Social structures are typically thought of as “patterned social arrangements in society.” But in Luhmannian theory, the structures of social systems are not things like family relations, friendships, organizational chains of command, etc. They are expectations. Social structures are expectational structures. We can reduce it to one question: What comes next?
What does suggest for the concept of structural coupling? A simple example of structural coupling is when a physician is paid for treating a patient. Here the medical system and the economy structurally couple. The doctor expects to be paid, and the patient expects to be charged (or expects her insurance company to be charged). But neither doctor nor patient expects to discuss the doctor’s while in the examining room. That is left to nonmedical office staff. The medical function system only distinguishes between illness and non-illness (or normal health). Yet, the medical system could not function without payment–unless everything else in the society was free.
The reason, according to this view, that social structures do not consist of patterns social arrangements is that society does not consist of people; it consists of communication.
All of this reflects the Luhmannian emphasis on time. For instance, he reframed the study of law in terms of time, arguing that the function of the legal system is the stabilization of normative expectations.