Actuary X is correct, within the modern definition of
idealism, where the
opposite of idealism is either
pragmatism or
cynicism (which is another
noble tradition perverted by modern usage). However, the proper opposite of
idealism is
materialism.
An idealist, in the classical usage, is one who puts ideas over and above the material world. The origin of this view is in Plato's ideal forms. Despite the demise of this definition in common usage, it is still more or less true of most people and groups that are commonly described as idealistic. When you consider the repercussions of this view in the material world, the excesses and downsides of excessive idealism, in either the classical or modern sense, become clear. This, I believe, is what Dman sees in idealistic groups. If idealism is taken in its pure form, it can be extremely destructive. Noble ideas are a piss-poor substitute for a full belly and a warm bed.
Materialism, on the other hand, sees the world as only a collection of physical objects. I won't belabor the downsides of pure materialism -- it's off-topic for this node and for the most part a collection of truisms. Money and things are a piss-poor substitute for such things as love, justice, and fulfillment. One of communism's worst failings is that it is consciously materialistic -- Karl Marx called his way of analyzing the world dialectical materialism. Human ideals and emotions are discounted completely in classical Marxism and, to the degree practical, in communist practice.
The sensible path is the middle one, pragmatism. Pragmatism is, when not a code-word for pure materialism, a sensible blending of the materialist and idealist conceptions of the world. This is the point to which capitalism has evolved in practice. Both idealism and materialism have an enormous amount to recommend them, and much to hate. An ad-hoc blending provides the most livable societies.
To respond, however quickly, to
wharfinger:
Rational self-interest does exist often enough to be statistically significant. The problem that I believe that you are referring to is the distinction between what appears rational at the time to the person in question and what a true dis-interested observer. People will, in almost all cases, behave with what is rational self-interest according to their knowledge. It is common to be short on information, or to have incorrect information. I'm defining information here in a very broad sense, to include personal beliefs, past experience, and the whole ball of wax that makes up our brains.
The problem of incomplete and incorrect information is why rational self-interest does not save the world automagically.