First used scientifically by Freud, overdetermination refers to a non-reductionist account of human experience and believes that the constitution of each thing is given by the totality of cause and effects that give it instantiation. In Freud, that includes things not remembered, elements of the sub-conscious that act without being recognised. To understand something we can not just look at the most visible and obvious causes, and overdetermination is a counter to these totalities of simple determination. Representation is problematised by the presence of non-visible or not articulated sentiments must be included in the cognition of various aspects of social life, especially subjectivity.

More generally overdetermination refers to multiple coexistent and complexly integrated structures, non-transparent modes in which certain types of activity express themselves. It is a counter to the idea of single determination of a whole and in its structuralist use is a counter to functionalist ideas of static harmonious wholes. Thus overdetermination is used to refer to multiple causality - or for instance in Freud a hysterical symptom or in Althusser a revolutionary situation – or just the social itself.

In Althusser the need for a concept of overdetermination appears in Marxism at the point where the ideology of simple and general contradictions is undermined by historical process. "If, as in this situation, a vast accumulation of 'contradictions' comes into play in the same court, some of which are radically heterogeneous – of different origins, different sense, different levels and points of application – but which nevertheless 'merge' into a ruptural unity, we can no longer talk of the sole, unique power of the general 'contradiction'... This means that if the 'differences' that constitute each of the instances in play (manifested in the 'accumulation' discussed by Lenin) 'merge' into a real unity, they are not 'dissipated' as pure phenomena in the internal unity of simple contradiction. The unity they constitute in this ‘fusion’ into a revolutionary rupture is constituted by their own essence and effectivity, by what they are, and according to the specific modalities of their action. In constituting this unity, they reconstitute and complete their basic animating unity, but at the same time they also bring out its nature: the 'contradiction' is inseperable from the total structure of the social body in which it is found, inseperable from its formal conditions of existence, and even from the instances it governs; it is radically affected by them, determining, but also determined in one and the same movement, and determined by the various levels and instances of the social formation it animates; it might be called over-determined in its principle."

"I am not particularly taken by this term overdetermination (borrowed from other disciplines), but I shall use it in the absence of anything better, both as an index and as a problem, and alos because it enables us to see clearly why we are dealing with something quite different from the Hegelian contradiction." - from For Marx p.100-101 (Verso)

N.B – ‘ruptural unity’ – what is such a thing? Maybe this is a quantum idea. Hmmm.

The concept of overdetermination has been used by Resnick and Wolff in the formation of a non essentialist marxian political economy.

Reading

Contradiction and Overdetermination; notes for an investigation – Louis Althusser (in For Marx) Read (Marxists.org)