This book is based on collective research carried out during the 1980s. This edition appears ten years after the original publication in French. Since then we have experienced many changes. In the late decade, disciplines have changed, as have the societies being researched. The outbreak of AIDS in Africa and the industrial world is not the least of these major and influential changes. The reader today will be sensitive to these changes and this research maintains its value as an intellectual endeavour and a useful model.