Here’s what the National Association for the Deaf has to say.
“Deaf” and “deaf”
According to Carol Padden and Tom Humphries, in Deaf in America: Voices from a Culture (1988):
We use the lowercase deaf when referring to the audiological condition of not hearing, and the uppercase Deaf when referring to a particular group of deaf people who share a language – American Sign Language (ASL) – and a culture. The members of this group have inherited their sign language, use it as a primary means of communication among themselves, and hold a set of beliefs about themselves and their connection to the larger society. We distinguish them from, for example, those who find themselves losing their hearing because of illness, trauma or age; although these people share the condition of not hearing, they do not have access to the knowledge, beliefs, and practices that make up the culture of Deaf people.
Padden and Humphries comment, “this knowledge of Deaf people is not simply a camaraderie with others who have a similar physical condition, but is, like many other cultures in the traditional sense of the term, historically created and actively transmitted across generations.” The authors also add that Deaf people “have found ways to define and express themselves through their rituals, tales, performances, and everyday social encounters. The richness of their sign language affords them the possibilities of insight, invention, and irony.” The relationship Deaf people have with their sign language is a strong one, and “the mistaken belief that ASL is a set of simple gestures with no internal structure has led to the tragic misconception that the relationship of Deaf people to their sign language is a casual one that can be easily severed and replaced.” (Padden & Humphries)
National Association For The Deaf