John Callaghan is a professor of sport and is an expert on his subject in many more ways than one. The head of the department of exercise science at the University of Southern California is adept at whatever sport he plays.

In his early twenties he turned down an offer to have an extended trial with Liverpool FC (who had Jimmy Melia and Ronnie Moran on the books) and then played first-class rugby for London Irish. Even now at 64 Professor Callaghan is a good enough tennis player to make the last eight for his age group in the United States National Tennis Championships.

Originally from Crosby on Merseyside, Professor Callaghan has followed an academic path crisscrossing the United Kingdom and the United States for a number of years. "California is something of a paradise for someone who is both academic and interested in sports," Professor Callaghan said in his office overlooking the Cromwell Athletic Field at USC, where he used to watch top athletes such as Daley Thompson warming up for the 1982 Olympics.

Professor Callaghan started his academic career doing teacher training at St Mary's College, at Strawberry Hill, London before moving on to Loughborough to do a diploma in physical education.

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To get a good breadth of experience in teaching Callaghan then taught at a tough Liverpool comprehensive and a public school before, in 1965, making his first academic sortie to USC. He took a sabbatical from London and coached the USC soccer team. In the California sunshine, he also improved his tennis. Then after another year back in London he returned to USC to do a four-year doctorate in socio-psychological aspects of sport, during which time he also coached both the USC soccer and rugby teams.

One more academic posting back to Strawberry Hill in 1973, as head of a department, kept him in UK for a while but now he has been on the faculty of exercise science - an offshoot of sports medicine - at the 110-year-old USC since 1981.

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The department is split into three sections, physiology (the study of cardio-vascular movements, etc), bio-mechanics and sports pyschology. A faculty spokesman said: "Exercise science is the study of human movement. Specifically it is the study of how people move, why some people are able to move faster or for a longer period of time than others (endurance) and why they fatigue. "We do a lot of cross-cultural comparisons and I concentrate on the socio-psychological areas," Professor Callaghan said.

"The psychology of sport should by rights come under the social sciences faculty but instead it comes under the natural sciences faculty to keep the exercise science department as a whole. Certainly in this department I am the most social sciences-based of all the staff. This set-up seems to work fine.

"Study in my area attempts an analysis and understanding of behaviour in sport and physical activity, along with an assessment and evaluation of the pyschological varieties involved in performance."

During the last soccer World Cup in the US Professor Callaghan was asked to appear on a national cable channel to give his views on football hooliganism in Europe. Because of his academic background in sports sociology, and his birthplace - near Liverpool - he was considered to be an "expert".

He said: "Everyone here was fearful of hooliganism but in the end it wasn't a problem, probably because England didn't qualify. The English are the world champions of hooliganism, as we all know, and I've always argued that it arose from the inequalities of the class system in the UK."

Professor Callaghan said the various studies he had carried out suggested that the unfairness of the English educational system was largely to blame for the early growth of hooliganism. "Because of the inequality of opportunity in the British educational system - especially when we had the 11 plus - a lot of kids realise early on they will never grow up to be anything and so they take it out on anybody and anything; and this leads to vandalism and hooliganism, the one opportunity for them, in their terms, to show they are worth something."

There is a specialist research group at USC looking into hooliganism in general and Professor Callaghan has fed in his expertise on soccer hooliganism.

But his real specialisation is on sports people's "motivations" and the different philosophies of sport shown by the Europeans and British as opposed to the Americans.

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The transatlantic differences in sport were brought home to Professor Callaghan when he studied for his doctorate.

"There are different feelings on either side of the Atlantic about the competitive demands made by sport," explained Professor Callaghan. "The American philosophy has always been much more aggressive and this mainly stemmed from the frontier spirit. I think that the Americans have always seen themselves as pioneering and pushing back the barriers. This made them more competitive for many years, but I think that materialism and big money sponsorship for sport in Europe has whittled away many of the differences."

Professor Callaghan cites the examples of tennis and cricket in England and how the "olde worlde" gentility in the sports has been pushed out, in the main, because of the pressures and incentives brought in by more money. He said that unfortunately "the days of amateurism are almost over now".

Professor Callaghan says he has felt at home working on the USC campus from day one and still gets a buzz from working there.

With ,000 students USC is the largest private university in the US and the fifth-biggest in the land overall (University of Michigan, with 40,000 students is the biggest).

Professor Callaghan said: "American universities like USC are big, on-going, dynamic institutions. They are always innovatively thrusting ahead. I think that universities this side of the Atlantic are, like Americans themselves, never frightened of making mistakes. I think too that there is tremendous academic freedom here and great sense of camaraderie."

Professor Callaghan is very proud of the university's place in the local - mostly ghetto - communities.

"We do a lot for the local under-privileged communities and run camps for the neighbourhood youngsters every summer on the campus and our students do a lot of voluntary work in the ghettoes.

"I think because of this the local community has a lot of respect for the campus, despite it looking like an oasis of green opulence against poor surrounding districts."

Professor Callaghan said the respect was never more evident than during the Rodney King riots when businesses around the campus were burnt down but, despite being at the mercy of the rioters, the USC campus was left alone.

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"We are an integral part of the community and even the rioters appreciated that," he said.