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Rosenthal went on to study precisely that—what expectations mean for our children.10 In one line of research he showed that teachers’ expectations greatly affect their students’ academic performance, even when the teachers try to treat them impartially. For example, he and a colleague asked schoolkids in eighteen classrooms to complete an IQ test. The teachers, but not the students, were given the results. The researchers told the teachers that the test would indicate which children had unusually high intellectual potential.11 What the teachers didn’t know was that the kids named as gifted did not really score higher than average on the IQ test—they actually had average scores. Shortly afterward, the teachers rated those not labeled gifted as less curious and less interested than the gifted students—and the students’ subsequent grades reflected that.

But what is really shocking—and sobering—is the result of another IQ test, given eight months later. When you administer an IQ test a second time, you expect that each child’s score will vary some. In general, about half of the children’s scores should go up and half down, as a result of changes in the individual’s intellectual development in relation to his peers or simply of random variation. When Rosenthal administered the second test, he indeed found that about half the kids labeled “normal” showed a gain in IQ. But among those who’d been singled out as brilliant, he obtained a different result: about 80 percent had an increase of at least 10 points. What’s more, about 20 percent of the “gifted” group gained 30 or more IQ points, while only 5 percent of the other children gained that many. Labeling children as gifted had proved to be a powerful self-fulfilling prophecy. Wisely, Rosenthal hadn’t falsely labeled any kids as being below average. The sad thing is that such labeling does happen, and it is reasonable to assume that the self-fulfilling prophecy also works the other way: that branding a child a poor learner will contribute to making the child exactly that.

HUMANS COMMUNICATE VIA a rich linguistic system whose development was a defining moment in the evolution of our species, an innovation that remade the character of human society. It’s an ability that seems to be unique.12 In other animals, communication is limited to simple messages, such as identifying themselves or issuing warnings; there is little complex structure. Had Hans, for example, been required to answer in complete sentences, the gig would have been up. Even among primates, no species naturally acquires more than a few signals or combines them in anything but a rudimentary manner. The average human, on the other hand, is familiar with tens of thousands of words and can string them together according to complex rules, with hardly any conscious effort, and without formal instruction.

Scientists don’t understand yet how language evolved. Many believe that earlier human species, such as Homo habilis and Homo erectus, possessed primitive language-like or symbolic communication systems. But the development of language as we know it probably didn’t occur until modern humans came into the picture. Some say language originated one hundred thousand years ago, some later; but the need for sophisticated communication certainly became more urgent once “behaviorally modern” social humans developed, fifty thousand years ago. We’ve seen how important social interactions are to our species, and social interactions go hand in hand with the need to communicate. That need is so powerful that even deaf babies develop language-like gesture systems and, if taught sign language, will babble using their hands.13

Why did humans develop nonverbal communication? One of the first to seriously study the issue was an English fellow, spurred by his interest in the theory of evolution. By his own assessment, he was no genius. He had “no great quickness of apprehension or wit” or “power to follow a long and purely abstract train of thought.”14 On the many occasions when I share those feelings, I find it encouraging to review those words because that Englishman did okay for himself—his name was Charles Darwin. Thirteen years after publishing The Origin of Species, Darwin published another radical book, this one called The Expression of the Emotions in Man and Animals. In it, Darwin argued that emotions—and the ways they are expressed—provide a survival advantage and that they are not unique to humans but occur in many species. Clues to the role of emotions therefore can be found by examining the similarities and differences of nonverbal emotional expression across various species.

If Darwin didn’t consider himself brilliant, he did believe he possessed one great intellectual strength: his powers of careful and detailed observation. And, indeed, though he was not the first to suggest the universality of emotion and its expression,15 he spent several decades meticulously studying the physical manifestations of mental states. He watched his countrymen, and he observed foreigners, too, looking for cultural similarities and differences. He even studied domestic animals and those in the London Zoo. In his book, Darwin categorized numerous human expressions and gestures of emotion and offered hypotheses about their origin. He noted how lower animals, too, display intent and emotion through facial expression, posture, and gesture. Darwin speculated that much of our nonverbal communication might be an innate and automatic holdover from earlier phases of our evolution. For example, we can bite affectionately, as do other animals. We also sneer like other primates by flaring our nostrils and baring our teeth.

The smile is another expression we share with lower primates. Suppose you’re sitting in some public place and notice someone looking at you. If you return the gaze and the other person smiles, you’ll probably feel good about the exchange. But if the other person continues to stare without any hint of a smile, you’ll probably feel uncomfortable. Where do these instinctual responses come from? In trading the currency of smiles, we are sharing a feeling experienced by many of our primate cousins. In the societies of nonhuman primates, a direct stare is an aggressive signal. It often precedes an attack—and, therefore, can precipitate one. As a result, if, say, a submissive monkey wants to check out a dominant one, it will bare its teeth as a peace signal. In monkey talk, bared teeth means Pardon my stare. True, I’m looking, but I don’t plan to attack, so PLEASE don’t attack me first. In chimpanzees, the smile can also go the other way—a dominant individual may smile at a submissive one, saying, analogously, Don’t worry, I’m not going to attack you. So when you pass a stranger in the corridor and that person flashes a brief smile, you’re experiencing an exchange with roots deep in our primate heritage. There is even evidence that with chimps, as with humans, when a smile is exchanged, it can be a sign of friendship.16

You might think a smile is a rather shoddy barometer of true feelings because, after all, anyone can fake one. It’s true that we can consciously decide to exhibit a smile, or any other expression, by using the muscles in our faces in ways we are practiced at doing. Think about what you do when trying to make a good impression at a cocktail party, even though you are miserable about being there. But our facial expressions are also governed subliminally, by muscles over which we have no conscious control. So our real expressions cannot be faked. Sure, anyone can create a posed smile by contracting the zygomatic major muscles, which pull the corners of the mouth up toward the cheekbones. But a genuine smile involves contraction of an additional pair of actors, the orbicularis oculi muscles, which pull the skin surrounding the eye toward the eyeball, causing an effect that looks like crow’s-feet but can be very subtle. That was first pointed out by the nineteenth-century French neurologist Duchenne de Boulogne, who was an influence on Darwin and collected a large number of photographs of people smiling. There are two distinct neural pathways for these smile muscles: a voluntary one for the zygomatic major, and an involuntary one for the orbicularis oculi.17 So a smile-seeking photographer might implore us to say “cheese,” which nudges our mouths into the smile position, but unless you’re the kind who actually rejoices when asked to speak the word “cheese,” the smile won’t look genuine.