In a recent post I pointed to a research article showing how perception
can be distorted by learning and how this can account for why listening
alone tests of audio gear show no differences when the active item is not
known but differences are thought to exist.

http://www.eurekalert.org/pub_releas...-wbb060905.php

Now we have another research article showing how peer pressure can
likewise cause us to percieve something when it doesn't exist either. In
a test situation a person gets to know a group of other people he is told
are also being tested. They are not but are part of the test by acting as
though testees and by all agreeing among themselves about some perception
situation. The person being tested is shown something which is really
different then what the group is saying and when really tested while using
a brain scan percieves in the way the group says it is when it really is
not.

http://www.nytimes.com/2005/06/28/sc...gewanted=print

Both add together and account perfectly to explain why listening alone
testing causes previously percieved differences in known gear to disappear
when a test is done when which bit of gear is active is not known. This
is exactly the same as in any other test using human perception and is
exactly why sighted testing of audio gear is worthless in the face of the
above examples of how much perception can really be changed to distort
what is real. The entire subjective testing enterprise is an illustration
of what the research shows about distorted perception and how learning and
peer pressure can be the source of that distorted perception and why
people will swear on a stack of bibles that some amp/wire really really
sounds different then some other amp/wire.