Many people are familiar with Occam's Razor. Not as many as should be, unfortunately, so perhaps a brief introduction is in order.
William of Ockham (ca. 1285-1349) was a mediæval English philosopher and Franciscan monk. William was a minimalist, idealising a life of poverty and battling with the Pope over the issue. He was excommunicated by Pope John XXII. He responded, like all good rant fiends, not by rolling over and saying "mea culpa", but by writing a treatise demonstrating that Pope John was a heretic.
What is known as Occam's razor was a common principle in mediæval philosophy and was not originated by William. Rather, his name became associated with it because of his frequent usage of it.
The principle is succinctly stated as the principle of unnecessary plurality, or "Pluralitas non est ponenda sine necessitae". Plurality should not be posited without necessity. These days it is usually interpreted as meaning "the simpler the better", or "the fewer assumptions required for a supposition, the better that supposition is". Or, as one of my friends once put it, "If it walks like a duck, swims like a duck and quacks like a duck then it is not likely to be Otherkin." It is a principle frequently used outside of ontology to aid in choosing from amongst theories with equal explanatory power. When giving explanatory reasons for something, do not posit more than is necessary. Perhaps Von Daniken is right, and extraterrestrials did teach ancient peoples art and engineering, but we do not need to posit alien visitations in order to explain the feats of ancient people.
Let us look at the evidence in support of our Otherkin friends, or, rather, let us look at what they are attempting to explain by giving themselves non-human selves in human bodies.
The people who refer to themselves as Otherkin of all the various sorts claim a number of things, and I have no real problem with the experiences which are part of that claim. They claim to have insistent self-images that are not human, to have recovered memories of far and distant places very different from this one, to have independently similar memories of places like the Elf homeworld, to suffer from discomfort when in contact with iron, to communicate using telepathy, to have exceptional empathic senses, to be taller than average, to have slightly differing physical make-up in some cases, to be able to travel to Other realms and communicate with Other beings, to have a sense of having lived before in many other lives, to have difficulties blending in and understanding people, to find themselves performing involuntary actions that are not human in nature, and to sometimes be able to recognise others of a similar nature to themselves at a glance.
So let us look at the assumptions that have to be made in order to partake of this explanation.
This is just a preliminary list. Given time, I am sure there are lots more assumptions that are inherent in this particular explanation.
Now. I am not one to deny the validity of psychic experiences, nor the existence of other realms in this world, nor even the existence of another phase world, so I will ignore those assumptions, despite having little empirical evidence other than personal experience, and I often have doubts about that (a good thing, as doubt is the foundation of all learning). I will not question the existence of other races, nor that it is possible to travel between these worlds (although not using the principle contained within the appalling physics I have seen touted as an explanation).
I will offer some resistance to the idea that it is possible for one of these Others to live in a human body. Not only because most humans take up all the space within themselves and then some quite successfully, but because I have never met an Other who would want to confine himself and his abilities to flesh in such a way. I will let this one pass, however, because self-image and belief is everything in the subjective universe and thus if they wish to consider theselves to be Others in human form, then that is what they can be. They are entitled to that belief (although belief is the ultimate barrier to learning).
However. I do object to the idea that any such Other would be able to mimic humans so successfully that humans would not notice. Even observant ones, and I am observant. These Otherkin seem to be astonishingly human even when attempting to prove that they are not, and this is most strange if they are indeed what they say they are. The last assumption I will also let slip, for the time being, for state specific memory can be a real problem - although I will mention now that not one of the Other I have ever come across has ever had any difficulty. Mainly because the whole phenomena of change of state of consciousness is a peculiarly human, and most definitely this world thing. I also point out that one of the explanations for their presence here has been given as study. They are here to study us. If this truly is state specific memory, then what use will it be, for it will be difficult for them to remember what they have learned when they return? Or should we posit another assumption and say that their otherworldly selves are more capable of remembering than the human bound forms are?
So let us look at a simpler explanation. The similar memories can be attributed to a shared interest in all things Otherworld, a fascination with the likes of Tolkien, Tir Na nOg, the Sidhe, Elves, being beautiful, being different. Shared interest and shared impulses, shared reading materials and shared conversations are enough to explain the more detailed aspects. The more generic aspects can be traced to basic human archetypes, to images and feelings and dreams that have thrilled and haunted man for a very long time. Not because of any otherworldly influence, but through his own ancestral past. Just because a vision is shared, it does not mean it has to have a non-human origin - indeed it is more likely to mean that the origin is very much human. It is yourselves.
The feeling of displacement and alienation has been part of a portion of the human psyche since time immemorial. A greater sensitivity to the more esoteric realms has often gone hand in hand with this feeling of alienation. But humans are capable of profound sensitivity, particularly the "lessened" sensitivity that these Otherkin claim is a result of being trapped inside a human shape. A feeling of being something other than human is often produced as a result of psychological alienation - the sense of being a misfit causes the psyche to produce an explanation as a form of self-defence, and what could be more attractive than an explanation that you feel different because you are different, different to the point of not really being human, but being something else?
The physical effects - they are the first to admit that most of them look like humans, and some humans can look like them. The iron could be psychosomatic - if stigmatics can bleed, then Otherkin can feel antipathy towards iron. Indeed, all of the experiences have been well documented in humans who claim no otherworldy inheritance. I happen to be allergic to iron, cadmium and nickel. I have never claimed to be an Elf (although there are those out there who say I am merely denying my rightful heritage).
It is possible that aliens came and taught art and science to ancient peoples, but we do not need to posit their existence to explain the achievements of our ancestors. It is possible that these people truly are the product of a strange masochistic portion of an otherworldly race that wished to surrender skills, abilities, memories, family, beauty and happiness to be misfits in a world that would never be able to accept their existence even if they were to reveal it to us after all. But we do not need to posit the existence of Elves or others in human form to explain the feelings and experiences of these people. They are easily explained by the theory that these are people who reject the great potential of the human mind, the beauty it can achieve and the happiness that can await it, whether consciously or unconsciously. Until there is evidence other than the subjective experience of those who talk about it incessantly, until I meet someone I recognise as being an Other in human form, until I meet anyone who has objectively determined that this is what is happening, then I will prefer the latter explanation. If it looks human, acts human, talks human, reads human and was born of human parents, then all the evidence points to it being a human, and further assumptions are unnecessary unless there is profound reason to make them.
Incidentally, the corollary to Occam's Razor is Hanlon's Razor. Do not attribute to malice that which can be adequately explained by stupidity....
Just a thought.