Many people claim to be tone-deaf (technically called amusia) although it has been estimated that only about 4% of the population actually have the condition. This disparity between actual and claimed amusia is probably because it's an excellent way of avoiding singing in public - I've used it on many occasions. However, researchers have found the condition to be incredibly specific.

It seems that 'congenital amusics' have no problems in hearing music or other environmental sounds. They generally have no perceptual problems and can, paradoxically, detect pitch changes in speech that they cannot detect in music. Music seems to require a much more fine-grained differentiation of tones than any other form of auditory perception.

Amusia may be similar developmentally to dyslexia. In populations that speak tonal languages like Vietnamese and Cantonese, amusia is almost unknown - perhaps practice at a young age makes perfect. Recent research suggests that children can be trained to discriminate tones but adults are normally stuck with their disability.

However much practice you get at a young age, the skill can be affected by brain damage. Neurologists at the University of Tokyo report the case of a 62 year old professional tango singer who lost her singing ability after a stroke that affected a tiny part of her brain. After a period of recuperation from the stroke she experienced no other deficits in cognitive functions, other than amusia - a cruel fate for a professional singer.