Let’s say you’re redecorating your home, and, with a bit of Wes Anderson whimsy, you install two pendulum clocks on your wall. In what’s been a mystery since 1655, they will eventually synchronize, and start swinging through the seconds in perfect unison. Physics is just starting to understand why. Even weirder: People do it, too.
Richard Palumbo, senior research scientist at Northeastern University’s Computational Behavioral Science Lab, gave me the pendulum example when I asked him about interpersonal synchonry, how psychology describes the oddly magical, yet deeply normal, tendency for humans to fall into rhythm with one another, from their gestures down to their heart rate. You don’t even have to speak, evidently: In a recent paper, Palumbo and colleagues found that if you sit a couple face-to-face and ask them not to talk, just staring at each other for 15 minutes is enough to get their levels of skin conductance to sync up, three times more than if they were sitting back to back. (Skin conductance — when the skin literally becomes a better conductor of electricity, thanks to the activity of sweat glands — is a common way to track how physiologically aroused someone is.) “We interpreted it to suggest that quietly sitting next to a partner is sufficient to generate synchrony, so it could be happening all the time without even talking,” he wrote to Science of Us in an email. “I thought that was pretty wild.” It’s also pervasive: Put people in pairs, small groups, or organizations, and they start syncing up.
This represents a new direction for research psychology, which has so long been preoccupied with the individual. It reflects a quiet shift afoot in the discipline’s fundamental assumptions: Perhaps the brain expects to be in relationship with others, since that makes staying protected and finding resources so much easier. “How many things do we do in our lives individually? Almost nothing,” Erika Siegel, a postdoc at the University of California, San Francisco, explained in an email. But buildings, moon missions, democracies: These require relationships, and by studying synchrony, you gain access to dynamics between people. It’s a way of studying “humans as interpersonal animals,” she says: Whether it’s exchanging glances, sharing heart rates, or mirroring gestures, these are different ways of “greasing the cogs of the social world,” she said.
The earliest and best-documented variety is behavioral synchrony. Emily Butler, who directs the Health and Interpersonal Systems research group at the University of Arizona, says that moving in sync creates feelings of trust and closeness between people. There are two main forms of synchrony: in-phase, where you behave in the same way as the other, like crossing the same leg as your friend does when you sit down on the couch; and anti-phase, where you take turns. “Talking has an anti-phase pattern,” Butler says. “I talk to you, you listen, then we switch.” It can be super simple: In experiments, just tapping on a table at the same time as someone else makes them twice as likely to help you, while sharing bodily movements builds rapport and bridges out-groups. Relatedly, dancing prompts cooperation.
This behavioral synchrony doesn’t just happen among individuals, but in groups; organizational psychologists think that it may “enable groups to mitigate the free-rider problem and more successfully coordinate in taking potentially costly social action.” Thus why troops march in formation and churchgoers sing in unison: Choreography breeds closeness.
More recently, research has come into vogue on emotional-subjective and physiological synchrony — meaning being mad or glad at the same time as someone or sharing similar levels of heart rate, skin conductance, or other such biomarkers of arousal. For example, when there’s greater physiological synchrony between a therapist and a patient, the patient feels more like they’re getting an empathetic treatment from their therapist.
Butler and others have argued that emotions aren’t just intrapersonal interior states, but interpersonal, shared experiences. The synchrony that comes with that is not necessarily a good thing. If a married couple is more physiologically synchronized during an argument, they’re more likely to be dissatisfied with their relationship; other research indicates that couples tend have similar levels of cortisol, a hormone associated with stress. It looks like a negative feedback loop: Negative responses drive negative responses, and the relationship deteriorates. Yet synchrony also plays a part in acts of mutual cooperation, like listening and compromising. Because of all that, Butler wrote in a 2015 review, synchrony itself is a poor indicator of relationship quality: “You can be revving reach other up or calming each other down,” she says. For good or for ill in a romantic relationship, synchrony magnifies whatever dynamics are going back and forth between people.
This is especially seen in mother-and-child relationships. The effects start young: A study of 69 mothers and their 12-to-14-month-olds found that when mothers were given an experience of social rejection, their babies showed significantly higher heart rates than the control group. (Attachment theorists will note that even before a baby can speak, they’re learning what soothes and stresses Mom.) Synchrony also looks to be a useful indicator of how attuned a parent is to their kid, with positive results: A 2015 study of 7- to 12-year-olds with behavior problems found that when the kids and the mothers had greater synchrony during difficult discussions, they were better able to “repair” their relationships. Not having synchrony with the mother presents a host of risks to babies and the adults they will become.
The vast majority of this research is associative, so it’s hard to tease apart what causes what: Are the couples in fraught relationships escalating their arguments because their arousal levels are high or because their partner just isn’t listening, or a combination of both? (In any case, the phrase “getting on my nerves” acquires deeper meaning.) Butler says that way more experimental studies will need to be done to get to causality. Compounding that difficulty, the mathematical models that psychology is bringing in are technically demanding; fitting with that synchronized pendulum example, many come from physics. However, your average psych Ph.D. may not have the skills to do that kind of quantitative analysis, so expect lots of psychologists collaborating with computer scientists if they’re going to unravel synchrony.
While it might truly end up being the case that moving in sync literally gets people’s brains in sync, science still doesn’t know why — or how — it happens. “If it’s the clocks on the wall, they’re attached by physical connections,” Palumbo says. “It’s less clear if you’re talking about the beating of someone’s heart.”
Still, Palumbo sees lots of examples in the popular culture: Think about a football coach getting his team “psyched up” before a big game. In the course of the pregame pep talk, a coach isn’t just dispensing advice; he’s also getting his heart rate and cortisol going, bringing his players along with him. (Research on firewalking in Greek villages, of all things, indicates that the deeper the bond you share with someone, the more your physiological arousal mirrors theirs.) That difference in arousal levels between coach and player has something to do with the collective hyping; a mild-mannered ball coach would be, at a physiological level, less exciting.
While it’s still early in the research, some studies indicate that interoception, or your felt sense of your interior states, may be a gateway to synchrony. “The reasoning goes: If you have more access to the sensations from your body, it may be easier for you to simulate the body states of others, making you more empathic or socially sensitive,” says Siegel, the UCSF postdoc. While it’s not definitive, it falls in line with what the function of synchrony may be. “If the goal … is social facilitation,” she says, “then people who have an easier time simulating the experiences of others may synchronize with others more easily.” Or the better you feel yourself, the better you can feel for — and maybe share feelings with — others.