Overdetermination, the idea that a single observed effect is determined by multiple causes at once (any one of which alone might be enough to account for the effect), was originally a key concept of Sigmund Freud's psychoanalysis.
Freud wrote in The Interpretation of Dreams that many features of dreams were usually "overdetermined," in that they were caused by multiple factors in the life of the dreamer, from the "residue of the day" (superficial memories of recent life) to deeply repressed traumas and unconscious wishes, these being "potent thoughts". Freud favored interpretations which accounted for such features not only once, but many times, in the context of various levels and complexes of the dreamer's psyche.
The concept was later borrowed for a variety of other realms of thought.
In contemporary analytic philosophy an event or state of affairs is said to be overdetermined if there are two or more distinct, sufficient causes of it. Whereas there may unproblematically be recognised many different necessary conditions of the event's occurrence, no two distinct events may lay claim to be sufficient conditions, since this would lead to overdetermination. A much used example is that of firing squads, the members of which simultaneously firing at and 'killing' their targets. Apparently, no one member can be said to have caused the victims' deaths, since they would have been killed anyway. Overdetermination is problematic in particular from the viewpoint of a standard counterfactual understanding of causation, according to which an event is the cause of another event if and only if the latter would not have occurred, had the former not occurred. In order to employ this formula to actual complex situations, implicit or explicit conditions need to be accepted to be circumstantial, since the list of counterfactually acceptable causes would otherwise be impractically long (e.g. the earth's continued existence could be said to be the (necessary) cause of one drinking one's coffee). Unless a circumstance-clause is included, the putative cause to which one wishes to draw attention could never be considered sufficient, and hence not comply with the counterfactual analysis.
The Marxist philosopher Louis Althusser imported the concept into Marxist politics in an influential essay, "Contradiction and Overdetermination". Drawing, in an unusual combination, from both Freud and Mao Zedong, Althusser used the idea of overdetermination as a way of thinking about the multiple, often opposed, forces active at once in any political situation, without falling into an over-simple idea of these forces being simply "contradictory." Brewster, in Althusser et al's Reading Capital defines overdetermination as such:
"the representation of dream thoughts in images privileged by their condensation of a number of thoughts in a single image (condensation), or by the transference of psychic energy from a particularly potent thought to apparently trivial things ... [For Althusser] overdetermination of a contradiction is the reflection in it of its conditions of existence within the complex whole."
Thus, for Althusser's reiterating of Marxist thought, overdetermination is what [concealed/unaccepted] "determinant contradictions", or capital-economic incongruities (i.e., abstract labour resulting in "isolation" -- the class struggle), which are analogous to Freud's "potent thoughts", apply to instances that are more really, slight, understandable. An instance of a popular riot calling for revolution could exemplify this. The event has to it, in capitalist culture, an over-application (determination) of agitation. The determinant contradictions (the reasons for popular revolt) are not addressed and so their great mass is "displaced" onto the singular event.
Another conception of overdetermination is present in the later writings of Jean Baudrillard. His derives from Althusser's conception, but breaks with it just as Baudrillard breaks with structural Marxism in general.