Speaking with a long-departed loved one may be the result of fear or suggestion rather than genuine spirit contact, research suggests.
Illusionist Derren Brown faked a sťance
UK psychologist Professor Richard Wiseman looked at controlled "sťance" experiments.
He found many ways that people strive to interpret a medium's comments to apply to themselves, whether the facts are accurate or not.
Mediums may also tailor readings by picking up on subtle clues like what we wear, he said.
He will recreate a Victorian sťance at the Dana Centre in London on Thursday.
Professor Wiseman said there were several strategies that might underlie an "apparently" accurate reading by a medium.
He said people tended to believe very general, sweeping statements applied to themselves directly when they wanted to.
"I think the mediums are fairly sincere, but the person is reading a lot into what are fairly ambiguous comments," he said.
He said often people who went to mediums were seeking reassurance or help for a stressful period of change in their lives such as a recent bereavement.
"We want to believe that the statement is true, that it applies to us. So we tend to buy into it."
An example is if the medium said "The spirits are talking about the younger woman who has passed away", this could refer to a young child, a teenager or even someone who had died in their forties.
The degree to which a client thinks through these alternative interpretations would then influence whether they believe the statement applies accurately to them.
He said people tended to remember the "hits" - the things said by the medium which matched their own needs - and ignore the "misses".
It is not only the person going for the reading who influences the outcomes.
He said mediums might be able to unconsciously gain and react to information from things like how the client is dressed and how they respond to the medium's questions.
Professor Wiseman set up a controlled sťance with five subjects and mediums and found no evidence of genuine psychic ability.
He presented this research, which has been submitted for publication in a peer-reviewed journal, at a convention of the Parapsychological Association in Vienna in August.
In a hoax earlier this year on live TV, Illusionist Derren Brown convinced people he was contacting the deceased using a ouija board.
Brown said he wanted to encourage people to question sťances.
Professor Wiseman said he did not think most mediums were tricksters, but he said because people did seek their help for bereavement issues this could be good or bad.
A spokeswoman from Cruse Bereavement Care said: "When people are bereaved they feel very isolated, very lonely and very questioning, and they will make their way towards areas where they think they might find answers.
"That's a perfectly understandable thing to do, but amongst those people who appear to provide help and support or answers there will be people who are perhaps not so benign in their intentions.
"Therefore, it's important for bereaved people to be very cautious and very careful where they seek support."