If you’ve ever watched “The Sopranos” or the 1997 movie “Donnie Brasco,” you might have wondered whether killing people is fun. After all, it seems fun to watch, at least on TV. Or maybe it’s not the killing that’s fun. Maybe it’s the planning, the camaraderie, the petty squabbles, and the disdain for normal bourgeois existence that holds our attention. Demanding protection money and dealing drugs seems fun, too. If you’ve had such thoughts, have I got the social scientist for you. Beginning in 1989, Sudhir Venkatesh, now a professor at Columbia University, followed around a Chicago gang leader, identified only as J.T., for about six years, ostensibly to complete his sociology Ph.D. at the University of Chicago. Though Mr. Venkatesh did draw on his experiences in his dissertation, and in several much-discussed academic papers, the final result of his sojourn is “Gang Leader for a Day: A Rogue Sociologist Takes to the Streets” (Penguin Press, 320 pages, $25.95). If you’ve read the best-selling “Freakonomics,” Mr. Venkatesh should be a familiar figure: He was the guy who hung around young drug dealers, discovered that most of them don’t earn very much money and still live with their mothers, and asked why anyone would get into the business in the first place.

I opened this book expecting to learn about why crime is high, how the drug trade works, or why so many people seem to make dysfunctional lifestyle choices. That’s not what I got. Most of all this is a story of male friendship and bonding — that’s right, I mean the bonding between the researcher and the criminal.

J.T., the gang leader at the center of the story, and of Mr. Venkatesh’s research, becomes wrapped up in the idea of having his own biographer. Eventually it became his obsession that Mr. Venkatesh record the details of his life, including the shakedowns. In part, this was J.T.’s narcissism, and in part he needed the motivation of an observer. Most of all, J.T. seemed to enjoy having an audience: “I realized that he had come to rely on my presence; he liked the attention, and the validation,” Mr. Venkatesh reports. None of J.T.’s underlings were qualified for the role of courtier, but the highly intelligent and nonjudgmental Mr. Venkatesh was perfect.

Others are not quite so generous toward Mr. Venkatesh. Ms. Bailey, one neighborhood figure, told him: “You want to act like a saint, then you go ahead … But you are also hustling. And we’re all hustlers … You’re a hustler, I can see it. You’ll do anything to get what you want. Just don’t be ashamed of it.” Mr. Venkatesh himself does not shy away from telling us about the more lurid appeal his project had for him: “By now I had spent about six years hanging out with J.T., and at some level I was pleased that he was winning recognition for his achievements. Such thoughts were usually accompanied by an equally powerful disquietude that I took so much pleasure in the rise of a drug-dealing gangster.”

That disquietude did not always prevail. If you’re wondering where the title of the book comes from, Mr. Venkatesh himself spent one day running J.T.’s rounds and collecting payoffs; J.T. wanted to show the researcher just how hard his job was. And indeed, he succeeded. Not surprisingly, Mr. Venkatesh starts to feel that other scholars are “living in a bubble,” and at times these feelings turn to anger at his profession.

Slowly but surely Mr. Venkatesh starts to realize his legal liability. If he is abetting illegal activity, or if he knows of planned crimes, he is potentially an accessory and subject to arrest and imprisonment. “Four years deep into my research, it came to my attention that I might get in a lot of trouble,” Mr. Venkatesh writes, but even this not-so-streetwise reviewer had that worry by page 25 of the narrative. Mr. Venkatesh also realizes that his command of so much information puts him in a dangerous position. The criminals know that he can be forced to testify against them, and eventually Mr. Venkatesh has to tell them that, unlike a journalist, a researcher has no confidentiality protection from the probing hand of the law. It surprises this reader that he survived the experience without even as much as a good beating; presumably this was due to the protection of J.T., who vouched for him. It is disquieting to learn that the danger is not just from the criminals: Gang members warn him, above all else, not to write about the police. They tell him that the police don’t want to be watched or chronicled, and that, yes, they will retaliate.

It is interesting that so many of the criminals wanted to cooperate with the researcher at all. Many of them thought that if they shared information they might be able to find out what their competitors were making. And it seems that many of them wanted to please J.T. by helping out his friend. Still others may have liked the idea of lying to a figure from the outside establishment. Most of all, they liked the idea that someone took them seriously as businessmen. By the end of the story, Mr. Venkatesh and J.T. have met their respective fates. Mr. Venkatesh has become a successful academic, now teaching at Columbia and having what may well become a best-selling book. He is already one of the best and most respected social scientists in the United States.

His, subject, too has moved on. J.T. grew tired of running a gang, particularly when the crack trade dried up and with it a lot of the business. He tried managing a dry cleaning business and then started a barber shop, which failed. For a while, he tried to market himself as a consultant for higher-ups in the drug economy. Right now he seems to be living off his savings. The two men see each other every now and then, but they don’t seem to have established their previous rapport.

When it comes to understanding the world, biography is truly the underappreciated method in the social sciences. The life of the individual reveals what is otherwise hidden in abstract numbers or faceless questionnaires. Mr. Venkatesh is to be applauded for his path-breaking work and his compelling exposition. He’s lucky that he didn’t have to pay a high price, but by the end of the story the reader is wondering whether someone else might have, due to Mr. Venkatesh’s unintended encouragement of J.T. Yes, evil really can be attractive, and the biographical achievement here is splendid, but when I return to the thought of encouraging and feeding the ego of a gang leader for six years running, I can’t bring myself to be attracted to this book.

Mr. Cowen is professor of economics at George Mason University.