SUMMONED by a frantic phone call, they arrive at the bar to find two men fighting. Pulling the protagonists apart, the gang then starts systematically to beat one of the men up. As the victim lapses into unconsciousness, and the beating shows no sign of coming to an end, Simon Winlow realises he has no choice but to risk blowing his cover.

He has spent months getting to know these men, winning their trust, in the name of research into the role of violence in working-class culture. Now, his new friends are on the point of killing a man in front of him.

"These are men who are highly skilled in violence, they are punching and kicking this man and I think he has passed out. I think they're going to kill him and I think I'm going to be the first sociologist who is witnessing a murder.

"You don't want the violence to be directed towards you, but you don't want to be ambivalent about someone losing their life. I had a knotted stomach and myself and another person who was there said 'he has had enough'.

'As soon as the words left my mouth I remember regretting it. It created a tension in the air, but they stopped." Putting himself in dangerous situations became a regular occurrence for Dr Winlow, 28, a lecturer in criminology at Teesside University, over the course of his research into how changes in society over the past ten to 20 years have changed the role of violence and crime and its place in working-class culture.

Frustrated by traditional academic analysis of the subject, Dr Winlow saw his own background, North-East working-class, as an ideal opportunity to get close to the people involved, and find out what went on at first-hand.

Over the course of four-and-a-half years, he trained and worked as a bouncer at pubs and clubs throughout the North of England, observing his new colleagues and their attitudes to violence. He explored the links between some doorstaff and crime, from protection rackets to cigarette smuggling and, in a less covert role, interviewed and studied people involved in organised crime.

"I felt I had access to these people and not to research them would be a criminal waste," he says. "I had all the tools at my disposal to do this piece of research, it was a job waiting to be done and it related to my own upbringing.

"It is okay for middle-class, liberal academics to say that violence is a bad thing and people who engage in violence are pathological and evil, but that doesn't help us understand how it happens in the first place.

"We have got to go out and understand how these people think and understand the world as they see it. Without that, we are just left in our ivory towers, discussing what these people get up to, and it isn't good enough."

His research grew out of a PhD thesis, carried out while at Durham University, and now into a book, arguing that bouncers represent a new form of masculinity, replacing traditional industries as a test of manhood.

"Society has changed intrinsically, over a period of ten to 20 years, with the virtual collapse of industrial production, and this has forced men to change. A knock-on effect is that criminal practice changes. Increasingly, there is the development of entrepreneurial crime, for financial reward, rather than crime as a means to gain repute and respect.

"The occupation of being a bouncer fills the vacuum left by all the collapsed industries. It takes in working-class males who, in generations gone by, would have worked in manufacturing industries, and now they are controlling the growing night-time economy. I was trying to gain some kind of empathy with the way they felt about their work and themselves as men."

Long periods of his time as a bouncer were spent hanging around with nothing to do, but at least giving him a chance to talk to his new colleagues and discover their attitudes to their job. But the potential for trouble was always there, and, on occasion, it erupted in spectacular fashion.

"I found it fairly stressful. If you can imagine being surrounded by thousands of, often very drunk, young people, you are going to feel that violence is never far from the equation.

"You might go six or seven months without having to throw a single person out of a bar, your presence alone serves to keep order, but then, one evening, world war three starts."

And, when violence did break out, Dr Winlow had to be careful not to blow his cover, while at the same time playing the part of a bouncer.

"The best way of describing it is trying to organise a peripheral route. There were some bouncers who were always the first into a fight, but I tried to be involved in the periphery, controlling the crowd. You can't just stand there and watch, particularly if this was research to find out how bouncers organise their occupation.

"On occasions, I would be involved. If a fight starts, you pick your way through the crowd, which is a job in itself, and separate the two people and drag them to the door. This can be a lot more complex than it sounds, because you have a mass of bodies." As well as his work as a bouncer, he used contacts from his childhood to gain access to people involved in organised crime and, in the main, was up front about his intentions.

"Violence is absolutely central to the characters of some of these people, it is at the forefront of their every waking moment. They're trying to cultivate this persona of being tough, somebody to be reckoned with, somebody who commands respect." But, whereas this may have been true for violence throughout the ages, the economic changes of the last 20 years have prompted an important addition, according to Dr Winlow.

"Violence, and the reasons why it occurs, are generally the same. It is how it is employed that has changed. Violence is being used as an asset in the market place. It still happens because of individual conflict, because of issues of masculinity, but now it has this added factor where you can use it to gain financial reward.

'It is a generalisation to say that people are involved in this kind of activity to get money. It is a part of it, but it may merge with issues of status and power and respect, which are essential to most of society."

But, while he may have considered it essential to witness the violence to be able to understand it, that did not stop him from being deeply shaken by some of the things he saw. In one particularly gruesome incident, a girl was glassed with a bottle in the face, losing four teeth and ripping her lip open.

"I was disturbed by what I saw, but you have got to keep going out there. I would go to work as a bouncer and I was dreading it and desperate to get away at the end of the night. It was worse when violence happened but it was a constant threat.

"I never enjoyed witnessing violence. You want to watch but, while watching, my stomach would be turning. It was difficult, but I was trying to produce some worthwhile research. How on earth can you understand violence unless you go out and discover it?"

* Badfellas: crime, tradition and new masculinities, by Simon Winlow, is published by Berg.