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In order to understand the nature of the intertwin relationship, certain psychological processes have been postulated. These processes, peculiar to twins (and possibly to other children of multiple births), are concurrent and frequently conflict with the usual developmental processes. Commencing with a state of psychological syncytium, a gradual transition takes place as the infants begin to distinguish between self and nonself: primary intertwin identification occurs as a result of visual incorporation. Constant confrontation with a mirror image retards differentiation of self and twin, and inhibits clear delineation of body boundaries. The continuing primary intertwin identification resembles in effect the continuation of primary identification with the mother in so far as it prolongs the mutual interdependence, brings about retardation of ego identity, and interferes with object relations to others.
Ordinarily these difficulties appear to be overcome in the course of development. However, in some instances when the primary intertwin identification is emphasized by one or more of the four factors—the cultural attitude, the parental attitude, the degree of similarity, and economic pressures—the effect may be pathogenic. Implicit to this conclusion is the assumption that prevention of pathogenic development must occur through measures taken to counteract the influence of those four factors.