Smile and the world smiles with you, but why?

“We are connected in ways we don’t consciously know, but which are absolutely essential for communication,” said psychologist and author Daniel Goleman at a March 14 talk on social intelligence sponsored by the John F. Kennedy School of Government’s Center for Public Leadership. “There is a subterranean emotional economy that’s part of any interaction.”

To illustrate his point, he described a scenario involving three 11-year-old boys in a New Haven, Conn., public school who are heading off to a soccer match. Two of the boys are accomplished athletes; the third is pudgy and not very coordinated. One of the jocks starts to taunt the pudgy kid, telling him that he is about to be embarrassed on the playing field. The tension mounts. But instead of responding to the jock’s put-downs with anger, the pudgy kid employs a “put-up strategy.” Laughing at his own lack of coordination, he compliments the jock’s athletic prowess. “Would you teach me how to dribble better?” he asks. The pudgy kid’s split-second calculations constitute a social alchemy, transforming a fistfight into an occasion of emotional bonding.

Experientially, we’ve always known how positive and negative emotions alike can rapidly spread through a group. But thanks to the discoveries that have taken place in the field of social neuroscience over the past decade, “we now realize why emotions are contagious,” said Goleman. “We understand the mechanisms” that enable emotions to spread.

From one-person to two-person psychology

Goleman’s 1995 bestseller “Emotional Intelligence” examines competencies within us as individuals — for example, the attributes of self-awareness and self-management — which enable us to read our own emotions, accurately assess our personalities, keep disruptive emotions under control, and be trustworthy, flexible, and optimistic. Weaving together a number of independent strands of research, including work that revealed how emotions are regulated by the brain, “Emotional Intelligence” makes the case that, although cognitive ability as measured by IQ is a strong predictor of which jobs or professions people may enter, emotional intelligence is what sets apart the star performers from the merely average in a given job or profession.

Since that book’s publication, many of the most interesting advances in neuroscience have had to do with two-person psychology. We now know that the brain is structurally designed for sociability: Whenever we interact with another person — whether in the boardroom or the bedroom, the classroom, the marketplace or the playground — a brain-to-brain connection, or neural bridge, is formed.

“A previously unknown class of neurons — mirror neurons — acts like a neural Wi-Fi system, monitoring everything the other person is saying and doing,” explained Goleman, who received his Ph.D. in psychology from Harvard. These neurons serve a synchronizing function, activating the same part of our brain that is being activated in the speaker’s brain. This is what keeps a conversation on track. For example, said Goleman, “In the buildup to a first kiss, these synchronizer cells are what tell you, ‘Now.’” But if you miss that signal, he continued, “and ask, ‘Is now OK?’ — you’ve blown it.”

A scientific illustration of the neural bridge that is established when two people relate can be found in the work of Harvard Medical School psychiatrist Carl Marci, who studied the psychotherapist-client interaction by measuring each person’s sweat response (a proxy measure for the person’s emotional responses) throughout a conversation. The videotape of the session depicts each person’s reading as a line that undulates in response to rising and falling emotions.

When patient and therapist aren’t connecting, “the two lines move like jittery birds, their ups and downs on private trajectories,” wrote Goleman in his 2006 book, “Social Intelligence: The New Science of Human Relationships.” But when the patient and therapist are in sync, “the lines fly like birds in formation, a graceful ballet of coordinated movement. When two people feel rapport, the gliding lines reveal, their very physiology attunes.”

Loren Gary is the associate director of Leadership Development and Public Affairs for the Center for Public Leadership at the John F. Kennedy School of Government.