I also remember hearing that classical music played to young children and even those still in the womb (with pillow speakers for mom's belly) tended to make children smarter. This might be kind of the same thing, that children with parents that care enough to try this might also be more nurturing, interested parents. Could be the environment is better, not just the music. Or it could be that the patterns in the music, the complex mathematical relationships, or maybe even just the stimulus causes better neural growth. I know that children that are deprived of stimulus don't develop as well, so this may be related to that.