Most popular music is not written by the person who performed it.
I think that varies by genre. It's certainly true around pop music. But I don't think so much for, say, metal.
On the other hand, having recently watched a few episodes of VH1's "Clasic Albums" series (at least I think that's what it was), I was really surprised to see the amount of creative input that came from the album's producers. I was surprised to see that the melody of Kiss's "Beth" was written by the producer, and that Bob Rock did so much with Metallica (unfortunately?) in building the songs for The Black Album, albeit it out of riffs written by the band.
It does vary, but I didn't want to pigeon hole pop. Mostly because it happens in a lot of other genres such as country/wester and even folk. It also occurs more in places such as Nashville, Austin and Memphis where there is a strong songwriting tradition.
I think that varies by genre. It's certainly true around pop music. But I don't think so much for, say, metal.
On the other hand, having recently watched a few episodes of VH1's "Clasic Albums" series (at least I think that's what it was), I was really surprised to see the amount of creative input that came from the album's producers. I was surprised to see that the melody of Kiss's "Beth" was written by the producer, and that Bob Rock did so much with Metallica (unfortunately?) in building the songs for The Black Album, albeit it out of riffs written by the band.