Multiple Authors

Sometimes I look at a book that is by multiple authors and wonder just how much that other author really contributes. There are times when having two authors works well as with Douglas Preston & Lincoln Child, but there are now books that seem to be from a major author with someone else. I wonder how much the major author really contributed.
For example: Judge & Jury by James Patterson and Andrew Gross. I wonder how much James Patterson really wrote. I'm sure that sometimes lesser known authors are taken under the wing of a widely known author to give them some exposure and then the lesser known author publishes their own books and is recognized for having written with the major author.
Hm.