A Mathematical Model for the Behavior of Individuals in a Social Field
  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''. 
Dirk Helbing