I don't think sophisticated Marxian theory (including that of the MANIFESTO, if I remember correctly, which allows individual bourgeois to change class allegiance) pretends to be able to explain the behavior of indivdiduals. It's more of a form of structural analysis, which is partly expressed statistically. More generally, it simply says that each person's "preferences" are highly influenced by their biographies...
However, what about a Freudian theory to explain the behavior of psychoanalysts? (do male shrinks seek out female patients for sexual reasons?)
-- Jim Devine "Segui il tuo corso, e lascia dir le genti." (Go your own way and let people talk.) -- Karl M., paraphrasing Dante A.