What Makes Humans Unique?

Cecilia Heyes on what separates humans from other apes

“Humans fly spaceships, split atoms, and edit genes. No other animal has even invented the wheel,” says Max Bennett in A Brief History of Intelligence. While there are continuities between the mental abilities of apes and humans, human intelligence is plainly radically different from that of our animal cousins.

What accounts for the difference? Answering this question is one of the goals of evolutionary psychology and evolutionary anthropology. There is widespread agreement that humans are different because they possess cognitive mechanisms that are specialized for the problems that humans routinely face, like learning by imitation, interpreting the minds of others, using language, and understanding moral codes. While these abilities are present in a rudimentary form in apes, they developed into a much more elaborate form in the human lineage.

So where do these specialized cognitive mechanisms come from? The overwhelming consensus among evolutionary theorists is nativism: we are born with cognitive instincts that are specialized for solving these problems. We come into the world with one cognitive instinct that solves the problem of imitative learning, another for using language, another for morality, and so forth.

In her 2018 book Cognitive Gadgets: The Cultural Evolution of Thinking and in other writings, the evolutionary theorist Cecilia Heyes attacks this nativist consensus. Heyes does not deny that human minds have specialized abilities. However, she believes that these specializations are products of learning rather than instincts. What accounts for human uniqueness is not “big special” cognitive instincts but “small ordinary” changes to the primate brain that gave humans an enhanced capacity for learning. She calls these learned specializations “cognitive gadgets” to distinguish them from cognitive instincts. Heyes examines theories of imitative learning, theory of mind, language, and morality and finds in all cases that the cognitive gadget theory explains the facts better than nativism. She concludes that human uniqueness is not due to cognitive instincts, but rather to our possession of powerful domain-general intelligence and to cultural evolution.

Imitative learning as a gadget

Cultures consist of a vast and complicated set of practices that humans need to learn if they are to survive and thrive. If you are an Inuit living in the icy Arctic tundra, you need to learn how to make shelters out of snow and ice, how to find the breathing holes of seals in the ice so you can hunt them, how to make harpoons out of caribou antlers, how to carve lamps out of soapstone, and a host of other practices. Inuit who do not learn these skills are not likely to thrive, or even survive.¹

One of the primary ways by which humans learn is imitation, and the dominant theory is that humans have a cognitive instinct that is specialized for imitation. There have been two arguments for the nativist theory. The first is that some experimental results suggested that newborn infants spontaneously imitated adults before they had any opportunity to learn imitation.

The second is that imitation seemed such a difficult problem that psychologists doubted that infants could learn how to solve it unaided. Take the picture below. The little boy is imitating the adults by walking with his arms crossed behind his back. However, while he can see the arms of the adult, he cannot see his own. The boy must learn to connect the way that his arms feel with the way that others’ arms look, a very difficult cognitive problem, especially because the boy cannot see his arms. In the words of the psychologists Andrew N. Meltzoff and M. Keith Moore, imitation requires an ability to “connect the felt but unseen movements of the self with the seen but unfelt movements of the other.”

Meltzoff and Moore began arguing in the 1970s that, because of the difficulties inherent in imitation, it was not plausible that babies could learn how to do it themselves. Rather, such learning supposedly required an innate cognitive instinct, dubbed “active intermodal mapping,” that linked sensory perceptions with motor movements. This type of “poverty of the stimulus” argument is quite common among nativists. The logic is that children lack the information necessary to learn complex skills, so an innate mechanism is necessary.

Heyes begins her critique of Meltzoff and Moore’s theory by asking how the active intermodal mapping instinct is supposed to work. What rule set does this instinct use to accomplish its goal? What stimuli elicit the instinct and how? She finds in this case and all the others she examines that an answer has never been given. Nativists’ cognitive instincts are “black boxes,” the functioning of which is never explained in anything like a satisfactory manner.

Heyes then moves on to whether the facts justify the instinct theory. Psychologists always had difficulty replicating the finding that newborns imitated adults: some results were positive, some negative. Then, in 2016, a definitive study came out that seemed to decisively refute the theory that newborns come into the world able to imitate. Psychologists presented infants aged nine weeks or less with various facial expressions, like happy face, sad face, tongue protrusion, and mouth opening, and gauged whether the infants imitated these expressions. Except in the case of tongue protrusion, the results were all negative. The tongue protrusion result is explained by the fact that infants just tend stick out their tongues when exposed to any surprising stimulus, whether it’s an adult sticking out their tongues or flashing lights.

Meanwhile, Heyes and others developed non-innatist theories of imitation. According to Heyes’ own theory, associative sequence learning, infants learn to map perceived actions with motor movements when they occur together. If an infant can observe itself making a grasping movement while it is grasping, it will form an association between the way grasping looks and the way that it feels.

