No, headmate was not created by MPD/DID systems.

lb-lee:

I’ve done some cursory digging into the term ‘headmate’ and I find Astraea
referencing it in their glossary in 2008, as specifically from
soulbonders, but I can track it back earlier without too much trouble.

For instance, someone in the soulbond_domain
comm on LJ uses it in July 16, 2006.  I’m willing to bet I can find
earlier than that, on say, the old soulbonding comm, but the dates are
so far back, I’d have to plow through each entry individually over the
course of like four years, at the height of the comm’s popularity.

I’m finding a good few LJs, both comms and individual, that list ‘headmate’ in their interests from ‘07 and ’08, (link here
for those who want to retrace my steps) and how convenient, a lot of
them just never updated after a month or too!  All of them seem to be
soulbonders, though I also see mention of ‘daemons,’ which might be a
His Dark Materials thing?  Anyway, more of these folks start showing
membership in non-SB multi communities, but these are LJs that died in
2010 or later, and no telling when they last updated their interests.

AHA!  Found another comm post for someone using the term headmate– this time from July 20, 2005.
 The system using it IDs as a multiple system, and it looks from their
profile that they were a “natural” multiple.  Interesting twist!  I also
see it from what looks to be a soulbonder on the 31st of July, same
year, here.
 I’m not finding a core source, but a lot of the old soulbonding sites
seem to have died, unfortunately. : That’s probably all I’m going to
find unless I got hardcore on looking this stuff up.

Regardless, anyone claiming that ‘headmate’ was coined by DID systems is lying.  I can find absolutely NO evidence for that.  All the evidence I’ve found (which is admittedly pretty sloppy) points to it coming from soulbonders and natural multiples.  I’m willing to bet that the word is logical enough that it’s cropped up spontaneously in many circles, but regardless, no, DID guys.  We didn’t create it.

Would love to see more research on this, but honestly, plowing through ollllld-ass LJ comms entry by entry is more energy than I’m willing to spend on that.  If someone can track a DID system using the term before July 2005, THEN maybe I’ll believe it, but right now?  No.

–Rogan