When you run your hands through your lover’s hair, you’re probably not thinking about your place in the social hierarchy. Give your team-mate or colleague a pat on the back after a setback, and the chances are you’re not consciously seeking to change the mix of signalling chemicals in their brain. It may not seem like it, but these socially important rituals and others like them predate the time our species first walked the African savannah.
Human behaviours that involve physical social contact have a lot more in common with social grooming activities we typically associate with other species than we might initially think. When rhesus monkeys or chimpanzees pick through their friends' fur, they're not just helping them remove dirt and parasites from hard to reach spots. There is undoubtedly a hygienic benefit, but this behaviour, which animal behaviour researchers call “allogrooming”, has far greater significance. The gelada baboon, for example, spends 17% of its waking hours doing this when just 1% would be sufficient to achieve good hygiene, according to one estimate. Allogrooming is the currency of what primatologist Frans de Waal calls the "marketplace of services" in chimpanzee life: it defines the social hierarchy, which in turn dictates access to food, sex, and social support.
For example, one chimpanzee is more likely to share food with another that has previously groomed it. Grooming also serves to ease tensions in a chimp troop following an aggressive situation. One of the most complex forms of reconciliation among chimpanzees occurs when two rival males reach a point of stalemate, neither backing down nor escalating the aggressive interaction. Sometimes, a female breaks the deadlock and eases the tension by grooming first one male, and then the other, until the two become relaxed enough to end what amounted to an angry staring contest.
According to anthropologist Robin Dunbar, this works because grooming stimulates the release of endorphins – opiates produced by the brain that trigger feelings of relaxation by lowering the heart rate, reducing overt nervous behaviours like scratching, and even bringing on sleep. Female chimps that use grooming as a peacekeeping strategy may also experience their own rush of endorphins and enjoy many of the same benefits.
Humans, lacking the fur of our more hirsute evolutionary cousins, had to find a replacement for allogrooming. Like grooming, gossip establishes and maintains our place in the social hierarchy. Also like grooming, the social information that makes up gossip is itself a form of currency in human culture. Or, at least, that's the theory put forward by Dunbar. He argued, in his book Grooming, Gossip and the Evolution of Language, that the faculty of language allowed our species to substitute gossip for grooming.
But grooming, and related forms of social physical contact, hasn't gone away entirely. While we humans don't make a habit of picking through our friends' hair for parasites – nurses searching for lice on the first day of summer camp notwithstanding – the truth is that gossip hasn't completely replaced our need for physical touch. Indeed, words don't always make adequate tools for communicating our feelings. Far more can be said by a heartfelt hug or squeeze on the shoulder after a friend suffers the loss of a relative than through words.
In the same way, one's love and desire for a partner can be conveyed with a seductive stroke far more effectively than even the words "I want you" ever could. Indeed, Dunbar writes, "the physical stimulation of touch tells us more about the inner feelings of the 'groomer', and in a more direct way" than words are able. And those forms of touch stimulate within us the same endorphin release that chimpanzees enjoy during social grooming.