The elementary unit of social life is the individual human action. To explain social institutions and social change is to show how they arise as the result of the actions and interaction of individuals. This view, often referred to as methodological individualism, is in my view trivially true. (13)Max Weber is often identified as the modern originator of theory of methodological individualism. (Weber's student Joseph Schumpeter was the first to use the concept in print.) Weber's reason for advocating for MI derived from his view of action as purposive behavior, and his view that social outcomes need to be explained on the basis of the purposive actions of the individual actors who constitute them. So MI began with a presupposition about the unique importance of rational-intentional behavior in social life. Weber insisted on a rational actor foundation for the social sciences. And this prepared the ground for a joining of forces between methodological individualism and rational choice theory.
The emphasis on methodological individualism sometimes reflected a strong disposition towards eliminative reductionism with respect to social entities and properties: the early twentieth century exponents like J.W.N. Watkins wanted to find logical formulations through which social terms could be eliminated in favor of a logical compound of statements about individuals. And what was the motivation for this effort? It appears to be a version of the physicist’s preference for reduction to ensembles of simple homogeneous "atoms" transported to the social and behavioral sciences. This demand for reduction might take the form of conceptual reduction or compositional reduction. The latter takes the form of demonstrations of how higher level properties are made up of lower level systems. The conceptual reduction program didn't work out well, any more than Carnap's phenomenological physics did.
In addition to this bias derived from positivist philosophy of science, there was also a political subtext in some formulations of the theory in the 1950s. Karl Popper and JWN Watkins advocated for MI because they thought this methodology was less conducive to the "collectivist" theories of Marx and the socialists. If collectivities don't exist, then collectivism is foolish.
Another phase of thinking was more ontological than conceptual. These thinkers wanted to make it clear that social things, causes, and structures depended on the activities of individuals and nothing else. Another way of putting the point is to say that social entities are composed of ensembles of individuals and nothing else. Their concern was to avoid the social analog of vitalism -- the idea in the life sciences that there is some special "sauce" of life activity that is wholly independent from the molecular and physical structures that make up the organism. Essentially this crowd wants to hold that the properties of the whole are fixed solely and completely by the physical structures that make it up. The theory of supervenience pretty well captures this ontological position: no differences at the upper level without some difference at the lower level. (This position doesn't imply its converse statement: if two physical systems differ then their upper-level systems must differ too. This is the point of multiple functional realizability.) The position does rule out some forms of emergentism, however. The idea of microfoundations comes into this line of thought. If we make a claim about the structural or causal properties of an upper-level thing, we need to be confident that there are microfoundations that would show how this feature comes about. In the strongest case, we need to actually provide the microfoundations.
There is another important stream of MI thinking that derives from a set of ideas about how higher-level facts ought to be explained: they should be explained on the basis of demonstrations of how the upper-level entity is given its properties by the organized system of elements from which it is comprised. This is essentially what the analytical sociologists seem to demand, by insisting on the logic of Coleman's boat. This approach privileges a certain kind of explanation--constructive or compositional explanations.
There is one aspect of the tradition that I haven't mentioned yet: the idea that we can carve out the individual as separate from and prior to the social -- a view sometimes referred to as "atomistic". In classical physics the analogous claim is supportable. Sodium atoms are homogeneous and interchangeable. But it is not plausible in the human world. Social facts intertwine with the mind and actions of individuals all the way down. So from the start, it would seem that the program of MI should be formulated in terms of reduction from the big-social to the small-social, not the non-social.
So what kinds of social claims do these various formulations rule out?
All of them rule out spooky holism, those social theories that claim that social entities exist that are wholly independent of the features of individuals.
Several of them rule out strong emergentism -- the view that there are social properties that could not in principle be derived from full knowledge about the states and properties of the constituent individuals.
They by and large rule out explanatory autonomy for the social level. This is the idea that there might be fully satisfactory causal arguments that proceed from statements about the properties of one set of social factors and the explains another set of social outcomes on this basis. (The ontological thesis does not have this implication.)
As Heath argues in his SEP essay, they rule out macro-level statistical explanations and what he calls micro-level sub-intentional explanations.
In my view, the only claims about methodological individualism that seem unequivocally plausible today are the ontological requirements -- the various formulations of the notion that social things are composed of the actions and thoughts of individuals and nothing else. This implies as well that the supervenience claim and the microfoundations claim are plausible as well.
But to concede that x's are composed of y's does not entail the need for any kind of reductionism from x to y. And this extends to the idea of explanatory reduction as well. So methodological individualism does not create valid limits on the structure of social explanations, and meso-level explanations are not excluded.
So it seems as though we can now draw several conclusions about the field of methodological individualism. The ontological thesis is roughly true, but it is compatible with a range of different ideas about within- and cross-level explanation. So reductionism doesn't follow. The micro-level can't be a hypothetical pre-social or non-social individual. Finally, there is no reason to associate the plausible core of MI theory with one specific theory of action, the rational-intentional theory. As pragmatist sociologists are now arguing, there are compelling theories of the actor that do not privilege the model of conscious deliberative choice.