This article is from the Dissociation FAQ, by Discord (email@example.com) with numerous contributions by others.
In many (if not most) cases, severe trauma at an early age (by early, we
mean before, say age 5), although there are definitely exceptions to this.
Please bear in mind that "severe" is an entirely subjective term.
In people who developed their multiplicity as a result of trauma, the
trauma was very likely some sort of abuse: physical, sexual, psychological,
emotional, or religious/magickal. Usually this abuse began at a very
early age and was long-standing.
Other trauma, such as witnessing a death, or the abuse of someone close
to you, or possibly just living in a family where there is a great deal
of negative emotion expressed, could be sufficient.
Some people who developed multiplicity as a child did so because they had
a dissociative role model, perhaps a parent.
Some people explore identity or alternate identity games, like role
playing, acting, pretending, or alternate social structures to the
point where they begin to question their original identity. In some
cases, these identities can take on aspects, experiences, and problems
which are essentially identical to those experienced by multiples who
experienced trauma. This effect seems to be most prevalent in people
who explore in their adolescence.
Some multiples are unaware of any initial trauma. Whether such trauma
exists and the memories blocked or whether no such trauma exists is
usually impossible to determine.
It is also possible to partition one's adult life so rigidly as to
create the effect of multiplicity. This sometimes happens if the
person uses multiple names, has multiple residences, multiple jobs,
multiple social contexts, or simply separates work and social life
distinctly. After a period of time, each facet grows its own
relationships, experiences, and skills which may or may not overlap
into other life contexts. This rigid partitioning can also closely
resemble the experience and presentation of multiplicity.