In The Language of Psychoanalysis, Jean Laplanche and Jean-Bertrand Pontalis give the following definition of complemental series: "Term used by Freud in order to account for the aetiology of neurosis without making a hard-and-fast choice between exogenous and endogenous factors. For Freud these two kinds of factors are actually complementary—the weaker the one, the stronger the other—so that any group of cases can in theory be distributed along a scale with the two types of factors varying in inverse ratio. Only at the two extremities of such a serial arrangement would it be possible to find instances where only one kind of factor is present" (1967).
The concept is most clearly explained by Freud in the Introductory Lectures on Psychoanalysis (1916-17a [1915-17], p. 347 and note). The endogenous factor corresponds to the fixation points specific to each person (and determined by that person's hereditary constitution and childhood experience), while the exogenous factor corresponds to frustration. Freud returned to this question, apropos of trauma, in the last part of Moses and Monotheism (1939a [1934-38]): "In this way we reach the concept of a sliding 'complemental series' as it is called, in which two factors converge in fulfilling an aetiological requirement. A less of one factor is balanced by a more of the other" (p. 73).
The concept of "complemental series" thus appears in Freud's work in relation to two key themes, neuroses and traumas, a fact that underscores its importance. It would be interesting to look at this concept from the standpoint of Freud's renunciation of his neurotica in 1896, that is to say, the change he introduced in the etiology of the neuroses from the theory of a real trauma to that of an imagined trauma.
As we know, Sigmund Freud may never have abandoned the theory of the real trauma, and there is a sense in which the concept of "complemental series" testifies to the very real effort he made to reconcile internal and external factors and thus transcend the opposition between external reality and psychic reality. In a way the complemental series foreshadows our more modern and still debated polyfactorial model of pathological etiology.
See also: Constitution; Internal/external reality; Psychic causality; Psychogenesis/organogenesis.
Freud, Sigmund. (1916-17a [1915-17]). Introductory lectures on psycho-analysis. SE, 15-16.
——. (1939a [1934-38]). Moses and monotheism: three essays. SE, 23: 1-137.