However, some actions, like the boy’s in the picture above, are perceptually opaque, meaning that babies cannot see themselves performing the action. Another type of perceptually opaque action is facial expressions, as we cannot see our own expression without a mirror. Other types of learning are necessary to develop imitation of perceptually opaque actions. For example, adults often imitate babies’ facial expressions, and from this type of imitation, a baby can learn to associate the motor movement of smiling with the perception of a smiling person. Another way is by interacting with mirrors. Heyes argues throughout the book that “poverty of the stimulus” justifications for nativism fail because there are many opportunities for infants to learn imitation and other abilities from their environments.

Heyes reviews a wealth of evidence that the non-innatist theory is correct. For example, because adults have learned to associate the sight and feeling of walking, the part of the brain involved in walking tends to fire when we see someone walking. However, that part of the brain does not fire in infants who merely see someone walking. Rather, the association develops only once infants themselves walk while they see others walking.

Other evidence shows that adults are quite poor at imitating facial expressions and don’t get any better at it until they can test imitations in a mirror. Of course, actors and impersonators learn their craft through lots of effortful practice in front of a mirror rather than through any innate imitation instinct. Through these various types of learning, we eventually end up with a cognitive gadget that is highly effective at imitation.

How cognitive gadgets evolve

On Heyes’ theory, cognitive gadgets are the products of evolution, but not of genetic evolution. Rather, they evolve through cultural evolution. Suppose that there are two groups of humans, A and B, and group B invents a beneficial cultural practice that group A does not know about. The practice might permit B to use language more subtly, or to interpret the minds of others more accurately, or teach children important cultural information to infants more effectively. All else being equal, you would expect group B to outperform group A due to this advantage. Group B would have more babies with a higher survival rate and be more effective in intergroup competition. Also, B might get converts from A due to their superior culture. Eventually, you would expect that A’s superior cultural practice would eventually eliminate B’s. In this way, cognitive gadgets could get built up across generations. This is a theory of evolution in which different variants compete against each other. However, these are not genetic variants, but cultural variants.

What makes humans unique?

Heyes’ theory gives her an original and persuasive perspective on the evolution of human uniqueness. Nativists are forced to defend the theory that a suite of “big special” cognitive instincts evolved in the six million years since the human lineage diverged from that of other apes. It is not impossible that this happened, but Heyes proposes a more plausible theory that a number of “small ordinary” changes to the primate brain gave humans the capacity for learning that accounts for our cognitive gadgets.

What changes do you need to make to the brain of an ape to get a human capacity for learning? Heyes lists a number of them, which she calls the human “genetic starter kit.” The first is reduced aggressiveness and increased tolerance for others. Dozens of humans can sit together on an airplane peaceably, but that would not be the case for other apes. If you tried to pack chimpanzees together like that, chaos would erupt as the apes started fighting each other. Humans show reduced androgen reactivity compared to chimpanzees, which makes us less prone to physical conflict. Reduced aggressiveness is one of the many traits that humans share with domesticated animals. Indeed, one theory has it that humans are apes who domesticated each other.

Along with lower aggression comes increased social motivation. Humans are more likely to seek out social contact and find it rewarding than apes. Humans crave social approval and from infancy adopt behaviors that yield social rewards like nods, smiles, warm touches, and kind words. We also enjoy provoking responses in other people through our own behavior. Heyes suggests that the oxytocin system may be responsible for our strong social motivation.

Humans are also different from chimpanzees in the amount of attention they pay to faces and voices. Newborns look at images that resemble faces longer than at images that do not, and prefer the vocalizations of humans and other apes to synthetic sounds.

Cognitively, humans show enhanced associative learning and executive function relative to apes. While associative learning is an ancient ability that emerged hundreds of millions of years ago, humans can learn more quickly and store more associations than other animals. This hypothesis is difficult to test and elaborate, but one study showed that baboons can learn four times as many associations as pigeons, and, because of our brain size, humans are likely to be much more abundant learners than baboons. Associative learning is an important factor in intelligence, subtlety of social cognition, and precision in categorization.

Finally, humans differ from other apes in their executive functions. Psychologist Adele Diamond defines executive functions as:

a family of top-down mental processes needed when you have to concentrate and pay attention, when going on automatic or relying on instinct or intuition would be ill-advised, insufficient, or impossible…. Using [executive functions] is effortful; it is easier to continue doing what you have been doing than to change, it is easier to give into temptation than to resist it, and it is easier to go on “automatic pilot” than to consider what to do next.

