THE kind of research Valerie and Gerald Mars undertake gives food-lovers plenty to swallow. They study culture and society by what and how people eat. ''Food has always defined society. It can be used to separate people or control them,'' said Gerald Mars, a British anthropologist. Gerald and his wife Valerie, an historian, were in Hongkong recently to give a lecture and seminar at Hongkong University. She lectured on food, and he on cultural theory. Gastronomy is their vocation. She's in the middle of writing a magnum opus on the dining habits of Victorians. ''In upper-middle class Victorian society, food was used to control children,'' said Valerie. ''Since children are unpredictable and parents didn't want something unexpectedto happen, they forced children to learn manners before they were invited to the dinner table with adults. Victorians structured everything.'' Gerald collaborated with his wife on a recent project - a year-long study of 37 families in urban and suburban London. She wanted to learn how people in the same income bracket regarded food and dining. He applied research methods and classified people into four groups according to the way they related to food and mealtimes: the enclavists, traditionalists, the competitive eaters and fatalists. Enclavists. Vegans belong to this group. They are people who are almost defensive about food. They are anti-authority and believe the world is against them. Seating at mealtimes is random and there's always a place for everyone. Competitive. Devotees of what's hot, what's trendy and fashionable in food belong to this group. Food becomes status symbols. One way to demonstrate it is by a massive display of eclectic dishes from various cuisines. Set meal times are regarded as a good idea but rarely practised. Seating is random but there is always competition for the best chair. Traditionalists. This group obeys the rules and regards mealtimes as sacred. Promptness at meals is paramount and everyone sits in their proper place. Consistency is revered over creativity in cooking and foods are prescribed by tradition. Fatalists. This group has no control over food choices. Habits are capricious and eating is ''catch as catch can''. The unemployed belong to this group. Every generation has their food taboos, said Valerie. An upper-crust Victorian wouldn't be caught dead eating food on the street while a street urchin during the times may have celebrated finding a penny by unabashedly eating whatever he could buy or steal. An example of contemporary competitiveness is cutlery. During a recent trip to the United States, the couple saw differences in how tables were set in California and New York. In California, the silverware arrived with each new course. But in New York, the table was set with every piece required for an entire meal. ''If you were unfamiliar with which piece to use first, you could feel threatened or out of place. It's a type ofculinary one-upmanship,'' Valerie said.