One group may view that it is competition with the other group and may work harder than than they would under normal circumstances. This generally is applied to the control group "taking on" the treatment group.The effect is named for the classic American folk hero about a mighty steel-driver on the crews that built the railroads across the country. One day, the owner of the railroad buys a steam-powered hammer to do the job and John Henry challenges the machine in order to save jobs of his work-crew. In a tough mano a machino contest John Henry bests the machine but dies in the process.
I had my own little John Henry effect. My son has begun riding a bike. We go out to the trail, he bikes and I run along-side him. He is only seven, his bike still has training wheels, but he still peddles faster then I run. I am inspired, this is man against machine. I push myself to new limits. My heart pounds, my legs strain, my breath is ragged. It is MAN vs. MACHINE! AGE vs. YOUTH! I run like the wind (or at least a gentle breeze.)
And today, in defense of honor - I kept my son in sight for 87 seconds.
Strictly speaking, this may not be an illustration of the John Henry Effect, or it might be...
The fact is I didn't do very well in the class, which is why I volunteered to sing John Henry, Steel Driving Man in class for extra credit.
Yes it was kind of embarrassing, but it was also good preparation for my thesis defense (currently projected for 2017.)