According to the most popular model among cognitive scientists, there are three executive functions: inhibition, working memory, and cognitive flexibility. Inhibition is the ability to override automatic responses when we judge them likely to be ineffective. Inhibition enables us to suppress awareness of some stimuli so that we can focus on others, as we do when we attend to one person’s voice at a noisy party. Inhibition also is what allows us to control our behavior and stay on a task that we choose so that we don’t give into easy temptations.

Working memory is our ability to hold information in our minds and work on it. Working memory is essential to doing math in our heads. Imagine multiplying 73x58—this requires you to store the product of 8x3, then add it to the product of 8x70, and so forth. Working memory is also essential to understanding any process that unfolds over time: to understand the plot of a movie, you have to store what happened previously.

Cognitive flexibility is the executive function that enables people to switch between different mental tasks or between different parameters for solving the same mental task. Cognitive flexibility permits us to visualize an object from a different perspective, to appreciate how someone else sees a situation, to come up with a new strategy for tackling a difficult intellectual problem, or to abandon one pursuit when an unexpected opportunity to do something else important comes along.

The executive functions are highly dependent on another. To switch to a new method of tackling a task, you must inhibit your prior method. To perform complex mental tasks that require storing a lot of information in working memory, one must focus one’s attention on it, which means inhibiting distracting factors.

While other apes also possess executive functions, there is good reason to believe that humans’ are significantly more powerful. For example, a review of evidence on working memory finds that, whereas humans are typically capable of holding seven pieces of information in their heads, for apes it is only two or three. Chimpanzee working memory performance is comparable to that of a five-year-old child.

In essence, Heyes believes that you get human cognition by making some minor tweaks to an ape brain—reduced aggressiveness, preference for faces, and so forth—then hooking that modified brain up to a supercomputer, and letting cultural evolution do its work.

Why intelligence matters

According to Heyes, enhancements in associative learning and executive function were essential to the evolution of cultural beings. From the account of imitative learning, it is clear why she believes this. We learn to imitate by forming associations between internal states and external movements, and such tasks require executive functions to carry out. The role of executive functions is particularly obvious in theory of mind cognition. Our interpretations of the mental states of others often require us to integrate multiple perspectives: “A believes that B believes that C believes that X is true, but C does not believe X is true, and B knows that C does not believe that, so A is mistaken in two ways.” Such chains of reasoning strain the capacity of our working memory, as they require us to store and reason about multiple pieces of information, but we can understand this scenario if we make an effort. A chimpanzee would not have the mental capacity to understand this scenario.

Aptitudes for associative learning and executive functions are also essential components of intelligence, and IQ scores are correlated with both. Although she never raises the point explicitly, Heyes assigns far greater importance to intelligence in the evolution of human cognition than the nativists do. For example, the nativist evolutionary anthropologist Joseph Henrich devotes a whole chapter of his book on the evolution of cognition, The Secret of our Success, to diminishing the role of intelligence in the evolution of cognition. He even cites a study that found that chimpanzee working memory is comparable to that of humans. However, the broader evidence has found that human working memory is much more powerful than that of chimpanzees.

The reason for the difference between Heyes and the nativists on this point stems from the difference in their broader approaches to human cognition. The nativists believe that cognitive instincts are domain-specific in that they are specialized for solving particular cognitive problems. Intelligence, however, is a domain-general ability that could be compared to the processing power of a computer. Just as a computer with a relatively powerful processor will, all else being equal, run all software programs faster than a less powerful computer, so a more intelligent animal will tend perform all cognitive tasks more effectively than a less intelligent one. On the cognitive gadget theory, domain-general intelligence is crucial to amplifying the capacity for learning that, on Heyes’ view, sets humans apart.

The cognitive gadget theory is attractive because it provides a more detailed account of the workings of cognition than the nativist theory and because it fits the experimental findings on cognition better. Another strong point of Heyes’ theory is that it better accounts for human behavioral flexibility. Humans left Africa 80,000 years ago and colonized most of the earth, which required learning rapidly how to adapt to radically different environments. This adaptation required the kind of flexibility that intelligent learning provides. As Heyes says, “Distinctively human cognitive mechanisms need to be nimble, capable of changing faster than genetic evolution allows, because their job is to track specific, labile features of the environment” (p. 208). It is not clear that an organism that was heavily dependent on innate specializations would have had the flexibility required to adapt so effectively to different environments.

The question of what makes humans unique is far from settled. Even if Heyes turns out to be wrong, hopefully her sharp critique will spur nativists to refine and enhance their theories of cognition.

1. Joseph Henrich, The Secret of our Success: How Culture Is Driving Human Evolution, Domesticating Our Species, and Making Us Smarter (Princeton: Princeton UP), pp. 22-27.