As psychology matures, and the understanding of the human mind is expanded, more and more models of typical human development emerge.
It is generally believed that the infant mind is a series of impressions and state-dependencies: being hungry, feeling pain, needing attention, being scared, being fed, being nurtured, being uncomfortable, etc. A human does not form a mature and integrated personality until later in childhood or even in the teen years. Some models even suggest that humans do not gain full agency and self-awareness until their twenties.
When these states are validated, and when caregivers react appropriately to these states being fed when hungry, being comforted during pain, being interacted with and touched when lonely, protected when frightened, etc. a child is given a secure basis to explore personhood and develop ideas, thoughts, give voice to needs and desires, develop preferences, etc. and bring all of these varied states as they change and mature with development into one person-ality.
When the milestones of life are interrupted in any way, from the womb and onwards, with neglect of these needs, with additional threats from caregivers, with physical, mental and emotional violations, with erratic caregiver behaviors, even with floods of anxiety chemicals in the womb itself the child's psyche may not emerge with a single sense of personhood or one person-ality. Each state is left to fend for itself as it were, a trial-by-fire of development of thought, strategy, ideas, and adaptability but shuffled amongst the varied folds of the states. Additional states may be induced in such a tumultuous environment that would not have been needed in a properly nurturing environment: the need to play up to abusers, the need to lie, the need to protect oneself, the need not to cry, etc. Some states may get stuck at various stages of development, unable or unwilling to progress later becoming Littles.
Meanwhile, the struggle for survival continues hampered, unsupported or improperly supported by caregivers and the environment. Humans are survivors if nothing else. The child unwittingly allows (indeed needs to allow) their varied states to adapt to an unpredictable, volatile, unstable, lonely, or threatening situation becoming hyper-vigilant to cues from the environment and other people, while being highly malleable in response to external events and situations.
Multiples may experience this disruption of development exceptionally early in childhood development, or even before being born through anxiety chemicals, drugs, sounds or even physical battery of their mother while in the womb. When experienced "early enough" in development (which can vary from person to person), there isn't even a hint of the "one person" they were potentially to become had the trauma not occurred. The trauma was experienced so young there was never any one person-ality emerging in the first place.
Given this model, one could say we're all born multiple, but when normal development is interrupted we remain multiple and all these various states remain segregated to optimize our chances of survival in a hostile developmental environment, or in an environment where we are best off relying solely on oneself.
Even then, experiencing trauma at a very young age or in the womb does not predispose a person to definitely becoming multiple. There's still the potentially genetic predisposition to dissociation to consider which allows for more compartmentalization of both physical impressions and memories and if there are stable cornerstones of a baby or child's life, they still may develop one or more stronger person-alities that appear to be essentially well-adjusted or "normal".
Other models or explanations of being "born multiple"
Some multiples have had the experience of others sharing the body with them since birth, whether or not they were traumatized in any way. (Some remember being multiple before they were abused, or being abused or punished because they were multiple.)
Many such multiples feel that plurality is simply a natural variant of the human mind-- something which, like sexual orientation or gender identity, is either inborn, genetic, or hardwired in at such an early age as to make changing it later in life impossible.
Others have offered spiritual explanations for their multiplicity. Some see multiplicity as the result of persons from past lives sharing one's current body. Others see it as the result of having channeled someone in before birth.
Other models constructed with contributions from Shiu of Amorpha.