Human life is composed of a continuous flow of activity. Besides the infinite variety of overt actions and expressions that impact the social and physical environment, it also has a more covert side in the mental activities of experiencing, perceiving, thinking, feeling, and imagining. These mental activities are part of the flow, although they cannot be observed directly by others and have no direct impact on the environment. The scope of human activity thus ranges from dreaming (Klinger, 1971) to preplanned, intentional acts. The psychology of motivation is specifically concerned with activities that reflect the pursuit of a particular goal and, in this function, form a meaningful unit of behavior. Motivational research seeks to explain these units of behavior in terms of their whys and hows.

Questions pertaining to the whys of human activity address its purposes from a variety of perspectives; for example:

• Can different units of behavior be assigned to one and the same class of goals and differentiated from other classes of goals?

• How do these classes of goals evolve in the course of an individual's development, and which individual differences exist in this regard?

• Why is it that specific situational conditions prompt people to choose certain goal-oriented activities over others, and to pursue them with a certain amount of time and energy?