“Normal people have some musical ability—if I play you a piece of music and I miss a note, you would know something wrong with that. Amusics can’t [tell],” says Psyche Loui, a neurology instructor at Beth Israel Deaconess Medical Center and Harvard Medical School in Boston. “The main compliant is that they cannot sing in tune.”
Anywhere from 4 to 9 percent of the population suffers from amusia. It’s difficult to obtain solid estimates because people dubbed tone-deaf earn the distinction because of terrible singing, not because they have been tested for amusia. (And these are real tests, including this one from the Music and Neuroimaging Laboratory, where Loui works http://musicianbrain.com/pitchtest/).
Loui says experts remain unsure about what causes amusia, but most believe a combination of environmental and genetic factors lead to disruptions in the brain, contributing to “unawareness and poor memory for sounds, especially pitches.”