John Henry Effect

John Henry effect is an experimental bias characterized by the control group’s unconscious or conscious reactive behavior. This is caused by the research participants’ awareness that they are a part of the control group and that when compared with the experimental group, may be perceived as a disadvantage. Hence, they may be motivated to over perform. This effect is attributed to John Henry, an American steel driver who worked himself to death when he heard that his output was being compared to that of a machine. This legendary incident happened in the 1870s and the term was introduced by Gary Saretsky in 1972.

For instance, a group of psychology students are conducting an experiment about the effect of chocolate on short-term memory, a respondent who is informed that he is a part of the control group knows that his experimental counterpart is given a chocolate bar while memorizing. He was then induced to do his best on the memory tasks to overcome the possible disadvantage.