In their 1990 work, Gottfredson and Hirschi introduce a new and comprehensive theory of crime. At the time, crime researchers tended to focus on environmental factors that led to crime, not on the criminals themselves, and were inclined to think about crime only from their particular academic perspective. Ideas about what caused crime, and how to prevent it, were often in conflict. Gottfredson and Hirschi believed that criminology should return to its roots and examine the ways in which crime is connected to human behavior. They developed a theory of crime focused on self-control, proposing that all crime can be explained by the amount of self-control a person can exercise.