context: Radio stations played Top 40 hits regardless of genre; although, most were in the same genre until the mid-1970s when different forms of popular music started to target different demographic groups, such as disco vs. hard rock. This evolved into specialized radio stations that played specific genres of music, and generally followed the evolution of artists in those genres.
question: ఏ దశాబ్దంలో జనాభా సమూహాల ఆధారంగా శైలులు విడదీయడం ప్రారంభించాయి?
answers: 1970s