Clench your left hand into a fist. What happened to your right hand when you did it?

If you’re like most people, the answer is nothing. But, surprisingly, not everyone can do this. Some people make “mirror movements”, where moving one side of the body, particularly the hands, causes the other to move unintentionally. Clench the left fist, and the right one closes too. Doing things like playing the piano or typing are very difficult. In 2002, a Chinese man with the disorder failed to get into the military because he couldn’t use the monkey bars.

Young children sometimes make mirror movements but they almost always grow out of it by the age of 10. The only exceptions tend to be