Whether someone is an individualist or a collectivist is a matter of psychology, not ideology.
I don't agree with that. Individualism has a clear meaning as an ideology. That does not take away from the fact that a person's psychology can reflect individualistic or authoritarian traits. They are different subject areas despite the abstract rendering of a common theme in each one.
The failure to recognize this can lead to further faulty reasoning, like that shown in saying, "Libertarians are collectivists -- at least some of them." That ignores the fact that acceptance of the principles of libertarianism means being an individualist in the political realm. And with that said, it is a kind of conflation to say that they are collectivists. Collectivism in politics means the initiation of force and is, therefore, contradictory to individualism in politics (respect for individual rights). Fuzzy thinking.
"Individualism regards man—every man—as an independent, sovereign entity who possesses an inalienable right to his own life, a right derived from his nature as a rational being." Ayn Rand, Virtue of Selfishness
In that statement Rand touches on metaphysics, epistemology, morality and politics.
"The mind is an attribute of the individual. There is no such thing as a collective brain." Ayn Rand, For the New Intellectual
In that statement, Rand displays an intellectual orientation towards the individual (as opposed to a group) based upon the mind.
Individualism can be looked at inside the political, the cultural, the moral, the epistomological and the psychological subject areas. If knowledge were imagined to be a large, layered structure of some sort, then "individualism" would be seen as a vertical swath running through many of the horizontal layers.
Individualism is most important in the political realm because it can recognize our individual rights - implementing the moral concept that our lives are an end in themselves. (Or its political importance can be due to collectivism (individualism's absence), for violating those rights - treating individuals' lives as a means to some other end).
Individual rights are, by definition, moral principles. And morality is a more fundamental level than politics - being the level upon which politics rests.
Individualism also exists in parts of the culture quite apart from politics. Are organizations organized around a recognition of individual efforts and results? Are individual choices seen as important?
Epistemologically, individualism arises out of choice - that place where human nature, epistemology and the beginnings of morality intersect. Groups don't choose - individuals do. Choice is an individual mental exercise.
Hopefully, choices are born of sound reasoning, but too often, sick cultures, bad premises, and/or neurotic defensiveness of an individual will encourage emotionalism and a substitute of political correctness and/or peer pressure in place of individualism (independent critical reasoning) in the psychological process of choosing.
So, individualism is also a proper subject for psychology.