Lit crit appears actually more objective than many other disciplines, in that when one is looking at the same book in the same printing of the same edition together with other people, one is sharing the same data with others (at least in recent literature -- older works generate more fascinating difficulties in terms of editing, attribution, etc. than recent ones do, before lit critics can agree on what data to be studied); disagreements mainly lie in interpretation of the same data. In economics, sociology, psychology, etc., researchers often disagree with one another upon what data are relevant, how they should be collected, etc. It's much harder to be "on the same page," so to speak.
Now, saying that psychology is a social science like economis, sociology, etc. doesn't solve the problem here. Historical materialism, first of all, asks what were the historical conditions of emergence of such disciplines as "economics," "sociology," "psychology," etc. as well as of their objects of study (e.g., "the market," "civil society," "abstract individual," etc.). Empirical data gathered in each discipline are sometimes of interest to historical materialists, but such data for analytical reason are to be relativized within dialectical reason (which asks how these institutions such as "psychology" arose and what roles they have played in history). "Psychology" didn't always exist, just as wage labor didn't always exist. The "Unconscious," "psyche," etc. are variants of a notion of universal and timeless human nature (like the idea of the soul in Christianity), and, for this reason, Marxists should not think of them as _explanatory tools_; on the contrary, the emergence and trajectory of such concepts is _what is to be explained historically_.
Yoshie