Agency is a concept used in philosophy and sociology to refer to the capacity of an agent to act in a world. In philosophy, the agency is considered as belonging to that agent even if that agent represents a fictitious character, or some other non-existent entity. The capacity to act does not at first imply a specific moral dimension to the ability to make the choice to act, therefore moral agency is a distinct concept. In sociology, an agent is an individual engaging with the social structure; the structure and agency debate concerning the level of reflexivity that agent may possess.
Human agency is the capacity for human beings to make choices and to impose those choices on the world. It is normally contrasted to natural forces, which are causes involving only unthinking deterministic processes. In this respect, agency is subtly distinct from the concept of free will, the philosophical doctrine that our choices are not the product of causal chains, but are significantly free or undetermined. Human agency entails the uncontroversial, weaker claim that humans do in fact make decisions and enact them on the world. How humans come to make decisions, by free choice or other processes, is another issue.
The capacity of a human to act as an agent is personal to that human, though considerations of the outcomes flowing from particular acts of human agency for us and others can then be thought to invest a moral component into a given situation wherein an agent has acted, and thus to involve moral agency. If a situation is the consequence of human decision making, persons may be under a duty to apply value judgments to the consequences of their decisions, and held to be responsible for those decisions. Human agency entitles the observer to ask should this have occurred? in a way that would be nonsensical in circumstances lacking human decisions-makers, for example, the impact of comet Shoemaker-Levy on Jupiter.
In certain philosophical traditions (particularly those established by Hegel and Marx), human agency is a collective, historical dynamic, rather than a function arising out of individual behavior. Hegel's Geist and Marx's universal class are idealist and materialist expressions of this idea of humans treated as social beings, organized to act in concert.
Structure and agency forms an enduring core debate in sociology. Essentially the same as in the Marxist conception, "agency" refers to the capacity of individuals to act independently and to make their own free choices, whereas "Structure" refers to those factors (such as social class, but also religion, gender, ethnicity, subculture, etc) which seem to limit or influence the opportunities that individuals have.
Bandura, A. (2001). Social cognitive theory: An agentic perspective. Annual Review of Psychology, 52, 1-26. The Agency (1956). Describes the form of agency.