

As a child, my brother would frequently challenge me to a game he called punch-for-punch. He’d let me hit him in the arm if he could hit me back just as hard. We’d exchange blows until one of us—the loser—quit. It wasn’t a long game; dull punches soon became bruising wallops. Being several years younger and many pounds lighter, I’d often concede quickly, fearing the next blow that, despite the game’s equally-hard rule, always felt more forceful than the last.



The thing is, my brother and I were both playing by the rules—at least, we thought we were. Over the last seven years, three experiments have used a laboratory analog of punch-for-punch to show that our brains are wired to discount the sensation of our own actions. My brother, it turns out, had to punch me back harder to feel like he was hitting me with an equally-hard blow. This neurological phenomenon not only explains why we can’t tickle ourselves, but also might help avert delusional thoughts.



The first laboratory game of punch-for-punch took place in 2003. Neuroscientist Daniel Wolpert, working with colleagues at University College London, built a version of the game, which they called “tit-for-tat”. The researchers used two robotic levers capable of applying force and recording the amount of force applied to them. Pairs of subjects faced each other with the levers between them. They rested their right index finger on one of the levers and their left in a mold beneath the other. In this setup, a lever separated each subject’s right index finger from their partner's left. If one subject pushed down on a lever with their right finger, the lever applied a force to their partner’s left. Their partner could then return the favor in the same manner, the levers recording the forces each subject applied as the game progressed. Subjects were told to push back with the same amount of force and no more—tit-for-tat.



The tit-for-tat experiment, like my childhood game, didn’t last very long. In only four turns, the force subjects pushed back with rose rapidly, from a fraction of a Newton—barely a tap—to nearly five—a painful jab. But the test subjects were adamant that they were playing by the rules. They pushed back with the same amount of force they felt; the other subject, they said, must have been secretly told to double the force.



To figure out why subjects thought they were playing by the rules, Wolpert and his team had individuals play against themselves. The robot’s lever applied a force to a subject’s finger; the subject then had to apply the same amount of force back to their same finger by pushing down on the lever with their other hand. In my childhood game, this would be like my brother punching himself in response to my punch. He’d have no incentive to cheat because, well, why would he want to punch himself harder than he had to? But this, it turned out, is exactly what subjects did. Regardless of how much or how little force the robot applied, subjects always pushed back on themselves harder. Their brains, it seemed, were discounting sensations caused by their own actions; they had to push back harder to feel like they were applying the same amount of force applied to them.



The phenomenon observed in Wolpert’s study isn’t unique to games like tit-for-tat. We have all experienced it before, most noticeably during a self-imposed tickle. The sensation from a self-tickle pales in comparison to the sensation we feel when someone tickles us—and it should. Brain scans show that, compared to a real tickle, cortical areas that process sensation are less active during a self-tickle. For some reason, whenever we move, our brains predict the sensory consequences of our own actions and reduce them. So self-tickles feel lame, and my brother and I have to punch each other back harder to feel like we’re playing our game by the rules. Why would our brains do this?



Two years after publishing the tit-for-tat study, Wolpert, working with a slightly different group of researchers, re-ran the experiment. They reasoned that our brains might discount the feeling of our own actions to help us differentiate between self-generated and externally generated sensations. Put another way, reducing the sensation of our own actions makes alien sensations—say, a tap on the back—more noticeable.



To test the idea, the researchers had schizophrenics play tit-for-tat against themselves. Schizophrenics have trouble recognizing their own actions—that is, they often attribute their behavior to an alien source. Some can even tickle themselves. If our brains discount the feeling of our own actions to help us differentiate between self-generated and externally generated sensations, then a group of subjects who can’t make this distinction might simply be missing this sensory reduction. In that case, reasoned Wolpert and his team, schizophrenics should be better at playing tit-for-tat by the rules. And they were. When the robot pushed on the fingers of schizophrenics they were much better at pushing back on themselves with the same amount of force the robot had applied. Their brains didn’t discount the consequences of their own actions as much as the brains of healthy subjects did.



But the tale of the tit-for-tat experiment doesn’t end there. This past year, Wolpert, now working at Cambridge with another group of researchers, ran the tit-for-tat study a third time. Thirty healthy subjects were recruited. They played the game against themselves and completed a short survey designed to gauge delusional thoughts. The survey asked questions like, “Do you ever feel as if you have been chosen by God in some way? and “Are you often worried that your partner may be unfaithful?”—questions that, on their own, are endorsed by about one in four people.



Wolpert and his colleagues compared the survey results to subjects' tit-for-tat performance. They found that delusional thinkers, just like schizophrenics, were better at playing tit-for-tat by the rules—they were better at pushing back on themselves with the same amount of force the robot applied. A reduced ability to discount the sensory consequences of self-generated actions was not just a consequence of schizophrenia—it seemed to be, more generally, a characteristic of deluded thinkers.



Of course, unlike schizophrenics, who verbally attribute their actions to an alien source, it’s hard to say how such a deficit might cause someone to incessantly worry about an unfaithful partner or believe themselves to be the second coming. But what the tit-for-tat studies do suggest is that a small change in the way our brains interpret sensory information relates to a substantial change in our perception of the world around us—a result that has convinced me to take a second look at my punch-for-punch matches with my brother. I never won. But we always punched each other back a lot harder. That’s starting to seem like a good thing.