We only feel ticklish if we detect a sense of invasion or attack. If we try to tickle ourselves, our brains anticipate this "attack" and prevent us from feeling panicked. Strangely enough, even if a person knows that they are about to be tickled, the fear of being touched may cause the same "ticklish" reaction. Some people laugh even before being tickled.

我们只会在察觉受到侵犯或攻击时才会觉得痒。如果试着替自己哈痒，脑部会自动预感到这种“攻击”从而使我们感觉不到惊吓。奇怪的是，即使有人知道他要被哈痒了，担心被人触摸，也会引起同样“怕痒”的反应。有些人甚至还没被哈到痒就先笑了。