While the information here is great, I have a few qualms with the psychologists who made the study:
– The continuing pathologizing of schizophrenia (and other mental illnesses), implying schizophrenics are simply insane/dangerous and not at all spiritual. Many visionaries, artists, musicians etc. were schizophrenic or had schizophrenic-like tendencies.
– The inevitable co-opting/appropriation of the word shaman by ceremonial magickians and other occult practitioners not at all familiar with these practices. This is dealing with the dream and trance-states of indigenous folk magic and the word shaman is a culturally-loaded term. Just because you did some dream work does not make you a shaman. It seems the co-opting of this word is more for a pretense at prestige and egotism than any authenticity.
– This is predominantly Norse religion (Asatru) and there are no female shamans to speak of, but even though that’s par for the course I wonder if there is any gender stereotyping involved here.
– Even if there are similarities between shamanism and schizophrenia, this does not mean they are the same thing. If two different physical illnesses can appear the same with their symptoms, then why couldn’t it happen with mental states or altered states of consciousness?
– Pathologizing of shamanism and other religious/spiritual states as “mental illness” even if (or even though, as most of the time) they are not hurting anyone. Other scientists and hardcore atheists tend to do this as well and atheists were doing it before psychologists did or before even seeing what psychologists had to say about it, so this a trend that probably began with non-professionals anyway. There are a number of reasons why this is wrong, not least of which the fact that psychologists don’t all agree with each other and I suspect the ones conducting the study were “skeptical,” that is, walled off with doubt. Another is that people can have an irrational belief without being mentally ill. They can even have a delusion and even more commonly, have conflicting beliefs which they attempt to resolve with cognitive dissonance. Often these claims of mental illness – usually done by non-professionals – are used to cover up the real social issues. Common beliefs are not delusional. Beliefs which are conditioned into people since birth or from a young age are not delusional; children tend to believe whatever people tell them, even if they are irrational beliefs or maladaptive practices.
“When missionaries arrived in Africa, their reaction to witchcraft beliefs and practices was to treat them as ‘primitive’ and ‘backwards’. This was based in the modern Western rational assumption that witchcraft and belief in the supernatural was not ‘real’. For example, the Church Missionaries Society (CMS) in East Africa presented the fear of witchcraft, spirits and ‘superstition’ as a form of collective mental illness or weakness, employing it as an important symbol in the battle against heathenism. In doing so, the missionaries conformed, to an extent, with the tendency of psychiatrists, psychologists and colonial officials to pathologise ‘normal’ or traditional African behaviour. 19 In this sense, belief in witchcraft was equated with insanity and dealt with by colonial officials as such.”
— DeWan, Jennifer; David Lohan (2012-05-15). Open Secrets: An Irish Perspective on Trafficking and Witchcraft,”Witchcraft and the Colonial Encounter”
cf. Mental illness as a social construct – Blaming everything on mental illness – What this depression survivor hears when you call religion a mental illness – Faith is not a mental illness – Psychiatry’s oppression of young anarchists – The origin of witchcraft, shamanism and sorcery – Witchcraft and shamanism – Definition of “shaman”
I wonder what these guys would have had to say about the case of “demonic” possession/obsession witnessed by many people, including medical professionals. Would they say levitating and walking up the walls are symptoms of mental illness? I don’t think so.
Finally, carpeumbra asks this question: “What’s a better way for helping someone who hears voices, teach them methods of meditation so as to block out spirits and whispers of the dead, which concludes in them learning how to deal with that constructively (after all hearing voices is spoken of in every culture all over the world, and not in a negative light nor in a way that says that those people were unable to function in society), or telling them they are unnatural freaks and inject every drug known to mankind into their bloodstream, turning them into drug-dependent people with a self-worth smaller than an atom?”
Communication with the gods and the spirits of the land is something many people wish they could achieve. Some people believe the “Third- Eye” or “Spirit Sense” is something that certain people are born with, others believe everyone has this ability, and yet others believe it is a gift bestowed to you by the gods. Most Conservative Christians view magicians, those who speak to spirits, pagans, and sorcerers alike as demonic, devil worshiping fellows, but these people do not view themselves as such. Psychologists conducted a study connecting Shamanic trances with schizophrenia not demonic possessions and Satan, although I do believe that these trances do have some specific spiritual significance.
“It was first noticed by Silverman (1967) that Shamanic Cultures show the signs of voluntary trances and involuntary spiritual possession which link very closely with schizophrenia. He found there to be very little differences in the core psychological factors between both schizophrenia and shamanism.… Therefore a genetic role in shamanism must be looked at when discovering the origin of Schizophrenia. He furthers this claim by showing that religion appears to have meaningful connections to both shamanism and psychosis, thus supporting the notion that all three phenomena could have common origins.” 
In the Culture of the Late Iron Age Scandinavians, speaking to and understanding the will of the Gods was a part of everyday life. The Old Heathens, who where the people that worshipped the gods of Pre- Christian, Northern Europe, used many techniques to speak to the gods and learn as much as they could about what paths and choices would be best. Through offering sacrifices, which usually came in the form of human or animal blood being spilled, they could appease the gods, but this did not appease the hearts of the Norsemen, they needed to communicate with these gods and land spirits to deal with the harsh winters, warfare between opposing tribes, travel and child bearing. Sorcerers where called upon often in parts of Northern Europe to see what the future held, to curse an enemy, and for success in family affairs. It was part of every day life, not some demonic, satanic ritual. The Norsemen believed that women where naturally born with mystical powers of communication with the gods and could perform magic naturally. Magic was strictly a female art. Men who performed magic where considered to have lost their manly-hood, although it was looked down upon, it was not uncommon. There where two deities specifically called upon for the art of sorcery. One was a male deity, Odin, the god of Sorcery, the dead and warfare. The other, Freya, the goddess of lust, death, and magic.  She was the one who taught Odin many forms of his magic. She was the patron of many wives, and village women. Female sorceresses where usually called Volvas. These women where seers who saw what the gods wanted them too and spoke what the gods aloud them too. They practiced many forms of magic, casting and writing of the runes, and Seidr, which is a trance like magic.
