As social animals, we depend on others for survival. Our communities provide mutual aid and protection, helping humanity to endure and thrive.
“We have survived as a species not because we’re fast or strong or have natural weapons in our fingertips, but because of social protection,” said John Cacioppo, the director of the Center for Cognitive and Social Neuroscience at the University of Chicago. Early humans, for example, could take down large mammals only by hunting in groups. “Our strength is our ability to communicate and work together,” he said.
But how did these powerful communities come to exist in the first place? Cacioppo proposes that the root of social ties lies in their opposite — loneliness. According to his theory, the pain of being alone motivates us to seek the safety of companionship, which in turn benefits the species by encouraging group cooperation and protection. Loneliness persists because it provides an essential evolutionary benefit for social animals. Like thirst, hunger or pain, loneliness is an aversive state that animals seek to resolve, improving their long-term survival.
If Cacioppo’s theory is correct, then there must be an intrinsic biological mechanism that compels isolated animals to seek out companionship. Something in our brains must make it feel bad to be alone and bring relief when we’re with others.
Researchers at the Massachusetts Institute of Technology think they’ve found the source of that motivation in a group of little-studied neurons in part of the brain called the dorsal raphe nucleus. Stimulating these neurons drives isolated mice to find friends, according to research published earlier this year in the journal Cell. The finding provides critical support to Cacioppo’s theory and illuminates a deep connection that links specific structures in the brain to social behavior.
The new study — the first to link specific neurons to loneliness — is part of a growing effort to map out the genetics of social behavior and its underpinnings in the brain. “Over the last roughly 15 years, there has been a tremendous increase in the desire to understand the basis of social behavior, including caring for others, social rejection, bullying, deceit and so forth,” said Patricia Churchland, a philosopher at the University of California, San Diego, who studies the brain and social behavior. “I think we have a good idea for the evolutionary basis for caring and sharing and mutual defense, but the brain mechanisms are bound to be very complex.”
Together, Cacioppo’s work and the new findings from MIT are helping to move loneliness from the realm of psychology and literature to biology. “I think the bigger picture is not to understand why loneliness is painful but rather how our brain is set up to move us out of that lonely state,” said Steve Cole, a genomics researcher at the University of California, Los Angeles. “Instead of thinking about loneliness, we could think about social affinity.”
Gillian Matthews stumbled across the loneliness neurons by accident. In 2012 she was a graduate student at Imperial College London who had been studying how cocaine changes the brain in mice. She would give the animals a dose of the drug, place each one alone in a cage, and then examine a specific set of its neurons the next day. She did the same for a control group of mice, injecting them with saline instead of cocaine.
When Matthews returned to her mice 24 hours after dosing them, she expected to see changes in their brain cells, a strengthening of neuronal connections that might help explain why cocaine is so addictive.
To her surprise, both the drug-treated mice and the control mice showed the same changes in neuronal wiring. Overnight, the neural connections onto a certain set of cells had grown stronger, regardless of whether the animals were given drugs or not. “We first thought there was something wrong, that we had mixed up our procedure,” said Matthews, who is now a postdoctoral researcher at MIT.
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