The pioneer of this kind of analysis — of what is called the study of cultural microrhythms — is a man named William Condon. In one of his most famous research projects in the 1960s he attempted to decode a four-and-a-half-second segment of film, in which a woman says to a man and a child, over dinner: "You all should come around every night. We never have had a dinnertime like this in months." Condon broke the film into individual frames, each representing about 1/45th of a second. Then he watched — and watched. As he describes it:

To carefully study the organization and sequence of this, the approach must be naturalistic or ethological. You just sit and look and look and look for thousands of hours until the order in the material begins to emerge. It's like sculpturing… Continued study reveals further order. When I was looking at this film over and over again, I had an erroneous view of the universe that communication takes place between people. Somehow this was the model. You send the message, somebody sends the message back. The messages go here and there and everywhere. But something was funny about this.

Condon spent a year and a half on that short segment of film, until, finally, in his peripheral vision, he saw what he had always sensed was there: "the wife turning her head exactly as the husband's hands came up. From there he picked up other micromovements, other patterns that occurred over and over again, until he realized that in addition to talking and listening, the three people around the table were also engaging in what he termed "interactional synchrony." Their conversation had a rhythmic physical dimension. Each person would, within the space of one or two or three 1/45th-of-a-second frames, move a shoulder or cheek or an eyebrow or a hand, sustain that movement, stop it, change direction, and start again. And what's more, those movements were perfectly in time to each person's own words — emphasizing and underlining and elaborating on the process of articulation — so that the speaker was, in effect, dancing to his or her own speech. At the same time the other people around the table were dancing along as well, moving their faces and shoulders and hands and bodies to the same rhythm. It's not that everyone was moving the same way, any more than people dancing to a song all dance the same way. It's that the timing of stops and starts of each person's micromovements — the jump and shifts of body and face — were perfectly in harmony.

Subsequent research has revealed that it isn't just gesture that is harmonized, but also conversational rhythm. When two people talk, their volume and pitch fall into balance. What linguists call speech rate — the number of speech sounds per second — equalizes. So does what is known as latency, the period of lime that lapses between the moment one speaker Stops talking and the moment the other speaker begins. Two people may arrive at a conversation with very different conversational patterns. But almost instantly they reach a common ground. We all do it, all the time. Babies as young as one or two days old synchronize their head, elbow, shoulder, hip, and foot movements with the speech patterns of adults. Synchrony has even been found in the interactions of humans and apes. It's part of the way we are hardwired.

When Tom Gau and I sat across from each other in his office, then, we almost immediately fell into physical and conversational harmony. We were dancing. Even before he attempted to persuade me with his words, he had forged a bond with me with his movements and his speech. So what made my encounter with him different, so much more compelling than the conversational encounters I have every day? It isn't that Gau was deliberately trying to harmonize himself with me. Some books on salesmanship recommend that persuaders try to mirror the posture or talking styles of their clients in order to establish rapport. But that's been shown not to work. It makes people more uncomfortable, not less. It's too obviously phony.

What we are talking about is a kind of super-reflex, a fundamental physiological ability of which we are barely aware. And like all specialized human traits, some people have much more mastery over this reflex than others. Part of what it means to have a powerful or persuasive personality, then, is that you can draw others into your own rhythms and dictate the terms of the interaction. In some studies, students who have a high degree of synchrony with their teachers are happier, more enthused, interested, and easygoing. What I felt with Gau was that I was being seduced, not in the sexual sense, of course, but in a global way, that our conversation was being conducted on his terms, not mine. I felt I was becoming synchronized with him. "Skilled musicians know this, and good speakers," says Joseph Cappella, who teaches at the Annenberg School of Communication at the University of Pennsylvania. "They know when the crowds are with them, literally in synchrony with them, in movements and nods and stillness in moments of attention —" It is a strange thing to admit, because I didn't want to be drawn in. I was on guard against it. But the essence of Salesmen is that, on some level, they cannot be resisted. "Tom can build a level of trust and rapport in five to ten minutes that most people will take half an hour to do," Moine says of Gau.

There is another, more specific dimension to this. When two people talk, they don't just fall into physical and aural harmony. They also engage in what is called motor mimicry. If you show people pictures of a smiling face or a frowning face, they'll smile or frown back, although perhaps only in muscular changes so fleeting that they can only be captured with electronic sensors. If I hit my thumb with a hammer, most people watching will grimace: they'll mimic my emotional state. This is what is meant, in the technical sense, by empathy. We imitate each other's emotions as a way of expressing support and caring and, even more basically, as a way of communicating with each other.

In their brilliant 1994 book Emotional Contagion, the psychologists Klaine Hatfield and John Cacioppo and the historian Richard Rapson go one step further. Mimicry, they argue, is also one of the means by which we infect each other with our emotions. In other words, if I smile and you see me and smile in response — even a microsmile that takes no more than several milliseconds — it's not just you imitating or empathizing with me. It may also be a way that I can pass on my happiness to you. Emotion is contagious. In a way, this is perfectly intuitive. All of us have had our spirits picked up by being around somebody in a good mood. If you think about this closely, though, it's quite a radical notion. We normally think of the expressions on our face as the reflection of an inner state. I feel happy, so I smile. I feel sad, so I frown. Emotion goes inside-out. Emotional contagion, though, suggests that the opposite is also true. If I can make you smile, I can make you happy. If I can make you frown, I can make you sad. Emotion, in this sense, goes outside-in.

If we think about emotion this way — as outside-in, not inside-out — it is possible to understand how some people can have an enormous amount of influence over others. Some of us, after all, are very good at expressing emotions and feelings, which means that we are far more emotionally contagious than the rest of us. Psychologists call these people "senders." Senders have special personalities. They are also physiologically different. Scientists who have studied faces, for example, report that there are huge differences among people in the location of facial muscles, in their form, and also — surprisingly — even in their prevalence. "It is a situation not unlike in medicine," says Cacioppo. "There are carriers, people who are very expressive, and there are people who are especially susceptible. It's not that emotional contagion is a disease. But the mechanism is the same."