Functionalist theory originates in the work of the French sociologist David Émile Durkheim (15 April 1858–15 November 1917). The main characteristic of functionalist theories is that social phenomena such as crime and deviance are viewed as parts of an interlocking system rather than products of individual circumstances. Social institutions are seen as necessary to the functioning of society and act as parts of an interlocking organism shaped to maintain social order and stability.

What is defined as "crime" is is socially determined. All societies must define what is legitimate behavior and what behavior must be regulated or eliminated in order for the society for function. The specifics of which behaviors are considered crimes vary with the needs of the particular society at the particular moment. For example, dueling would have been legal in the Renaissance, but killing someone in a duel would be illegal now. Likewise, bans of opioids and other recreational drugs are relatively recent phenomena.

The act of defining crimes and arresting criminals creates a form of cohesion in society, making clear the nature of social norms. Society develops a sense of solidarity as the "law abiding" define themselves in opposition to criminals and use crime and deviance as a way of demarcating their values. Criminals and deviants, by testing the boundaries of social norms, can become agents of social change, as new behaviors (such as cyberbullying) become criminalized and old ones (recreation use of marijuana, for example) become decriminalized.

Structural-functionalism is a sociological theory that views society as a complex system, a sum result of all its parts working together and interacting according to the individual role of each within the system.  A functioning society, therefore, is a product of the interconnectedness of its norms, customs, traditions, and institutions.

Applying this concept to the idea of crime, or “deviance,” in sociological parlance, we get the idea that to a certain extent criminal activity is natural and essential to the healthy working of a society.  According to Emile Durkheim, crimes are important for social in four main ways.  First, it reaffirms cultural values and norms – he is quoted as saying “any definition of virtue rests on an opposing idea of vice:  There can be no good without evil and no justice without crime.”  So, by the existence of this deviant behavior, and recognizing it as deviant, a culture is reinforcing what constitutes desirable behavior within its society.  Related to this, Durkheim secondly state that criminal behavior defines the limits of moral behavior – in this way people learn where their boundaries are.  Third, when crimes are committed communities must work together both to establish and execute the consequences, thus creating a stronger bond of solidarity among the members of a society.  And finally, deviance can be a catalyst for social change by not only defining, but pushing the moral limits of a society.  As an example of this point, consider interracial marriages or freedom of religion – things that people were once persecuted for in Western society, but which have now, through deviation from the norm, have become common, accepted practices in the modern day.

According to Robert K. Merton’s Strain Theory in functionalism, deviance is a result of dissatisfaction with a society’s established means of achieving a goal.  Thus, crime is a different route to a similar goal.  For example, in what Merton calls innovation, a criminal who lacks opportunity could be disillusioned with the common societal goal of “making oneself,” and generating wealth, and so creates an alternate track to the same goal by stealing for money.  Similarly, mass deviance such as rioting or rebellion can come about from such a means/goal strain, in which those in power are not making enough headway on a certain problem through what are deemed “legitimate” means, and so a large group rejects the accepted methods and creates their own.  Rebellion can also result in a full-out rejection of general societal goals along with the means to achieve them, creating a “counterculture” within the existing society.

So according to structural-functionalism, society operates as an organism, with all its parts and organs working together to keep society moving normally.  Crime is not a breakdown in this organization, but a necessary part of it, in order to allow society to understand itself, its moral boundaries, and to grow.  Similarly, crime is not a breaking down of societal rules, but a direct result of these rules being incompatible with certain individuals’ ability to achieve cultural goals, which leads to deviance in an attempt to reach those goals.

Structural functionalists believe that every aspect of society plays an important role in that society.  They believe that crime, in the proper amount, is good for a society.  The main reason for this is that crime helps to define our values and to get people to adhere to those values.

When we identify and punish criminals, we are affirming our shared values.  We are saying that certain behaviors are outside of what is acceptable to us.  By doing so, we are also defining what is acceptable.  This helps us to retain our sense that all of us in the society are united and cohesive.  For this reason, crime is good so long as it does not reach levels that are too high.