to be highly unappetizing and associated with filth and disease. In many other cultures, however, insects are eaten with varying degrees of enthusiasm. Sometimes they are eaten in grub form, which is not very crispy, but adult insects, with their crunchy exoskeletons of chitin, are enjoyed in many cultures, often well-seasoned and fried to an extra-crispy state.

No human—no reasonably large-bodied primate, for that matter—can survive on a diet of insects alone. It is just not nutritionally practical. Some small prosimian primates, such as lorises, galagos, tarsiers, and the smaller lemurs, can get by on diets that are almost entirely insectivorous. Such primates are similar in size and habits to the original primates, which lived some 60 million years ago. Most paleoanthropologists believe that insects were an important part of the diet of those earliest primates. So we have insect-eating ancestors, even if some of us are not all that enthusiastic about eating them today.

Many people are also not all that enthusiastic about eating vegetables. Unlike fruits, many of which are sweet and juicy, enticing animals to eat them as a means of seed dispersal, the parts of the plant that we usually think of as vegetables often contain toxins to discourage animals from eating them. They are also relatively low in nutrient content. Now, many animals, including some major groups of primates, have evolved digestive specializations that allow them to get by on a diet of leaves and stalks. However, our closest relatives, the chimpanzees, are largely fruit eaters, consuming leafy vegetation and insects to supplement their preferred diet (meat is also eaten enthusiastically but in relatively small quantities).

Nonpreferred foods that an ape or monkey eats when other foods are not available are called "fallback foods" by primate ecologists. It is clear that humans and other primates have evolved preferences for sweet and salty tastes, and very likely also for fat and umami, the savory flavor associated with meat and some other foods. By definition, fallback foods do not push the preference buttons, since they are not preferred. However, all animals at some point face situations where preferred foods are not available or are insufficient. Attraction to some quality of the most available fallback foods would be adaptive. For primates like us, the attractive quality of likely fallback foods, such as insects and fresh vegetables, might be crispiness. Crispy may make relatively common, unpalatable fallback foods more acceptable, and an "innate" preference for crispy could be adaptive.

At some point in human evolution, maybe well over a million years ago, our ancestors discovered and learned how to use fire. According to the primatologist Richard Wrangham, this was one of the most momentous events in human evolutionary history. It opened up the potential diet of these early humans, allowing them to make better use of the whole animals they hunted or scavenged. No longer were they limited to the soft bits, such as brain, liver, and marrow; now they could cook (and thus tenderize) the fibrous muscle that makes up most of the nutrient content of an animal. Fire also allowed for the cooking of tough roots, another source of calories not exploited by other apes or hominids.

Cooking also made available to humans a whole new range of crispy and, maybe more important, intensely flavored foods. Heating facilitates the Maillard reaction, in which carbohydrates and amino acids combine to produce an array of flavors and aromas (and brown color). In dry-heat cooking, such as grilling, baking, and frying, the Maillard reaction occurs on the surface of the meat or vegetable, leading to enhanced flavors and a crispy crust. As Wrangham argues, the nutritional advantages provided by cooking undoubtedly made it a critical step in human evolution. But cooking also combined crispness with more-appealing flavors (compared with the crispy fallback foods), which may have helped the practice of cooking become even more adaptive.

Fallback foods and cooking provide the evolutionary foundations for the appeal of crispy foods, but what can we say about their appeal today in the developed world, where fallback foods and fire are not really a concern? One thing that crispy and crunchy foods do is enhance the sensory experience of eating. Obviously, when we eat, we use our senses of taste and smell, as well as the sense of touch as we assess the texture and "feel" of the food both in our hands and in our mouths. An underappreciated component of the eating experience is sound.

The external sounds of the eating experience are a concern to some restaurateurs. (Diners, assaulted by loud or horrible music, may wish it was a concern to more of them.) And, of course, many cultures dictate the acceptable amount of noise a diner should make while eating. While Western etiquette experts try to stamp out the "gross noises" of eating (see emilypost.com), in other cultures enthusiastic noisemaking while eating is a sign that the food is being enjoyed.

Crispiness may be more important for the sounds it makes inside our heads. The internal noises of chewing are always there when we eat, but actually these are typically the sounds we stop hearing. A common feature of all neural sensory systems is habituation—sensory neurons usually become less responsive with persistent exposure to a stimulus. When you put clothes on, after awhile your skin no longer feels that they are there. Similarly, there is habituation to the taste and smell of food as we eat.

Celebrated chefs such as Thomas Keller and Ferran Adrià combat sensory habituation by serving many small, varied dishes over the course of a long (and expensive) meal. As many regretful Thanksgiving eaters know, the variety of foods served in traditional feasts also make it possible to eat more by reducing the impact of sensory habituation.

