The John Henry Effect refers to the tendency for people based in a control group to perceive themselves at a disadvantage to the experimental group and work harder in order to overcome the perceived deficiency.
For example, in an office environment, some workers may be provided with a new CRM system while others use legacy tools to see if the new CRM actually confers a competitive advantage. Those using the legacy tools may try to redress the perceived disadvantage by, for example, working outside normal office hours.
Naturally the John Henry Effect can skew results, which is why those placed in a control group are often purposefully unaware of the fact that they are being compared, or the results will not truly show whether the new tool or method being introduced is advantageous.
The effect was named by G. Saretsky in 1972 after a – perhaps apocryphal – American steel driver in the late 19th century who, following the introduction of steam drills into the steel driving process, worked so hard to overcome his now considerable disadvantage that he died as a result.
A related term is John Henryism, which refers to coping strategies for long-term stresses and disadvantages that result in high physical and mental costs. Someone forced into manual labour, for example, may work extra hard to accumulate money to prevent their children having to do the same, but then suffer from arthritis later in life