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Chapter: Psychology: The Genetic and Evolutionary Roots of Behavior

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The Biological Roots of Smiling

All animals interact with other members of their species, whether as mates, parents, off-spring, or competitors.

The Biological Roots of Smiling

All animals interact with other members of their species, whether as mates, parents, off-spring, or competitors. And these interactions, in turn, usually depend on some kind of communication as each animal lets the other know about its status and intentions. Sometimes, the style of communications is species specific—pertaining to just one species—but often the communication involves signals shared by many types of animals. Thus, many mammals use the same “surrender” display to end a fight: They lie on their backs, exposing their bellies and their throats, as a way of communicating (roughly) “I know I’ve lost the fight; I’m giving in, and making myself completely vulnerable to you. Let’s not fight any more.”

Humans, too, have various ways of communicating their status and intentions. In many circumstances, of course, we use language—and so we can, with enormous preci-sion, convey our message to a conversational partner. We also communicate a great deal by our body position—how close we stand to another person, how we hold our arms, how we orient our bodies, and so on. Our faces convey still more information— including the various facial displays that express emotion.


Virtually all babies start smiling at a very young age. The first smiles are detectable in infants just 1 month old; smiles directed toward other people are evident a month or so later. One might think that babies learn to smile by observing others and imitating the facial expressions they see, but evidence argues against this proposal. For example, babies who are born blind start smiling at the same age as sighted babies, and—just like sighted babies—they’re most likely to smile when interacting with others and when they’re com-fortable and well fed. Likewise, one study compared the facial expressions of three groups of athletes receiving their award medals at the 2004 Paralympic Games (Matsumoto & Willingham, 2009). One group had been blind since birth; a second group had some years of visual experience but was now fully blind; a third group had normal sight. The study showed essentially no difference among these groups in their facial expressions.

Apparently, then, the behavior of smiling is something that humans do without a history of observational learning. On this basis, we might expect to find smiles in all humans in all cultures—and we do. In other words, the behavior of smiling is species general—observable in virtually all members of our species. In one study, American actors were photographed while conveying emotions such as happiness, sadness, anger, and fear. These photos were then shown to members of an isolated tribe in New Guinea, and individuals there were asked to pick the emotion label that matched each photograph. Then the procedure was reversed: The New Guinea tribesmen were asked to portray the facial expression appropriate to specific situations, such as happiness at the return of a friend or anger at the start of a fight. Photographs of their performances were shown to American college students, who were asked to judge which situation the tribesmen had been trying to convey in each photo (Ekman & Friesen, 1975).

Performance in these tasks was quite good—the New Guinea tribesmen were generally able to recognize the Americans’ expressions, and vice versa. Importantly, performance varied from one emotion to the next, with (for example) disgust and fear more difficult to recognize than sadness. The expression producing the most accurate identification, though, was happiness. Smiles are apparently a universal human signal (Ekman, 1994; Izard, 1994).


The behavior of smiling is not just shared across cultures; it is also shared across species. In other words, we find similar emotional expressions in animals with genomes simi-lar to ours. Thus, smiling is species general (found in the entire species), but it is not species specific (found only in one species).

Darwin himself was especially interested in this point and drew evidence from several sources, including his own observations of animals at the London Zoo. He eventually published his findings in a book entitled The Expression of the Emotions inMan and Animals (Darwin, 1872). Darwin’s observations made it clear to him that therewere different types of smiles—a point that has been well confirmed in more recent research. The smiles can be differentiated in terms of the situations that elicit them; they can also be differentiated by their exact appearance. And, remarkably, the various types of smiles can be identified as readily in other species as they can in humans.

One type of smile seems straightforwardly expressive of your inner state, and it’s pro-duced when you feel happy. This smile will be produced even if no other people are around, and it involves both a change in mouth shape (the corners of the lips are pulled upward) and a shift in the muscles of the upper face, surrounding the eyes. The latter shift creates the pattern often called crow’s feet—lines that radiate outward from the eyes (Figure 2.16).

The expressive smile obviously occurs in humans when events or stimuli please us, or when we hear a good joke. A similar expression occurs in young monkeys in the midst of play; many observers interpret it as a signal between monkeys that their pushing and tumbling is playful rather than aggressive. Smiles in humans also promote cooperation: A smile on someone’s face often signals their intention to cooperate and, at the same time, the sight of a smile tends to evoke positive feelings in the perceiver, making cooperation more likely.

A different sort of smile seems more polite in nature, and it’s rarely produced without an audience. In this smile, one pulls the corners of one’s lips upward but with little movement of the eyes. This sort of smile seems to function as a greeting and also as a means of defusing situations that otherwise might be tense or embarrassing (Goldenthal, Johnston, & Kraut, 1981). It’s also a smile people use when they wish to simulate happiness (e.g., when pretending to have fun at a dreadful party, or when trying to persuade someone they’re amusing when they are actually boring).

This polite smile can also be found in nonhuman primates, where it generally takes a form of drawing back the lips and revealing the teeth, but keeping the teeth plainly closed. In monkeys, this smile may be a gesture of submission at the end of a conflict, or it may be intended to avoid a conflict. It’s as if one monkey is saying to another, “Look—my teeth are closed; I’m obviously not preparing to bite you or fight you. So be good to me” (Figure 2.17).

What does all of this imply about the origins of smiling? The fact that smiles emerge with no history of learning (e.g., in individuals blind since birth) tells us that this behavior is strongly shaped by inborn (genetic) factors. This claim is certainly consis-tent with the universality of smiling (across cultures and across species), which sug-gests an ancient origin for this behavior: It’s likely that the smiles evident in American college students, in New Guinea tribesmen, or in playful monkeys were shaped by natural selection long ago, in the ancestors shared by all of these modern primates. Moreover, the obvious function of smiles in modern creatures supplies an important

clue as to why the behavior of smiling served our ancestors well—by providing a signal about emotions and intentions that facilitated social interactions. These observations in turn provide a clear indication of why this behavior was promoted and preserved by natural selection. All of these are key points whenever we try to establish—whether we’re focusing on smiles or any other trait—how and why the trait evolved.

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