Classical criminology is an approach to the legal system that arose during the Enlightenment in the 1700s (18th century). Philosophers like Cesare Beccaria, John Locke, and Jeremy Bentham expanded upon social contract theory to explain why people commit crime and how societies could effectively combat crime. The concepts continue to play a large role in the legal systems of many nations today, although the approach in the modern world tends to be a bit more flexible.
It is important to understand the context in which classical criminology was developed. During the Enlightenment, Europe was changing radically, with many nations emerging from feudal monarchies and radically reforming their laws. Across Europe, the law was wildly inconsistent and applied even more inconsistently. Judges and other legal officials often lacked extensive training, and prescribed punishments totally out of proportion to some crimes while ignoring others. Many people recognized the need for a more uniform and effective justice system, and this approach was the result.
According to the theorists, human beings are self-interested animals, but they are also extremely rational. While people will tend to do things that are in their own self interest, they also understand that some actions actually conflict with this, and many societies develop a social contract that dictates human behavior, with humans mutually agreeing to refrain from activities that hurt each other or society.
People also have free will, which means that they can opt to violate the social contract. For example, someone might steal or murder to accomplish a self-interested goal. By having consistent punishments in place that are proportional to the crime and applied rapidly, classical criminologists argue, the legal system will create deterrents to crime. Rather than committing a crime with a degree of uncertainty about the punishment, people in a nation with a clear and concise legal system will be well aware of the consequences of violating the law and the social contract, and they may think twice before committing crime.
One of the big problems with classical criminology is that it does not allow for extenuating circumstances. Someone who robs a business for profit is treated exactly the same as someone who robs a business in order to eat, and some people feel that this is inhumane. Others feel that the assumption of free will is also somewhat questionable, as people may be forced into making decisions as a result of their circumstances or socioeconomic class.