In epistemology, and more specifically, the sociology of knowledge, reflexivity refers to circular relationships between cause and effect, especially as embedded in human belief structures. A reflexive relationship is multi-directional when the causes and the effects affect the reflexive agent in a layered or complex sociological relationship. The complexity of this relationship can be furthered when epistemology includes religion.

Within sociology more broadly—the field of origin—reflexivity means an act of self-reference where existence engenders examination, by which the thinking action "bends back on", refers to, and affects the entity instigating the action or examination. It commonly refers to the capacity of an agent to recognise forces of socialisation and alter their place in the social structure. A low level of reflexivity would result in individuals shaped largely by their environment (or "society"). A high level of social reflexivity would be defined by individuals shaping their own norms, tastes, politics, desires, and so on. This is similar to the notion of autonomy. (See also structure and agency and social mobility.)

Within economics, reflexivity refers to the self-reinforcing effect of market sentiment, whereby rising prices attract buyers whose actions drive prices higher still until the process becomes unsustainable. This is an instance of a positive feedback loop. The same process can operate in reverse leading to a catastrophic collapse in prices.

In social theory, reflexivity may occur when theories in a discipline should apply equally to the discipline itself; for example, in the case that the theories of knowledge construction in the field of sociology of scientific knowledge should apply equally to knowledge construction by sociology of scientific knowledge practitioners, or when the subject matter of a discipline should apply equally to the individual practitioners of that discipline (e.g., when psychological theory should explain the psychological processes of psychologists). More broadly, reflexivity is considered to occur when the observations of observers in the social system affect the very situations they are observing, or when theory being formulated is disseminated to and affects the behaviour of the individuals or systems the theory is meant to be objectively modelling. Thus, for example, an anthropologist living in an isolated village may affect the village and the behaviour of its citizens under study. The observations are not independent of the participation of the observer.

Reflexivity is, therefore, a methodological issue in the social sciences analogous to the observer effect. Within that part of recent sociology of science that has been called the strong programme, reflexivity is suggested as a methodological norm or principle, meaning that a full theoretical account of the social construction of, say, scientific, religious or ethical knowledge systems, should itself be explainable by the same principles and methods as used for accounting for these other knowledge systems. This points to a general feature of naturalised epistemologies, that such theories of knowledge allow for specific fields of research to elucidate other fields as part of an overall self-reflective process: any particular field of research occupied with aspects of knowledge processes in general (e.g., history of science, cognitive science, sociology of science, psychology of perception, semiotics, logic, neuroscience) may reflexively study other such fields yielding to an overall improved reflection on the conditions for creating knowledge.

Reflexivity includes both a subjective process of self-consciousness inquiry and the study of social behaviour with reference to theories about social relationships.

The principle of reflexivity was perhaps first enunciated by the sociologists William I. Thomas and Dorothy Swaine Thomas, in their 1928 book The child in America: "If men define situations as real, they are real in their consequences". The theory was later termed the "Thomas theorem".

Sociologist Robert K. Merton (1948, 1949) built on the Thomas principle to define the notion of a self-fulfilling prophecy: that once a prediction or prophecy is made, actors may accommodate their behaviours and actions so that a statement that would have been false becomes true or, conversely, a statement that would have been true becomes false - as a consequence of the prediction or prophecy being made. The prophecy has a constitutive impact on the outcome or result, changing the outcome from what would otherwise have happened.