Picture two female chimpanzees hanging out under a tree. One grooms the other, systematically working long fingers through fur, picking out bugs and bits of leaves. The recipient sprawls sleepily on the ground, looking as relaxed as someone enjoying a spa day. A subsequent surreptitious measurement of her levels of oxytocin, a hormone associated with bonding and pleasure, would confirm that she is pretty happy.
And why not? Grooming appears to be a pleasurable way to spend time. Many species of apes and monkeys devote long chunks of the day to it. Among other things, grooming can curry favor and strengthen alliances so it is likely that of these two chimps, the female being primped is of equal or greater rank in the troop than the one doing the work.
There is another level of social complexity to this scene that researchers have only recently discovered. If any old troop mate is doing the grooming, hormone levels do not change much. But if it is an individual with whom the recipient has a close bond—including but not limited to kin—oxytocin levels will rise considerably. What matters most, in other words, is whether the chimpanzees are friends.
To most of us, the pleasures of friendship are familiar. Like this pair of chimps, we are more likely to relax and enjoy ourselves at dinner with people we know well than with people we have just met. Philosophers have celebrated the joys of social connection since the time of Plato, who wrote a dialogue on the subject, and there has been evidence for decades that social relationships are good for us. But it is only recently that friendship is getting serious scientific respect. Researchers from disciplines as diverse as neurobiology, economics and animal behavior are recognizing parallels between the interactions of animals and the habits of people at dinner parties and are asking far more rigorous questions about the motivations behind social behavior.
The early answers, though preliminary, are spurring a reappraisal of the importance of friendship as a biological and societal force. First, there is the apparent universality of friendship. “As we think more deeply about what friendships are, we’re starting to find them in other species,” says Lauren Brent, a behavioral ecologist at the University of Exeter, who studies sociality in rhesus macaques and killer whales. “That means there’s a story there that goes beyond humans and human society.”
There also appears to be a genetic basis to both our instincts toward sociability and our actual relationships that goes beyond family. And there is strong evidence that the absence of friendship can be toxic for our health whereas those with tighter social bonds live longer and enjoy more reproductive success. All of this means friendship has evolutionary origins, says Robert Seyfarth, a psychologist at the University of Pennsylvania who studies social relationships in baboons. “It suggests a basic propensity for the need for sociality in mammals.” Friendship then is not a luxury, it is an infrastructural necessity.
Just what constitutes friendship? If you think we are friends, but I think we are acquaintances, which are we? The variety of possible answers is one reason friendship went unexamined for so long. Scientists gravitated to the study of individuals because it meant fewer statistical headaches and more available data. Furthermore, if one is interested in evolution, it is also true that natural selection occurs when conditions favor individuals carrying particular traits. The beak size of Darwin’s finches, to take a famous example, changed bird by bird. It is harder to develop an evolutionary argument about connections between people, which are so much less tangible.
When researchers did look at bonds between pairs, or dyads in scientific terms, they studied mates or relatives such as mothers and infants. To consider relationships between individuals who are not related and do not have sex requires agreement on how to measure the properties of those bonds. The current working definition of friendship—a persistent positive relationship that involves cooperation over time—developed only recently and is based on the quality and patterning of interactions.
Most critically, friendship is sustained. You might have a pleasant interaction with someone on the subway but would not call that person your friend. But the neighbor with whom you regularly exercise and occasionally dine? That’s a friend.
Although researchers cannot ask a monkey to name his closest friends, they can observe, in natural environments, how and with whom he spends his time. By following individual animals closely over years and painstakingly recording every instance of vocalizing, grooming, cooperative foraging, and so on, behavioral ecologists have amassed volumes of data on social activity in certain animal populations.
In people, researchers prod subjects to list names of friends and identify social relationships. The top two “name generating” questions concern free time and discussing important matters: Who do you invite to the movies? And who do you call when you are sick, breaking up and changing jobs? There may well be more than one person on the list, and the names may change over time, but a 2014 study of phone calls made by college students over a year and a half showed that the number of close friends you have remains surprisingly constant. The researchers, at the Aalto University School of Science in Espoo, Finland, monitored 24 students as they transitioned from high school to college, a period when these young men and women met many new people. They found that specific friendships changed during this period, but at any given time, most individuals still leaned on roughly the same number of core companions—and the specific number was unique to each person.
Your entire social circle is relevant to the new friendship research. In his early career as a physician, Nicholas Christakis, now a sociologist at Yale University, became interested in the way one person’s illness might take a toll on another, especially a spouse. That led to the realization that pairs of people connect to other pairs, as he puts it, “to form huge webs of ties stretching far into the distance.”
