Placebo effects can also be interpreted as “thought traps”. That is, if you classify them as a special case of expectations. Expectations are common, well researched and, of course, undesirable in science. After all, those who have expectations about their experiments have as good as falsified the results!
Here are some interesting results that were already known in the 1970s:
In the field of perception: When a tape recording was played in which an expected phoneme was overdubbed with a cough, not one of the 40 test subjects noticed the cough. Neither, incidentally, did those who were given a cue or even knew the passage (Warren, R. M. (1970): Perceptual restoration of missing speech sounds. Science 167 (3917): S. 392-393.)
In the field of advertising: A similar effect is seen in randomly selected primary school children. They are recommended to teachers as being gifted. At the end of the year, the expectations were fulfilled: The “gifted” children gained 10-15 points on the same IQ test, because they were given more support by the biased teacher. Thus, not only is there an influence, but also a self-confirmation of originally false theories (self-fulfilling prophecies). (Rosenthal, R. (1969): Interpersonal Expectations: Effects of the Experimenter’s Hypothesis. In Rosenthal, R./Rosnow, R. L.: Artifact in Behavioural Research. New York: Academic Press. S. 182-277.)
The percentage of subjects influenced is also very high in many studies: 60 per cent of subjects and 70 per cent of experimenters are affected (Rosenthal 1969, p. 235). This makes it all the more worrying that blind testing is still not standard practice in medicine and biology.
This brings me to some really interesting results from placebo research:
First, expectations surprisingly work in all directions: if the doctor tells patients that the drug (placebo) will increase, decrease or stabilise airway resistance, then depending on the information in the same study, exactly these effects occur (Turner et al. (1994)): The importance of placebo effects in pain management and research. JAMA 271(20):1609-1614).
Second, placebos work in up to 70% of patients (according to Turner’s meta-analysis of 70 placebo trials mentioned above). It has been observed that even years of severe pain disappear and people can walk with crutches after placebo surgery (Moseley et al. (2002): A controlled trial of arthroscopic surgery for osteoarthritis of the knee. New England Journal of Medicine 347(2):81-88).
Third, expensive placebos help more than cheap ones (colours and shapes also play an important role) (Waber et al. (2008): Commercial characteristics of placebo and therapeutic efficacy. JAMA 299(9):1016-1017).
If you are interested in this topic, there are whole shelves/hard drives full of literature…
The question is, of course, why are our expectations so extreme? There is no single explanation for these very different phenomena.
But I interpret expectations as a reaction to the environment that people have to deal with. We have to constantly search through huge problem spaces to find the few applicable hypotheses that guide us in very different tasks and thus help us. It helps to be very selective and to approach possible solutions with strong preconceptions or expectations. This includes, for example, coming up with a strong hypothesis very early on and then trying to verify it (falsification would be far too time-consuming!).