  Related to an idea of Lewin, a mathematical model for behavioral changes
under the influence of a social field is developed. The social field reflects
public opinion, social norms and trends. It is not only given by external
factors (the environment) but also by the interactions of individuals. Two
important kinds of interaction processes are distinguished: Imitative and
avoidance processes. Variations of individual behavior are taken into account
by ``diffusion coefficients''.
