亨利效应指控制组意识到自己处于控制组且要和实验组竞赛的情况下，超常发挥的现象。

约翰·亨利是一个传奇的美国黑人铁路工人；一个铁钉打入木桩，一般工人需要抡3下锤子，而他只需要1下。当时为了评估打桩机的效率是，把他亨利为控制组，但亨利超长发挥，竟然赢了机器。但他也死于过于劳累。

最早引用是：Saretsky, G. (1972). "The OEO P.C. experiment and the. JohnHenry Effect", Phi Delta Kappan 53: 579–581，

后来在 Cook, T. D. & Campbell, D. T.(1979). Quasi-Experimentation:Design and Analysis Issues for Field Settings. Houghton Mifflin Company:Boston也有引用。

大家可以参看：http://www.jstor.org/stable/20373317。

特别提示：百度百科是错的http://baike.baidu.com/view/1196501.htm

更简单的可以参看维基百科的http://en.wikipedia.org/wiki/John_Henry_effect：

The John Henry effect is an experimental bias introduced  into social experiments by reactive behavior by the control group.

In a controlled social experiment if a control is aware of their status as members of the control group and is able to compare their performance with that of the treatment group, members of the control group may actively work harder to overcome the "disadvantage"of being in the control group.[1]

For example, if in an educational trial where the school classes who are in the treatment receive an extra support teacher, students who are in the control group may be induced to work harder to overcome that disadvantage.

The term was first used by Gary Saretsky(1972) to describe the behavior of a legendary American steel driver in the 1870s who, when he heard his output was being compared with that of a steam drill, worked so hard to outperform the machine he died in the process.[2]