You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Adult_contemporary_music

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:
In what decade did genres begin to diverge on the basis of demographic groups?

1970s