Christakis joined forces with James Fowler, a political scientist at the University of California, San Diego, (both were then at Harvard University) to study social networks of 3,000, 30,000 or more people. Using sophisticated computational techniques, they and others have established measures of connectedness that allow sophisticated mapping of these bonds. For example, they count how many friends I would name (“outdegree”) and how many friends name me (“indegree”) separately—thereby dealing with any mismatch in our perceptions of how close we really are. Their 2009 book, Connected: How Your Friends’ Friends’ Friends Affect Everything You Feel, Think, and Do, made the case that social connections up to three degrees of separation have a significant influence on such things as our weight as well as smoking, altruism, and voting behaviors.
The new way of thinking about friendship also blurs the long-standing distinction between friends and family by theorizing that the quality of a bond might be more significant than its origin. “The relationship with your spouse can be positive and supportive or it can be the most toxic that you have in your life,” says John Cacioppo, a psychologist at the University of Chicago who pioneered the study of the neuroscience of loneliness. By the logic, relatives and sexual mates can be considered friends, but only if the bond is rewarding. According to this view family might so often come first in part out of convenience. “Friendship is just a word for a persistent long-term social bond,” says Seyfarth. “Kinship provides an easy start to these bonds.”
Research in animals has been important in establishing the idea that a strong social bond—all by itself—may have evolutionary significance. Evolutionary theories are hard to prove. Animal research has the advantage of reducing the number of variables—such as the number and form of possible interactions—and most species have shorter lifespans than humans, making measuring generational change a simpler proposition. That can make it easier to tease out cause from correlation. In addition, findings that echo across species suggest biological rather than cultural origins.
To date, horses, elephants, hyenas, monkeys, chimpanzees, whales and dolphins have all been shown to form social bonds that can last for years. Studies of our closest living relatives—monkeys and apes—have been especially ground-breaking. Robert Seyfarth and Dorothy Cheney, a biologist at the University of Pennsylvania, have studied the same troop of baboons in Kenya for more than thirty years. When they began, primatologist Robert Hinde had already established that nonhuman primates had notable social relationships. One of the first things Seyfarth and Cheney did was use audio playback experiments to show that baboons were aware of the relationships of others. When a group of female monkeys heard an offspring’s distress vocalization, they often looked at the infant’s mother. “That suggests that the social relationships were not just a figment of our human imagination,” says Seyfarth.
Eventually it became evident, in two separate long-term studies of baboons, one led by Seyfarth and Cheney, the other by primatologist Jeanne Altmann of Princeton University, that those social relationships, carefully recorded over time, made a big difference in lifetime reproductive success. In 2003, Altmann and her colleague Joan Silk, together with Susan Alberts of Duke University, published a seminal paper in Science that was the first to explicitly link friendship of adult females to the proportion of their infants that survive to one year. In 2009 and 2010, Seyfarth, Cheney, Silk and their colleagues presented similar data. They also showed that baboons with stable friendships have lower stress, and that female baboons work to form new friendships when a close friend is killed by predators—an important piece of evidence in favor of the social bond’s overarching importance.
The striking and convergent results from the two studies surprised the researchers, who had expected dominance rank to confer the most advantage. It was not that rank was unimportant, but the critical factor was a close set of social bonds. “Primates have these long-term relationships,” says Seyfarth. “They are aware of the relationships in others, and these relationships have a direct impact on reproductive success.”
The Social Genome
A related evolutionary idea about humans has also generated interest. The social brain hypothesis, put forward by evolutionary psychologist Robin Dunbar of the University of Oxford and others, argues that the need for early humans to live in ever-larger social groups led to the enlargement of the human brain. Navigating the complexities of social life after all requires social attention, the ability to take others’ perspectives, the ability to communicate, and ultimately to cooperate.
The idea is rooted in the earlier observation that monkeys and apes had much bigger brains relative to body size than other animals and that this was probably due to their social lives. Archeological and fossil evidence to bolster the theory includes changes, though slight, in brain size between Neanderthals and modern humans, at the same time that social groups expanded both in size and, especially, in complexity. A corollary known as Dunbar’s number holds that no matter what your Facebook page says, each of us can only maintain a wider social circle of about 150 people. It turns out many forms of social organization from military brigades to average holiday card lists hover around that number.
If evolution is steering various species including our own toward pro-social behavior, it makes sense to seek evidence in the genome. Already, genetic variation has been identified in people with disorders that include social dysfunction like autism spectrum disorder and schizophrenia. And some genes in the dopamine and serotonin pathways have been consistently linked with social traits. “Genetics started with an understanding of how genes affect the structure and function of our bodies, and then our minds,” says Christakis. “And now people like us are beginning to ask how genes affect the structure and function of our societies.”
