A study of the structure of power was conducted by social psychologists John R. P. French and Bertram Raven in 1959.
They divided the concept of power is into five separate and distinct forms: coercive, reward, legitimate, referent, and expert.
In 1965 Raven revised this model to include a sixth form: informational.
For an individual to have "expert power" the need to have what others perceive as having knowledge, experience, and special skills or talents. Expertise can be established by actions, formal credentials, and reputation.
Since 1965 this theory has been expanded to include the belief that coercion and reward can have personal as well as impersonal forms. Expert and referent power can be both negative and positive. Legitimate power, in addition to position power, may be based on other normative obligations such as reciprocity, equity, and responsibility.