Perfectly wrong

Repeated washing of hands, an obsession with closed doors or windows, anxiety about cleanliness - these may all be the physical symptoms of a much more disturbing mental disorder. Liu Zhihua explains.

Xiao Ming suddenly covers his mouth with his hand, blinks hard and smiles.

"Sorry I repeated the last few words so many times," he says, stuttering as he nervously explains. "I couldn't control myself."