There's enough documented cases where we KNOW this is the case to think this is likely true for a very large proportion of them. Dick Francis is a major case in point; his wife wrote most of "his" books.
You have to wonder at the mentality of men who do this. What kind of narcissistic asshole do you have to be to pretend that a creation is yours?
Oh. Wait. The popularity of generated "Art" tells me exactly what kind of asshole you have to be: a run of the mill asshole.