Closed captions are invisible until decoded by a caption decoder, whether this decoder is a separate piece of equipment attached to the television set or is built into the television set itself. Closed caption technology was first developed to enable deaf and hard of hearing people to watch television. But closed captions are NOT just for deaf and hard of hearing people. While I'm not sure how extensive the research has been, at some indicates that people who leave their caption decoder on when they watch television improve their reading scores. Closed captioned television is clearly not a magic cure for the massive problem of illiteracy.
Illiteracy is too deep-seated and complex a problem to be solved with closed captions alone.
Also clearly, the presence of closed captions should not become an excuse to permit children to continue watching television to the harmfully excessive extent they often do. Closed captions, however, could be a valuable motivating tool in encouraging the development of reading skills. A few models may have decoders available in screens as small as nine inches. The bad news is that not everything is captioned, especially once you venture beyond prime-time television and major release movies. But far more is captioned today than was in 1980 when closed captions began, and more is being captioned all the time in part due to letters of encouragement from consumers of closed captions.