Why you can't tickle yourself

From a recent article in the New Scientist

Human brains are wired to underestimate the amount of force exerted on other people, a study of "tit-for-tat" experiments has revealed.

As well as qualifying the teary "she hit me harder" playground argument and explaining why we can't tickle ourselves, the discovery may provide insight into some self-delusional symptoms of schizophrenia.

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When the body makes a movement a message is sent to the brain warning it what to expect. In this way, the brain predicts and already knows the outcome of an action before it is performed. This explains why you cannot tickle yourself - your brain already knows what to expect and de-sensitises the signal from your skin.

Wolpert says this may help to explain schizophrenic symptoms involving delusion of control. "Some schizophrenic people generate a movement but can't predict what is going to happen. They might claim the movements are generated by other beings, such as aliens, although they are made by themselves," he explains.