Fairbairn gave splitting a structural role. In Endopsychic Structure Considered in Terms of Object-Relationships3 (1944) he traced the basic situation of the mind to the infant's frustration by its mother. The infant splits her into good and bad and internalises the unsatisfying object in order to control it. It then splits that object into a tempting side and a rejecting side and represses both, and the parts of the ego still attached to each are split off with them. The result is a central ego that represses two subsidiary egos: a libidinal ego bound to a needed or exciting object, and an internal saboteur, later the antilibidinal ego, bound to a rejecting object (see endopsychic structure and libidinal ego).
For Fairbairn the splitting of the ego and repression are one process, and its dynamic is aggression. Expressing hatred risks losing the good object, and expressing love to a rejecting mother risks humiliation. So the child turns a maximum of aggression against a maximum of libidinal need, a technique he called divide et impera. He read the dissociations of hysteria as splits of the ego and placed the schizoid position at the centre of psychopathology. His collected Psychoanalytic Studies of the Personality4 (1952) adds the moral defence, in which the child takes the bad aspects of its parents into itself so that the outer world can remain good.
Herbert Rosenfeld defined a pathology by the failure of splitting. In Notes on the Psychopathology of Confusional States in Chronic Schizophrenias5 (1950) he described confusional states as the outcome of a breakdown in the normal, structuring split between good and bad objects, and between the loving and hating parts of the self. When the split fails, love and hatred become mixed, and the patient fears that destructive impulses will overwhelm loving ones. Where envy predominates, the good object itself is attacked, so that good and bad can no longer be told apart. Rosenfeld placed confusion between splitting and integration. Klein took up the idea in Envy and Gratitude6 (1957).
Wilfred Bion, working within Klein's account of the paranoid-schizoid position, carried splitting further in Attacks on Linking7 (1959). What the psychotic part of the personality splits, empties and destroys is not only the object but whatever it feels to be a link between two objects. That link may be the breast between infant and mother, the parents in intercourse, the words between patient and analyst, or finally the patient's own capacities for thought, emotion and perception. Because emotions themselves link one object to another, they too come under attack.
Donald Meltzer distinguished a neighbouring process. In Explorations in Autism8 (1975) he described dismantling, in which attention lapses and the self comes apart along the lines of the senses. Like splitting it breaks experience apart, but it is passive rather than sadistic, and the senses come together again easily once attention returns.
Kernberg brought splitting into the diagnosis of personality disorders. In Borderline Personality Organization9 (1967) he argued that borderline patients do not occupy an unstable zone between neurosis and psychosis. Instead they share a specific and stable pathological organisation, whose central defence is splitting. Contradictory ego states and introjections are actively kept apart so that aggression does not contaminate the good images. Splitting is supported by primitive idealisation, projective identification, denial, and omnipotence with devaluation, and it feeds ego weakness in a vicious circle. Kernberg traced the organisation to excessive pregenital, especially oral, aggression, and linked the resulting failure to integrate images of self and others to identity diffusion. The paper joined Klein's ideas on splitting and projective identification to Edith Jacobson's ego psychology of self and object representations.
In Borderline Conditions and Pathological Narcissism10 (1975) Kernberg described the split material as units of self-image, object-image and affect. In borderline patients, early aggression prevents the loving and hating images of self and others from being brought together.