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The classic example of an observer effect is Clever Hans: a horse in Germany in the early twentieth century that could apparently do arithmetic. If fact it appeared that he could add, subtract, multiply, divide, work with fractions, tell time, keep track of the calendar, differentiate musical tones, and read, spell, and understand German. For instance you could ask Hans, “If the eighth day of the month comes on a Tuesday, what is the date of the following Friday”? or almost any question with a numerical answer. Questions could be asked both orally, and in written form. Hans would answer by stamping his hoof a number of times and was very often correct. If fact, Hans could do none of these things but he could pick up on unconscious cues from humans. If you asked him what 4 plus 5 was then he would start stamping his foot and you would be mildly interested at the start of this process but after he had stamped for the 9th time your interest level would have heightened. You are really interested in whether he stops at this point or not. Maybe you look intently at his stamping foot, or you hold your breath, or your body stiffens. Hans picked up on these cues of heightened interest and stopped stamping. This simple rule - stop stamping when people seem really interested? was misinterpreted as arithmetic competence. The secret of Hans? success was unmasked by a blind procedure. If the question was presented to Hans on a blackboard but the person recording Hans? answer was the only person around and could not see the blackboard, so could not see the question and thus know the answer, then Hans did badly. But if that same person could see the blackboard then Hans did as well as ever.
In the study of human behaviour, the Hawthorn effect is the observation that humans tend to perform a task better if they know they are being observed. In the early twentieth century, a factory called the Hawthorn Works commissioned an experiment to see which lighting regime improved worker productivity. What was found was that any time the lighting was changed in part of the factory as part of the study (no matter whether the change was towards lesser or greater illumination) then productivity went up but slumped again after the study ended. Similarly, you would probably drive more carefully if you were giving your new boss a lift shortly after starting a new job, compared to your normal driving.
Another observer effect that you can get in humans is the phenomenon that the greater the expectation placed on people the better they perform (the Pygmalion effect or Rosenthal effect). Imagine you gave the same spelling test to two classes of 11-year-old children. Imagine that you said to one class “this test is really for 13-year-olds but do your best” and to the other class “this test is really for 9-year-olds so should be easy for you”; if this effect occurs we would expect children in the second group to perform better on the test.
Further reading
Try to take as broad a view as possible of the potential for observer effects. To open your eyes, De Boeck HJ, Liberloo M, Gielen B, Nijs I, Ceulemans R (2007) The observer effect in plant science. New Phytologist, 177, 579?583 discuss how the responses of plants to being touched can lead to plants responding to how they are monitored in plant science investigations. Riley KJ & Steitz JA (2013) The ‘Observer Effect’ in Genome-wide surveys of Protein-RNA Interactions. Molecular Cell, 49, 601?604 emphasizes how small perturbations caused by measurement affecting the cellular environment can have significant cumulative effects when surveys are carried out across a whole genome in modern cell biology.