Emerging from social networking in early 2008 (though they've literally been around as long as the Internet), Multiple Systems, also known as Median Systems, are individuals who claim (falsely) to have a form of multiple personality syndrome, with said personalities being known as "Headmates." Those who claim to be Multiple Systems often claim to be such for attention, or to claim they are an oppressed minority. It is also known as Multiplicity or Plurality.
The most common definition of a Multiple System, taken directly from the Living Plural tumblr, is "A group of people sharing the same body, while still being individuals with their own personalities and interests." Those who hear this term can be forgiven, given this description, of accidentally conflating a Multiple system with Disassociative Identity Disorder, Multiple Personality Disorder, Schizophrenia, or Delusional Mania or Bipolar Mania. Proponents of Multiplicity espouse that their condition is a medical diagnosis, entirely separate from DID/MPD, Schizophrenia, or Delusional/Bipolar Mania, which they often refer to as "Trauma-Based Multiplicity."
Multiple/Median Systems refer to their alternate personalities as "Headmates." These headmates can be quite literally anything, from animals and plants to video game and television characters (Fictives), inanimate objects, and even real-life people (Factives). There is considerable overlap with the Otherkin commmunity. As quite literally anything can be a Headmate, and many such Multiple Systems invent new ones and discard old ones via convenience, there can be little question that the term is entirely a fabrication.
Are Multiple Systems/Headmates Real?
There has never been a single credible, peer-reviewed study conducted by reputable scientists proving the veracity of any claim espoused by the Multiple System community. Indeed, a number of Multiple Systems that have been closely-analyzed by professionals indicate that they are, as many have suspected, essentially faking their symptoms for attention.
A fascinating peer-reviewed research paper on Multiplicity showed the clear disconnect in the Multiple System community regarding psychiatric analysis; all reputable psychiatrists espouse integrating any splinter personalities when dealing with someone with MPD/DID in order to enable them to live a life with fewer problems down the line; such integration, to a Multiple System, would constitute murder.
The Multiple System community presents a distressing danger to those with legitimate psychatric issues. The danger is twofold:
First and foremost, Schizophrenia, Disassociative Identity Disorder, Bipolar Mania, and so on are very real conditions, with massive - often crippling - consequences for those who have them. In many cases, this condition has utterly destroyed their lives. The Multiple System community has essentially piggybacked onto a very real and extremely serious mental health condition, and does so, primarily, for attention. This marginalizes people with very real and often life-threatening conditions, essentially repackaging falsified mental health concerns as trendy, cool things to have, ignoring the staggering consequences these people face on a day-to-day basis.
Second, it poses a terrifying threat should people with these aforementioned conditions wander into the Multiple System community via Tumblr or the like. Suddenly, the guy with Delusional Mania who has been hearing the voice of God in his mind is being told that - yes, the voice is God, and they should totally do what that voice in their head's telling them to do. Suddenly, a person with very real and terrifying DID is being told how amazing and wonderful their outright crippling condition happens to be. Even under the best of circumstances, these people are being told how their condition is an awesome thing, potentially causing them to forego treatment entirely, and rendering them a threat to themselves and others.