Foundation series
Psychohistory has one basic, underlying limitation which Asimov postulated for the first time on the last page of the final book in the "Foundation" series: psychohistory only functions in a galaxy populated only by humans. In Asimov's Foundation series, humans form the only sentient race that developed in the entire Milky Way Galaxy. Seldon developed psychohistory to predict the actions of large groups of "humans". Even robots technically fall under the umbrella of psychohistory, because humans built them, and they thus represent more or less a human "action", or at least, possess a thought-framework similar enough to that of their human creators that psychohistory can predict their actions. However, psychohistory cannot predict the actions of a sentient alien race; their psychology may differ so much from that of humans that normal psychohistory cannot understand or predict their actions.