Originally Posted by Andrew H.
It doesn't matter if they know other authors personally: if an author has reviewed several other authors books, they may feel an obligation to review his books positively. Or an author reviewing a book may be concerned about being too critical about a book lest that author retaliate with a critical review of the first author's book. These incentives don't exist if the person reviewing a book isn't an author.
They do if the reader got the book for free as a result of a book promotion. Also they might be more favorable if both the reader and the author were born in the same city, or if the book got a good review from someone they like or respect, etc.