The basics of it are common sense. As infants we "internalize" our parents as emotional, mostly unconscious "images," and those affect us for the rest of our lives. And we always seek satisfaction of our drives in relation to others. In other words, the intersection between nature and nurture.
Now imagine a world in which women denigrate men (feminism) and men denigrate women (quite a lot of the Manosphere). What happened to these kids growing up to create people like this?
"Splitting" refers to seeing yourself as good and the other as bad. Sometimes it refers to seeing yourself as bad as the other as good. It's the basis of narcissism.
So what happens to infants, both male and female, if, as is common today, the father is weak or missing, and single mothers are automatically incompetent? Or what happens if the parents treat the girl as if she is a princess (say lead her to believe she is invulnerable) and what happens if the boy is in the same way sheltered too much?
The research on these problems is extensive.
This article is from Wikipedia.
"Object relations theory in psychoanalytic psychology is the process of developing a psyche in relation to others in the environment during childhood. Based on psychodynamic theory, the object relations theory suggests that the way people relate to others and situations in their adult lives is shaped by family experiences during infancy. For example, an adult who experienced neglect or abuse in infancy would expect similar behavior from others who remind them of the neglectful or abusive person from their past (often a parent). These images of people and events turn into Objects in the subconscious that the person carries into adulthood, and they are used by the subconscious to predict people's behavior in their social relationships and interactions.
"Internal Objects are formed by the patterns emerging in one's repeated subjective experience of the caretaking environment, which may or may not be accurate representations of the actual, external others. In the theory, Objects are usually internalized images of one's mother, father, or primary caregiver, although they could also consist of parts of a person such as an infant relating to the breast or things in one's inner world (one's internalized image of others). Later experiences can reshape these early patterns, but Objects often continue to exert a strong influence throughout life. Objects are initially comprehended in the infant mind by their functions and are termed "part objects." The breast that feeds the hungry infant is the "good breast," while hungry infant that finds no breast is in relation to the "bad breast." With a good enough facilitating environment, part object functions eventually transform into a comprehension of whole objects. This corresponds with the ability to tolerate ambiguity, to see that both the "good" and the "bad" breast are a part of the same mother figure.
"Melanie Klein's work tends to be most commonly identified with the terms "object relations theory" and "British object relations," at least in contemporary North America, though the influence of 'what is known as the British independent perspective, which argued that the primary motivation of the child is object seeking rather than drive gratification', is becoming increasingly recognized. Klein felt that the psychodynamic battleground that Freud proposed occurs very early in life, during infancy. Furthermore its origins are different from those that Freud proposed. The interactions between infant and mother are so deep and intense that they form the focus of the infant's structure of drives. Some of these interactions provoke anger and frustration; others provoke strong emotions of dependence as the child begins to recognize the mother is more than a breast from which to feed. These reactions threaten to overwhelm the individuality of the infant. The way in which the infant resolves the conflict, Klein believed, is reflected in the adult's personality.
"Fairbairn took a radical departure from Freud by positing that humans were not seeking satisfaction of the drive, but actually seek the satisfaction that comes in relation to real others. Klein and Fairbairn were working along similar lines, but unlike Fairbairn, Klein always held that she was not departing from Freudian theory, but simply elaborating early developmental phenomena consistent with Freudian theory.
"Fairbairn revised much of Freud's model of the mind. He identified how people who were abused as children internalize that experience. Fairbairn's "moral defense" is the tendency seen in survivors of abuse to take all the bad upon themselves, each believing he is morally bad so his caretaker can be regarded as good. This is a use of splitting as a defense to maintain an attachment relationship in an unsafe world."
The entire Wikipedia article is HERE.