Approaches to criminology in the twentieth century can broadly be divided into those that focus on structure than those that focus on process. The structural approach examines the relationship between crime and social structures or situations. For instance, the ecological school of criminology which arose in the 1920s at the University of Chicago focused on which areas of cities attracted the most crime and why. Researchers found that crime was often highest in areas that were in transition from residential to business use, since social networks were often disrupted in these areas.
Process-based theories of criminology attempt to explain how people become criminals. They include the theory of differential association, which claims that people favor criminal behavior when they associate with criminals and control theory, which holds that criminals are usually those who do not develop strong social and family bonds. This results in their having little attachment to others and failing to develop such controls as conscience, guilt, and shame, which would dissuade them from criminal conduct.