John Henry Effect

The John Henry effect is a bias that can distort the results of a controlled experiment when members of the control group become aware that their performance is being compared against an experimental group, and, out of a sense of rivalry or a wish not to appear disadvantaged, respond by working unusually hard to outperform or match the experimental group, thereby destroying the control group's usefulness as an untreated baseline for comparison. The researcher Gary Saretsky coined the term in 1972, naming the effect after the legendary nineteenth-century American steel driver John Henry, who, upon learning that his output was being compared against that of a newly invented steam drill, worked himself to the point of death in his effort to outperform the machine. The John Henry effect is considered a specific variant of the broader Hawthorne effect, in which any participant's behavior changes simply because they know they are being observed or studied, but it is distinguished by the particular competitive motive involved, arising specifically from the control group's awareness of an ongoing comparison rather than from the more general effect of being watched. Because the effect can seriously undermine an experiment's validity by artificially inflating the control group's performance and thereby narrowing or eliminating the apparent difference between groups, researchers commonly try to prevent it by keeping participants unaware of which group they have been assigned to, or unaware that a comparison between groups is taking place at all.