Criminal justice refers to the system by which justice is dispensed onto those who have committed a crime. It includes the crime a defendant allegedly committed, the law enforcement officers who arrested him, the court system that prosecutes and defends him, and how the defendant is punished if he is convicted. Law enforcement agencies, the courts system, and the detention and supervisory agencies for offenders all work together to maintain a society's rule of law. Criminal justice systems have existed in some form or other for centuries, although the forms they have taken have changed with time. There have always been acts deemed by a society to be unacceptable, but the way society has punished those behaviors (and who was empowered to do the punishing) have changed. For example, stealing has been punished for centuries by mutilation and branding, but imprisonment is a relatively "modern" invention that was not widely used until the 19th century. Watchmen, the ancestors of our modern police, have similarly existed for thousands of years in many different parts of the world. In England, watchmen were codified into law in the 13th century. Unlit streets brought danger after dark. These volunteer groups of men were responsible for patrolling the streets of their local town or parish at night to keep the peace, watch out for fires, and enforce curfew ordinances. An increase in both population and crime in the late 18th and early 19th centuries made the effectiveness of this volunteer, local-level force both inefficient and untenable, and they were replaced by the first paid, state-run modern police force, London's Metropolitan Police, in 1829, which cast the model for today's law enforcement agencies.
Criminology, closely related to criminal justice, is the study of criminal behavior on both an individual and societal level. While criminal justice is more so the apparatuses that society uses to react to crime, criminology is more concerned with the philosophical aspects of crime: it focuses on what leads people to criminal behavior, the most effective ways to prevent crime and rehabilitate offenders, and how society itself reacts to crime. Criminologists' work influences the criminal justice system by proposing new methods and ways of thinking in how to effectively deal with crime. Criminology as we understand it today arose in the 18th century from the work of social philosophers and the work of these early criminologists gave rise to three schools of criminology theory: the Classical, Positivist, and Chicago schools. The Classical school of the mid-18th century advocated for deterrence and swift punishments that were proportionate to the crime. While the Classical school acknowledged that people have free will and agency over their actions, the Positivist school argued that our behavior is informed by both internal and external factors that lie outside an individual's control. Arguing the nature side in the ageless nature vs. nurture debate, criminologists in the Positivist school looked for biological evidence of "criminal tendencies," such as brain defects or even a cleft palate. The work of Italian sociologist Cesare Lombroso (1835-1909), called the father of criminology, falls into the work of the Positivist school. Finally, the Chicago school rose in the early 20th century from the work of sociologists at the University of Chicago. The Chicago school argued that a person's social environment influences and ultimately causes criminal behavior. Today, criminology operates through a multitude of disciplinary lenses and theories. It is a codified discipline that brings together sociology, psychology, biology, political science, philosophy, and history to understand the root cause of crime and the best ways to prevent it.