Psychic studies may be influenced by suggestion:

From Mind hacks: The BPS Research Digest has discussed a recent study that analysed recordings of parapsychology experiments and has found that some of the positive findings may be due to experimenters unconsciously prompting the participants as they gave their answers.The experiments used the Ganzfeld technique where one participant has diffuse white light and auditory noise played to them, effectively blocking the key senses, while another tries to ‘send’ images to them through mental projection.

Afterwards, the ‘receiver’ tells the experimenter what images came to mind and the research team see if it matches what the ‘sender’ was trying to transmit.

Taken as a whole, these sorts of experiments show a weak but positive evidence for extra-sensory perception (ESP), but it’s not clear whether this isn’t just due to a tendency for some negative trials not being reported.

In this new study, psychologist Robin Woofit analysed the tapes of Ganzfeld experiments from the mid-1990s and found that experimenters were more likely to respond decisively to correct responses but give subtle cues (such as saying ‘mm hm’) to give more information when the response wasn’t initially accurate.

This suggests that some of the positive findings may be due to this subtle prompting which is known as the Clever Hans effect, after a horse who was thought to be able to do amazing calculations, until it was later discovered that he was simply clopping his hoof until his trainer responded in a positive way.

However, this also highlights another aspects of parapsychology – they do some of the most thorough experiments in psychology.

This new study was only possible because the researchers keep archived audio recordings of every experimental session, something that almost never happens for other psychology studies.

It could be that other experimental findings in psychology are influenced by the Clever Hans effect, but we’ll never know, because few labs keep such thorough records.

Try asking for the audio recordings of decade-old experimental sessions from other areas of psychology if you’re not convinced.

It sometimes strikes me as ironic that some scientists consider academic parapsychologists to be unscientific when they do often some of the most carefully designed studies in the literature.

The fact that these studies typically find no evidence of ESP doesn’t mean they’re not doing science, and in fact, they’re provided some of the best evidence against airy fairy notions of ‘psychic powers’.

UPDATE: This is an important clarification on the study from Christian, which puts a different spin on it:

The observed interaction effect occurred during the review phase, where the researcher goes through the images the receiver spoke out loud earlier as the the ‘sender’ watched the video clip. This is prior to the receiver’s attempt to choose the correct video clip from a few distractors.

The review generally follows the pattern of the researcher saying ‘you said you saw x’, the receiver say ‘yes’ or ‘no’ or maybe elaborates. It was those times the receiver elaborated, that experimenters appeared to have an influence – if they said ‘okay’ and moved onto the next item, then that was that, but if they went ‘hmm mm’ with an enquiring tone, then the receiver tended to ramble on a bit more and lose confidence in their imagery.

I don’t think it is clear that this would make positive results more likely, and could even make a negative result more likely. Remember too that these were double blind experiments, so it is not a case of the experimenters directing the receivers towards the correct imagery. It is possible though that a sceptical researcher could be more prone to the ‘hmm mm’ noises, and therefore would make their receivers less confident.

Link to BPSRD on parapsychology and suggestion.
Link to abstract of scientific study.