They believe that there resides in women an element of holiness and a gift of prophecy; and so they do not scorn to ask their advice, or lightly disregard their replies. In the reign of the emperor Vespasian we saw Veleda long honored by many Germans as a divinity; and even earlier they showed similar reverence for Aurinia and a number of others — reverence untainted by servile flattery or any pretense of turning women into goddesses.
—Germania, ch. 8
Trances induced by sound where a way that a Shaman would speak to the gods. These trances where called Seidr. Seidr is just one but is also the most difficult one to describe. It is a word used to describe actions ranging from shamanic magic (like that of spirit journeys, magical healing, demonic possessions, etc), to prophecy, channeling the gods or the gods’ voices through a human agent, performing magic that affects weather or animal movements, and also shapeshifting, like that of the ancient Berserkers, or skin changer.  It was believed that the Volva would go into a trance by listening to the beat of a drum, or the singing of a chorus, and her spirit would travel through the nine worlds, starting from Midgard, or the Human realm, down towards Hel, the land of the dead and up into the halls of the gods. As her Spirit traveled she could ask spirits and gods alike many things. She could interact with them as if they were real. Although there are few surviving texts and references to this type of magic, many historians and archeologist, modern day Heathens, who follow a religion called Asatru, and book writers are believed to understand how this ritual trance, and Seidr work goes.
“Seiðr was a solitary art, where the seið-witch was not a member of a coven, as in found in other European witch traditions, although a seið-practitioner might have attendants or a chorus to assist her in the practice of her magic. In a very few rare instances only do the sagas report a group of seið-workers practicing together, there they are usually kin folk, such as a pair of sisters, a father and his family, and the like.” 
The first time we hear the use of the word Seidr in Germanic texts, it is in the Völuspá (The völva’s spá), or Volvas Magic. It speaks of Gullveig, the mysterious seið-witch, who was sent to another tribe of gods as a peace offering. She is thought to be the goddess Freyja. This magic is just one of the many interesting religious and spiritual traditions of the ancient Northern Europeans. Another is the use of runes, and magical staves for purposes of the divine.
Not much is known about the origins of the Runic alphabet, which is traditionally known as Futhark after the first six letters of the alphabet. In Old Norse the word rune means ‘letter’, ‘text’ or ‘inscription’. The word also means ‘mystery’ or ‘secret’ in Old Germanic languages and runes had an important role in ancient magic and communication with the gods, but was also used to write letters, etc.  They were used throughout northern Europe, Scandinavia, the British Isles, and Iceland from about 100 B.C.E. to 1600 C.E. Tacitus, in Chapter X of his Germania, speaks of a form of divination used by Germanic tribes, which was a way to speak and understand what the gods wanted.
“To divination and casting of lots, they pay attention beyond any other people. Their method of casting lots is a simple one: they cut a branch from a fruit-bearing tree and divide it into small pieces which they mark with certain distinctive signs and scatter at random onto a white cloth. Then, the priest of the community if the lots are consulted publicly, or the father of the family if it is done privately, after invoking the gods and with eyes raised to heaven, picks up three pieces, one at a time, and interprets them according to the signs previously marked upon them.”
Odin, the Norse High God hung from the world tree, Yggdrasil, impaled on his own spear, for nine days and nights in order to gain the knowledge of runes. Nine was a symbolic number to the ancient Norsemen. He hung over a body of water and when the runes appeared in the mirrored reflection below him, he reached down and took them up, and the runic knowledge stayed in his mind. He eventually taught this knowledge to the goddess Freya. She, in turn, taught him the magic of seidr. Heimdall, the god who guarded the Rainbow Bridge, taught the runes to mankind.  It is interesting, that Odin was a male who learnt the runes, and that the magic of the runes was largely practiced by men, although it is more then likely that women, at least, knew something of the runes. Here we can see that the sagas record instances of females cutting runes in wood in order to work a spell:
When they reached the shore, she hobbled on by the sea as if directed to a spot where lay a great stump of a tree as large as a man could bear on his shoulder. She looked at it and bade them turn it over before her; the other side looked as if it had been burned and smoothed. She had a small flat surface cut on its smooth side; then she took a knife, cut runes upon it, reddened them with her blood and muttered some spells over it. After that she walked backwards against the sun around it and spoke many potent words. Then she made them push the tree into the sea, and said that it should go to Drangey and that Grettir should suffer hurt from it.—Grettis saga Ásmundarsonar, ch. 79Another type of sorcery, used by men is called Galdr which literally means “to sing” and refers to magical songs that were sung for certain spiritual purposes. Usually when we would see in the sagas, a women using Galdr, it was more in the way of a chant, and less then a song. The last type of sorcery I will mention is Spa, or in the English and Scottish way, spae. Spá is also referred to as spá-craft or spae-craft, and the women who practice it are often called spá-kona or spae-wife, spae-women and so on. Spá is defined as the art of determining ørlög, or fate, which is usually foreseen and created by the Three Norns, or goddesses of ones fate. Women foresee ones fate usually through intuition or personal belief. Many of the goddesses carry the art of spá. In one of the sagas, called Lokasenna we are told that Frigga knows all ørlögs, though she does not speak of them except to her maidens that attend to her; and that Gefjion, a minor goddess of farming, knows all ørlögs as well as the God Odin. 
3. Simpson, 183
4. Ellis-Davidson, 37-38