Over the last five years, Christakis, Fowler and their collaborators have published a series of papers on both cooperation and the possible genetics of friendship. The first examined data on 1,110 twins included in the National Longitudinal Study of Adolescent Health, in which participants were periodically asked to name friends. Christakis and Fowler’s team found that genetic factors account for nearly half of the variation both in how many times a person was named as a friend, and more surprisingly, in the probability that a person’s friends are friends with each other, a property known as transitivity. “That’s a bizarre result,” says Christakis. “If you have Tom, Dick and Harry in a room, whether Dick is friends with Harry depends not only on Dick’s genes or on Harry’s genes, but on Tom’s genes. How can that be? We think the reason is that people vary in their tendency to introduce their friends to each other. Some knit the networks around them together and some people keep their friends apart.”
A person’s social position, in terms of how central they are in their network, was also significantly heritable. By their analysis, 29 percent of the differences in a person’s likelihood to have a particular social role could be explained by genetics as opposed to environment.
In 2011, Christakis and Fowler used six available genotypes from the same database (excluding relatives this time) to test for genetic similarity among friends. They found that the old adage about “birds of a feather” was genetically based: friends did not just have similar traits; they resembled each other on a genotypic level beyond what you would expect from systematic genetic differences that might occur due to shared ancestry such as being European or Asian. They expanded on this work in a 2014 paper, on friendship and natural selection, and showed that a degree of correlation in genotypes that made friends the equivalent of fourth cousins. And they replicated the results with a second large database, the Framingham Heart Study. “Friends may be a kind of ‘functional kin,’” they surmised.
As part of this work, in a 2012 paper in Nature, they even mapped the social network of the Hadza hunter-gatherers of Tanzania, who live essentially as humans did 10,000 years ago. They showed that the Hadza form networks with a mathematical structure just like humans living in modernized settings, suggesting something very fundamental about the structure of friendship.
Lauren Brent was the first to apply Christakis and Fowler’s social network analysis to monkeys. Together with neurobiologist Michael Platt, formerly at Duke University and now at the University of Pennsylvania, she works with a colony of rhesus macaques on Cayo Santiago, an island off the coast of Puerto Rico, for whom there are extended genetic records. Their 2013 study found that the most sociable monkeys, those with the largest, strongest networks, tended to be descendants of similarly social macaques. More social monkeys also had greater reproductive success, meaning their babies were more likely to survive their first year. In a 2015 paper, they showed that social vigilance, the ability to observe and gather social information, had a heritability of 12 percent.
In her newest work, Brent is now exploring whether indirect connections—friends of friends—are as significant for animals as they are for humans. All of these studies are based on relatively small samples (dictated by the number of available animals), so they lack the power of Christakis and Fowler’s work, which used extremely large databases. “It remains to be seen how pervasive this is,” says Seyfarth, but he calls the results striking. “This is an exciting time for people interested in trying to map [social behavior] onto genetics.”
If friendship is so important, the next aim is to understand why by teasing out what exactly social bonds do for us. Our pair of grooming chimpanzees were very much like a real duo studied by primatologists Catherine Crockford and Roman Wittig of the Max Planck Institute for Evolutionary Anthropology in Liepzig. In their work, Crockford and Wittig found that the closeness of a pair’s bond would determine the amount of oxytocin circulating in a primate’s blood. The finding might represent an important first step, says Seyfarth. “This is starting to say that there’s something about interacting with individuals that you perceive as close friends that’s physiologically very rewarding.”
There are also clues in human physiological responses to social interaction. Several large longitudinal studies have shown that the strength of our social network can predict mortality to such a degree that strong ties may be as beneficial to our health as quitting smoking and more impactful than well-known risk factors such as obesity and physical inactivity. Studies of loneliness make it clear that a weak social network can be detrimental to our well-being.
If the new science of friendship can paint a clearer picture of how and why we make friends, researchers hope to use that information in a variety of ways. In an ambitious randomized trial involving 30,000 people in 160 villages in Honduras, Christakis and Fowler are exploring whether targeting influential individuals, identified through social network analysis, can be used to change health habits and reduce childhood mortality. On Cayo Santiago, Michael Platt and Lauren Brent hope to be able to establish normal variation in social behavior among macaques as a way of then studying behavior that falls outside that range. “One of the first things that seems to fall apart in autism is attention to others,” says Platt.
But of course the most straightforward result of this work would be to spark a deeper appreciation of just how important friends are in our lives. “Other individuals are in fact the source of some of our greatest joys,” says Cacioppo. And now we know that they do not just make us happy, they help keep us alive.