Why does tickling yourself not work?

When you move your hand, your brain sends commands to the muscles and also predicts what the result will feel like. In a sense, it already knows where the feather will go and how firmly it will touch.

When the prediction matches the real sensation, the brain treats the feeling as weaker. You still feel the touch, but the surprise of something suddenly touching you is missing, so it does not grow into a tickle. Someone else's hand, on the other hand, can move anywhere at any moment, so the same pressure feels stronger and makes you flinch.

Many people think only those with sensitive skin are ticklish, but the same person reacts very differently depending on who is doing the touching.

In 1999, researchers in Britain ran a clever experiment. When people moved a robot arm with their left hand, a second robot stroked their right palm, and the researchers added a tiny delay in between.

As the delay grew to 0.2 seconds, people rated the same touch as more and more ticklish, even though most of them did not notice the delay. Twisting the direction of the stroke also made it more ticklish. Even a touch your own hand creates feels like someone else's when it does not match the prediction. That is why, however hard you try, it is hard to tickle yourself into fits of laughter.

This principle shows up in daily life too. In a 2003 experiment, pairs of people were asked to press back on each other's finger with exactly the force they had just felt, and the force grew stronger with every turn. Because force you produce feels weaker, you press harder than you think even when you believe you matched it. The force grew considerably with each exchange.

This may be why playful nudges can turn into a real fight. Both sides believe they only gave back what they got, but each push is a little stronger. When playing around, it is safer to assume your own force is stronger than it feels.