Perhaps one reason that crispy foods have such an appeal lies in their ability to stimulate our hearing as well as our senses of taste and smell. Crispiness in and of itself stands apart from other food qualities; this texture can be pleasurable even when combined with flavors that are themselves not necessarily all that appealing. Chewing crispy foods is louder than chewing noncrispy foods. If habituation takes longer given a stronger sensory signal, then we should enjoy eating crispy foods for a longer period of time during any given bout of eating. Of course, numerous factors are important in determining what we like to eat, but it is not unreasonable to suggest that we might like a particular crispy food in part because we like the way it sounds in our own heads.

Spoken language makes a significant contribution to the sound environment in which most people live. When spoken or voiced internally, the sounds of words such as "crispy" and "crunchy" are evocative of the qualities they are describing—they are onomatopoeic. The etymology of "crispy" is complex; it seems to have originally meant curvy or wavy, but whatever its origins, it is most commonly used as an adjective for brittle foods. It is clear that the sound of "crispy" is not identical to the sounds that crispiness makes (which is true of most onomatopoeic terms, in fact), but for some reason it evokes that sound to our ears. Similarly, the word "crunchy," which is widely acknowledged to originate in onomatopoeia, evokes an even more profound, and perhaps less refined, sense of this quality.

Onomatopoeia may be one reason that "crispy" is such an effective menu term. Functional neuroimaging research on onomatopoeic terms has shown that when listeners hear those terms, they show activation in the parts of the brain that may also be active when they are actually experiencing the action or emotional state the term evokes. Similarly, hearing onomatopoeic terms associated with walking (of which there are several in Japanese) causes activation in the parts of the brain associated with the visual processing of body actions. Other imaging studies have shown that only thinking about doing a motor action will activate the parts of the brain that actually come into play when doing the action.

The implications for crispy and crunchy in all this now begin to take shape. Simply reading, hearing, or saying the onomatopoeic terms "crispy" and "crunchy" is likely to evoke the sense of eating that type of food. Presumably this feeling would be represented in the brain by activation of the mouth and tongue regions of the primary motor cortex (and of course, when a word is actually said, the motor regions of the mouth are being directly activated). "Crispy" might be such a compelling descriptive term because, in a sense, hearing or saying it strongly promotes the motor imagery of eating—a food item with the word "crispy" attached to it is in some ways already being eaten by its potential consumer. "Crispy" in a menu could be quite persuasive, especially when coupled with the fact that crispy foods are often quite palatable for other reasons.

So, are we any closer to understanding the "something" that makes crispy food innately appealing? Why are we crazy for crispy? The human species has numerous ancestors and relatives for whom a crispy insect was and is an attractive meal. Some of our kindred species feast on raw, crispy vegetables, and for those species for which leaves and stalks are not a first choice (and we humans would be in that category), a preference for those foods is quite useful if we need to survive on fallback foods. In short, we have an evolutionary legacy as primates that suggests that crispy and crunchy foods should be attractive to us, at least sometimes and under certain conditions.

With the advent of cooking, dietary conditions changed drastically. Crispy became available to our ancestors via the Maillard reaction. Cooking made the nutrients in meat and certain plant foods, such as tubers, more available to us and more palatable as well. Our ancestors who liked crispy cooked foods may have done particularly well in the reproductive sweepstakes, since cooking allowed greater access to a whole range of high-quality food items in varied environments. And so our innate liking for crispy, derived from our distant relatives, may have been reinforced in more recent evolutionary times.

Crispy foods may further, in various small ways, have a privileged place in the brain. They incorporate hearing into the sensory mix of eating, and it is very likely that the stronger and more varied sensory mix provided by crispiness staves off boredom and habituation while we eat these foods. And as we just discussed, the word "crispy" itself may increase the appeal of such foods, at least when we are contemplating making them part of a meal. That would be an unexpected consequence of having a brain that is wired for language while still profoundly influenced by cognitive processes that occur well below this higher cognitive level.

There are other possible reasons why crispy may be so appealing, of course. In the modern food environment, commercially produced crispy foods are ubiquitous and strongly promoted—and, at the same time, demonized as leading to obesity. These foods, or at least some of them, are "bad." But as many of us are aware, doing something bad, as long as it is not too bad, can be pleasurable in and of itself. Eating a bag of potato chips may be enjoyable not just because it delivers salt, fat, and carbohydrate in a nice, crispy package, but also because of the frisson of illicit pleasure it confers in a hectoring, contradictory nutritional culture.

How we think food and how we eat food are complex products of multiple histories. These cognitive, evolutionary, and cultural histories interact in unique ways in each individual, who brings to the table a personal history as well. Crispy foods are certainly not the only type of food that humans find appealing, and of course some people do not even like them. But the pervasive appeal of crispy is clearly something that emerges out of our multiple, interacting histories.

John S. Allen is a research scientist at the Dornsife Cognitive Neuroscience Imaging Center and the Brain and Creativity Institute at the University of Southern California. This essay is adapted from his new book from Harvard University Press, The Omnivorous Mind: Our Evolving Relationship With Food.