There are many people who believe that they can't "carry a tune in the bucket" or that they are "tone deaf". What they don't realize is that according to many Musicology studies these people are not actually musically deaf.
Amusia (music deafness) is the inability to recognize music. This can show up in people in either tonal (pitches/sounds) or rhythmic forms. Therefore, it is possible for someone to become musically deaf to tones or rhythmic patterns without becoming deaf to the other. Still there is a sub-level to these two types of musical deafness. As the mind uses rhythmical patterns and tonal patterns all over, complete musical deafness in either area is rare. Partial rhythmical deafness means that the listener is unable to hear certain patterns. A person with partial tonal deafness may have troubles hearing the differences between certain pitches or sounds.
Furthermore, there are some people who experience music deafness in both area. This does not mean they have Amusia. Those who are truly amusic are unable to recognize tones and rhythms as what they are. As a result, they do not experience music the same way as those without Amusia. For people with complete musical deafness, music is not music. They are unable to hear the differences in vocal pitches, the differences between singing and talking, and the differences in music and banging pots together. Those who deal with musical deafness (Amusia) have often described listening to music as being "painful", "a screeching car", and generally "unpleasant". To those with Amusia, music is nothing but jumbled sounds being thrown together.
So, what is the cause of Amusia? According to studies in Musicology, musical deafness is a result of a person's inability to differentiate between adjacent tones (tones that are next to each other) and semitones (tones that are only a half step apart from each other). Without these basic building blocks, those who are Amusic can not find a tonal center. For them not being able to hear pitch differences to find the tonal center is like trying to speak without having the syllables to form the word.
How does someone become amusic? Amusia is typically not something that someone is born with. It is most often acquired. This can be a result of brain trauma which impacts the auditory senses, hearing deafness, strokes, and more. There is still some mystery surrounding how some people become musically deaf, especially those who are born with amusia.
With all of this in mind, why is it that some people are not able to sing as in tune or are classified as not being able to "carry a tune in a bucket"? It is not because they have Amusia or are musically deaf. Rather those who have difficulties with tonal pitches have not trained their auditory faculties to recognize the subtle differences or they have a partial Amusia. They can still hear music and appretiate it; they just may not be able to produce it as perfectly.
In conclusion, Amusia can be either a partial or complete musical deafness. It can limit someone's tonal or rhythmical perception of music or both at the same time, or it can completely cut off someone's perception of music at all.
Imagine what it must be like to have Amusia and to be musically deaf. How would your life be different? In what ways would you be affected? Or pretend you're a scientist/doctor and come up with another cause (not listed in this post) for why some people might be born or develop musical deafness (Amusia).
For more information on Amusia, feel free to watch this short video from the author of Musicophilia: