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skewer. But although a rabbit just can’t take a joke, the brain regions responsible for a rabbit’s unconscious processing are not that different from ours.

In fact, the organization and chemistry of the unconscious brain is shared across mammal species, and many automatic neural mechanisms in apes and monkeys and even lower mammals are similar to our own, and produce startlingly humanlike behavior.²⁰ So although other animals can’t teach us much about ToM, they they can provide insights into some of the other automatic and unconscious aspects of our social tendencies. That’s why, while other people read books like Men Are from Mars, Women Are from Venus to learn about male and female social roles, I turn to sources like “Mother-Infant Bonding and the Evolution of Mammalian Social Relationships”—which, some say, serves to minimize the mammalian social relationships in my own life.

Consider this quote from that work:

Reproductive success in males is generally determined by competing with other males to mate with as many females as possible. Hence, males rarely form strong social bonds and male coalitions are typically hierarchical with an emphasis on aggressive rather than affiliative behavior.²¹

That sounds like something you’d observe hanging out at a sports bar, but scientists are discussing the behavior of nonhuman mammals. Perhaps the difference between human males and bulls, tomcats, and male sheep is not that nonhuman mammals don’t have sports bars but that, to nonhuman mammals, the whole world is a sports bar. Of females, those same researchers write:

The female reproductive strategy is one of investing in the production of a relatively few offspring … and success is determined by the quality of care and the ability to enable infant survival beyond the weaning age. Females therefore form strong social bonds with their infants and female-female relationships are also strongly affiliative.

That, too, sounds familiar. One has to be careful about reading too much into mammalian behavior “in general,” but this does seem to explain why it is mostly women who have slumber parties and form book clubs, and why, despite my promises to be affiliative rather than aggressive, they have never let me into either. The fact that on some level human and nonhuman mammals seem to behave similarly does not mean that a cow would enjoy a candlelight dinner, that a mother sheep wants nothing more than to see her babies grow up happy and well- adjusted, or that rodents aspire to retiring in Tuscany with their soul mates. What it does suggest is that although human social behavior is far more complex than that of other animals, the evolutionary roots of our behaviors can be found in those animals, and we can learn something about ourselves by studying them.

Just how programmed is the social behavior of nonhuman mammals? Take sheep, for example.²² A female sheep—a ewe—is by disposition rather nasty to baby sheep (or, as the meat industry likes us to call them, lambs). If a lamb approaches, wishing to suckle, the ewe will scream at it with a high-pitched bleat, and maybe throw in a head butt or two. However, the birthing process transforms the mother. It seems magical, that transformation from shrew to nurturer. But it doesn’t seem to be due to conscious, maternal thoughts of her child’s love. It’s chemical, not magical. The process is instigated by the stretching of the birth canal, which causes a simple protein called oxytocin to be released in the ewe’s brain. This opens a window of a couple hours’ duration in which the ewe is open to bonding. If a lamb approaches her while that window is open, the ewe will bond with it, whether it is her baby, her neighbor’s, or a baby from the farm down the street. Then, once the oxytocin window has closed, she’ll stop bonding with new lambs. After that, if she has bonded with a lamb, she’ll continue to suckle it and to speak soothingly to it—which in sheep talk means low-pitched bleats. But she’ll be her nasty old self to all other lambs, even to her own if it didn’t approach her during the bonding window. Scientists, however, can open and close this bonding window at will, by injecting the ewe with oxytocin or inhibiting her from producing it herself. It’s like flicking a switch on a robot.

Another famous series of studies in which scientists have been able to program mammalian behavior by chemical manipulation concerns the vole, a small rodent that resembles a mouse and encompasses about 150 different species. One of those species, the prairie vole, would be a model citizen in human society. Prairie voles mate for life. They are loyal—among prairie voles whose partner disappears, for example, fewer than 30 percent will shack up with someone else.²³ And they make responsible fathers—the males stick around to guard the nest and share in the parenting. Scientists study prairie voles because they are a fascinating contrast with two related species of voles, the montane vole and the meadow vole. In contrast to prairie voles, montane and meadow voles form societies of sexually promiscuous loners.²⁴ The males of those species are, in human terms, ne’er-do-wells. They will mate with whatever female is around, then wander off and leave her to take care of the kids. If placed randomly in a large room, they avoid others of their species, preferring to crawl off to some isolated corner. (Prairie voles, on the other hand, will cluster in little chat groups.)

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