What is amazing about these creatures is that scientists have been able to identify the specific brain characteristic responsible for the behavioral differences among vole species, and to use that knowledge to change their behavior from that of one species to that of another. The chemical involved is again oxytocin. To have an effect on brain cells, oxytocin molecules first have to bind to receptors—specific molecules on the surface membrane of a cell. Monogamous prairie voles have many receptors for oxytocin and a related hormone called vasopressin in a particular region of the brain. A similarly high concentration of oxytocin and vasopressin receptors is found in that region of the brain in other monogamous mammals. But in promiscuous voles, there is a dearth of those receptors. And so, for example, when scientists manipulate a meadow vole’s brain to increase the number of receptors, the loner meadow vole suddenly becomes outgoing and sociable like its cousin the prairie vole.25
Unless you’re an exterminator, I’ve probably now supplied more than you need to know about prairie voles, and as for lambs, most of us never come into contact with them except those accompanied by mint jelly. But I’ve gone into detail about oxytocin and vasopressin because they play an important role in the modulation of social and reproductive behavior in mammals, including ourselves. In fact, related compounds have played a role in organisms for at least seven hundred million years, and are at work even in invertebrates such as worms and insects.26 Human social behavior is obviously more advanced and more nuanced than that of voles and sheep. Unlike them, we have ToM, and we are far more capable of overruling unconscious impulses through conscious decisions. But in humans, too, oxytocin and vasopressin regulate bonding.27 In human mothers, as in ewes, oxytocin is released during labor and delivery. It is also released in a woman when her nipples or cervix are stimulated during sexual intimacy and in both men and women when they reach sexual climax. And in both men and women, the oxytocin and vasopressin that are released into the brain after sex promote attraction and love. Oxytocin is even released during hugs, especially in women, which is why mere casual physical touch can lead to feelings of emotional closeness even in the absence of a conscious, intellectual connection between the participants.
In the broader social environment, oxytocin also promotes trust, and is produced when people have positive social contact with others.28 In one experiment, two strangers played a game in which they could cooperate to earn money. But the game was designed so that each contestant could also gain at the expense of the other. As a result, trust was an issue, and as the game progressed the players gauged each other’s character. Each assessed whether his or her partner tended to play fairly, so both players could benefit equally, or selfishly, to reap a greater benefit at his or her expense.
The unique aspect of this study was that the researchers monitored the players’ oxytocin levels by taking blood samples after they made their decisions. They found that when a player’s partner played in a manner that indicated trust, the player’s brain responded to that show of trust by releasing oxytocin. In another study, in which subjects played an investment game, investors who inhaled an oxytocin nose spray were much more likely to show trust in their partners, by investing more money with them. And when asked to categorize faces based on their expression, volunteers who were given oxytocin rated strangers as appearing more trustworthy and attractive than did other subjects not administered the drug. (Not surprisingly, oxytocin sprays are now available over the Internet, though they are not very effective unless the oxytocin is sprayed directly into the target person’s nostril.)
One of the most striking pieces of evidence of our automatic animal nature can be seen in a gene that governs vasopressin receptors in human brains. Scientists discovered that men who have two copies of a certain form of this gene have fewer vasopressin receptors, which makes them analogous to promiscuous voles. And, indeed, they exhibit the same sort of behavior: men with fewer vasopressin receptors are twice as likely to have experienced marital problems or the threat of divorce and half as likely to be married as men who have more vasopressin receptors.29 So although we are much more complex in our behaviors than sheep and voles, people, too, are hardwired to certain unconscious social behaviors, a remnant of our animal past.
SOCIAL NEUROSCIENCE IS a new field, but the debate over the origin and nature of human social behavior is probably as old as human civilization itself. Philosophers of centuries past didn’t have access to studies like those of the lambs and voles; however, as long as they have speculated about the mind, they have debated the degree to which we are in conscious control of our lives.30 They used different conceptual frameworks, but observers of human behavior from Plato to Kant usually found it necessary to distinguish between direct causes of behavior—those motivations we can be in touch with through introspection—and hidden internal influences that could only be inferred.
In modern times, as I mentioned, it was Freud who popularized the unconscious. But though his theories had great prominence in clinical applications and popular culture, Freud influenced books and films more than he influenced experimental research in psychology. Through most of the twentieth century, empirical psychologists simply neglected the unconscious mind.31 Odd as it may sound today, in the first half of that century, which was dominated by those in the behaviorist movement, psychologists even sought to do away with the concept of mind altogether. They not only likened the behavior of humans to that of animals, they considered both humans and animals to be merely complex machines that responded to stimuli in predictable ways. However, though the introspection elicited by Freud and his followers is unreliable, and the inner workings of the brain were, at the time, unobservable, the idea of completely disregarding the human mind and its thought processes struck many as absurd. By the end of the 1950s the behaviorist movement had faded, and two new movements grew in its place, and flourished. One was cognitive psychology, inspired by the computer revolution. Like behaviorism, cognitive psychology generally rejected introspection. But cognitive psychology did embrace the idea that we have internal mental states such as beliefs. It treated people as information systems that process those mental states much in the way a computer processes data. The other movement was social psychology, which aimed to understand how people’s mental states are affected by the presence of others.
With these movements, psychology once again embraced the study of the mind, but both movements remained dubious about the mysterious unconscious. After all, if people are unaware of subliminal processes, and if one cannot trace them within the brain, what evidence do we have that such mental states are even real? In both cognitive and social psychology, the term “unconscious” was thus usually avoided. Still, like the therapist who doggedly brings you back again and again to the subject of your father, a handful of scientists kept doing experiments whose outcomes suggested that such processes had to be investigated, because they played such an important role in social interactions. By the 1980s, a number of now-classic experiments offered powerful evidence of the unconscious, automatic components of social behavior.