I've been thinking whether it would be possible to optimize music choice, and it seems to me there are a few relevant questions. I'd be happy to hear some thoughts or related research on this.

1. Is there a way to do randomized control trials of music? It seems like selection effects are inevitable, especially since individual engagement/attitude may be important for musical engagement. The only way  I can think of circumventing this is something like lobby music, so that people are neither in charge of their music choices, but also don't feel forced into the choice.
2. How do effects of music change for different combinations of listener moods and music styles? For example, listening to sad music might not make you sadder if you listen to it in a good mood, and it might give life a greater sense of meaning. Personally, I've noticed that listening to music that matches my mood frequently creates a vicious cycle.
3. Optimal spacing/variety? I've personally noticed that I get very giddy when I re-discover a song I know well but haven't listened to in a long time. Perhaps active song choice/browsing is slightly better for enjoyment than playlists?

Also, this is my first post, so I'd really appreciate any tips on style, content, or posting norms that I can improve on. Thanks!