This is the idea that the tools of computer-based simulation of the aggregate consequences of individual behavior can be a very powerful tool for sociological research and explanation.

Mechanisms rather than regularities or necessary/sufficient conditions provide the fundamental grounding of causal relations and need to be at the center of causal research. This approach has several intellectual foundations, but one is the tradition of critical realism and some of the ideas developed by Roy Bhaskar (link).

Hedström and others in the AS field are drawn to a broad version of rational-choice theory -- what Hedström calls the "Desire-Belief-Opportunity theory". This is a variant of rational choice theory, because the actor's choice is interpreted along these lines: given the desires the actor possesses, given the beliefs he/she has about the environment of choice, and given the opportunities he/she confronts, action A is a sensible way of satisfying the desires.

First, there is the idea that social outcomes need to be explained on the basis of the actions of individuals. Hedstrom, Demeulenaere, and their colleagues refer to this position as methodological individualism. It is often illustrated by reference to "Coleman's Boat" in James Coleman, Foundations of Social Theory (Coleman, 1990, 8) describing the relationship that ought to exist between macro and micro social phenomena (link). The boat diagram indicates the relationship between macro-factors (Protestant religious doctrine, capitalism) and the micro factors that underlie their causal relation (values, economic behavior).

provide explanations of social outcomes of interest based on the mechanisms that produce them; identify mechanisms at the level of the actors who make up those outcomes strive for realism in assumptions and hypotheses be pluralistic about theories of actor motivation and decision making processes; and build explanations from individuals to social outcomes.