Alpert is not, it should be said, an obnoxious know-it-all. It's easy to see how he could be, of course. Even Alpert is aware of that. "I was standing next to a kid in the supermarket who had to show his I.D. to buy cigarettes," Alpert told me. "I was very tempted to tell him I was diagnosed with lung cancer. In a way, that desire to be of service and influence — whatever it is — can be taken too far. You can become nosy. I try to be a very passive Maven… You have to remember that it's their decision. It's their life." What saves him is that you never get the sense that he's showing off. There's something automatic, reflexive, about his level of involvement in the marketplace. It's not an act. It's very similar to the social instinct of Horchow and Weisberg. At one point Alpert launched into a complicated story of how to make the best use of coupons in renting videos at Blockbuster. Then he stopped himself, as if he realized what he was saying, and burst out laughing. "Look, you can save a whole dollar! In a year's time I could probably save enough for a whole bottle of wine." Alpert is almost pathologically helpful. He can't help himself. "A Maven is someone who wants to solve other people's problems, generally by solving his own," Alpert said, which is true, although what I suspect is that the opposite is also true, that a Maven is someone who solves his own problems — his own emotional needs — by solving other people's problems. Something in Alpert was fulfilled in knowing that I would thereafter buy a television or a car or rent a hotel room in New York armed with the knowledge he had given me.

"Mark Alpert is a wonderfully unselfish man," Leigh MacAllister, a colleague of his at the University of Texas, told me. "I would say he saved me fifteen thousand dollars when I first came to Austin. He helped me negotiate the purchase of a house, because he understands the real estate game. I needed to get a washer and dryer. He got me a deal. I needed to get a car. I wanted to get a Volvo because I wanted to be just like Mark. Then he showed me an on-line service that had the prices of Volvos all over the State of Texas and went with me to buy the car. He helped me through the maze of all the retirement plans at the University of Texas. He simplified everything. He has everything processed. That's Mark Alpert. That's a Market Maven. God bless him. He's what makes the American system great."

What makes people like Mark Alpert so important in starting epidemics? Obviously they know things that the rest of us don't. They read more magazines than the rest of us, more newspapers, and they may be the only people who read junk mail. Mark Alpert happens to be a connoisseur of electronic equipment. If there was a breakthrough new television or videocamera, and you were a friend of his, you can bet you would hear all about it quickly. Mavens have the knowledge and the social skills to start word-of-mouth epidemics. What sets Mavens apart, though, is not so much what they know but how they pass it along. The fact that Mavens want to help, for no other reason than because they like to help, turns out to be an awfully effective way of getting someone's attention.

This is surely part of the explanation for why Paul Revere's message was so powerful on the night of his midnight ride. News of the British march did not come by fax, or by means of a group e-mail. It wasn't broadcast on the nightly news, surrounded by commercials. It was carried by a man, a volunteer, riding on a cold night with no personal agenda other than a concern for the liberty of his peers. With Hush Puppies as well, perhaps the shoes caught the attention of Connectors precisely because they weren't part of any self-conscious, commercial fashion trend. Maybe a fashion Maven went to the East Village, looking for new ideas, and found out that you could get these really cool old Hush Puppies at a certain thrift store, for a very good price, and told his friends, who bought the shoes for themselves because there is something about the personal, disinterested, expert opinion of a Maven that makes us all sit up and listen. And why are the Zagat restaurant guides so popular? Partly it is because they are a convenient guide to all the restaurants in a given town. But their real power derives from the fact that the reviews are the reports of volunteers — of diners who want to share their opinions with others. Somehow that represents a more compelling recommendation than the opinion of an expert whose job it is to rate restaurants.

When I was talking to Alpert, I happened to mention that I was going to be in Los Angeles in a few weeks. "There is a place I really like, in Westwood," he said, without hesitation. "The Century Wilshire". It's a European bed-and-breakfast. They have very nice rooms. A heated pool. Underground parking. Last time I was there, five, six years ago, rooms started in the seventies and junior suites were a hundred and ten. They'll give you a rate for a week. They've got an 800 number." Since he was, after all, the Ur-Maven, I stayed at the Century Wilshire when I was in L.A., and it was everything he said it was and more. Within a few weeks of coming home, I had — completely out of character, I might add — recommended the Century Wilshire to two friends of mine, and within the month two more, and as I began to imagine how many people of those I told about the hotel had told about the hotel, and how many people like me Mark Alpert had himself told about the hotel, I realized that I had stepped into the middle of a little Mark Alpert-generated, word-of-mouth epidemic. Alpert, of course, probably doesn't know as many people as a Connector like Roger Horchow, so he doesn't quite have the same raw transmission power. But then again, if Roger Horchow talked to you on the eve of a trip to Los Angeles, he might not give you advice on where to stay. Alpert always would. And if Horchow did make a recommendation, you might take him up on it or you might not. You would take the advice as seriously as you take advice from any friend. But if Mark Alpert gave you advice, you would always take it. A Connector might tell ten friends where to stay in Los Angeles, and half of them might take his advice. A Maven might tell five people where to stay in Los Angeles but make the case for the hotel so emphatically that all of them would take his advice. These are different personalities at work, acting for different reasons. But they both have the power to spark word-of-mouth epidemics.

The one thing that a Maven is not is a persuader. Alpert's motivation is to educate and to help. He's not the kind of person who wants to twist your arm. As we talked, in fact, there were several key moments when he seemed to probe me for information, to find out what I knew, so he could add it to his own formidable database. To be a Maven is to be a teacher. But it is also, even more emphatically, to be a student. Mavens are really information brokers, sharing and trading what they know. For a social epidemic to start, though, some people are actually going to have to be persuaded to do something. A good number of the young people who bought Hush Puppies, for instance, were people who once upon a time wouldn't have been caught dead in them. Similarly, after Paul Revere had passed on his news, you can imagine that all of the men in the militia movement gathered around and made plans to confront the British the following morning. But it can't have been an automatic process. Some people were probably gung-ho. Some may have doubted the wisdom of confronting a trained, professional army with a homegrown militia. Others — who may not have known Revere personally — might have been skeptical about the accuracy of his information. That almost everyone, in the end, fell in line is something that we would normally credit to peer pressure. But peer pressure is not always an automatic or an unconscious process. It means, as often as not, that someone actually went up to one of his peers and pressured him. In a social epidemic, Mavens are data banks. They provide the message. Connectors are social glue: they spread it. But there is also a select group of people — Salesmen — with the skills to persuade us when we are unconvinced of what we are hearing, and they are as critical to the tipping of word-of-mouth epidemics as the other two groups. Who are these Salesmen? And what makes them so good at what they do?