This may explain a lot of sparring sessions. From That Bully Did Hit You Harder:
“Given that I have two small children, I was just curious about when they get into squabbles, when both say the other child hit them harder, whether this could be explained by a predictive mechanism,” said Daniel Wolpert of University College London, who led the study.He and colleagues set up a series of “tit for tat” experiments in which they asked pairs of volunteers to take turns pressing one another’s fingers with equal force.
They were asked to press back equally, but they didn’t, Wolpert and colleagues report in Friday’s issue of the journal Science. “The forces rapidly escalate and they escalate by around 40 percent per turn,” Wolpert said in a telephone interview.
Interestingly, it’s not all “psychological”:
To see if physical rather than psychological mechanisms were at work, Wolpert’s team did another experiment in which the volunteers were put up against a machine — and the same thing happened.The team of neurologists and psychiatrists believe there is a reason for this — a mechanism that filters out your own perception of what your body is doing when executing basic movements.
“The key message is our findings show we are not as aware of our actions as we think,” Wolpert said.
Just before making a movement, a signal is sent to the brain to tell it what to expect. If the movement is what the brain anticipated, it filters it out, leaving you not fully aware of the force you are applying.
“It is a way of filtering out uninteresting information. It just happens to lead to this consequence,” Wolpert said.
“This mechanism also explains why you cannot tickle yourself — the brain already knows what sensation to expect and alters the brain activity responsible for the sensation accordingly,” said Dr. Sukhwinder Shergill, who also worked on the study.
“But when someone else tickles you there is no chance to adjust your brain perception, and you feel the full effects.”