the course of ego-development and object-relations depends on the degree to which an optimal balance between introjection and projection in the early stages of development can be achieved. This in turn has a bearing on the integration of the ego and the assimilation of internal objects.
Klein argues that the balance between introjective and projective processes is foundational to ego integration and healthy object relations, with any excess in either direction producing developmental disturbance.
, Envy and Gratitude and Other Works 1946-1963, 1957thesis