Introject

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Introjects are headmates who have an original source that their personality, identity, and memories may be based partially or fully on. They can join a system for any number of reasons: having similar traumas to an individual, seeing strengths in the individual that the collective would benefit from, etc.

The term can be considered medical terminology and as such may not be used by every system. It should be checked first if a system is comfortable being referred to as such or not.

Related Terms
Terms like tive, intive, and introtive were coined to be used as an umbrella term without the medical associations. Outsourced may also be used similarly.

Fictives are headmates with a fictional source, and factives are headmates with a non-fictional source.

Autojects are any introjected headmates from sources the system themself have created. Generally, any term ending in -tive/-ive or -ject can be specific kinds of introject, like paratives or altives.

See the whole list of outsourced and introject terms here.

History
"Introjection" is a concept used widely in psychology, describing the process of internalizing external qualities of someone or something, including values, attitudes, truths, or other parts of one's personality. This can occur for any number of reasons.

This concept is what the term "introject" is based on; the process of incorporating outside qualities of an individual (known or unknown) into the collective's mind. Not much research has been done on fictives, factives, or other forms of introjects, besides in cases of ritual abuse purposely creating them.