I recently recommended a book to a friend. We were chatting in Gmail. A few moments later he replied that the book had earned "only mediocre reviews on Amazon." I was a bit stymied. After all, I myself, his close friend of several years, had just endorsed the book to him specifically. I added a few more reasons for the book's worth. He seemed nonplussed. So far, I don't know if he will have a look at the book.
I mention this in order to ponder the nature of rationality. On the one hand, it is respectably rational to go by a close friend's specific endorsement of a book, or, say, a movie or a restaurant. On the other hand, this being a largely stochastic, statistical world, it is respectably rational to heed the statistical pattern of yea's and nay's for the book, film, or restaurant. But which method of discernment is more rational? Isn't my friend being irrational by ignoring my personal recommendation, specifically raised to target his known interests, in favor of a basically anonymous pool of reviewers from who knows what kind of background? Or, isn't he being irrational by just taking my word for it and not betting more in line with "the odds" (i.e., if the average readers gives it a 6 out of 10, he'll probably give it a 6, and that would be a waste of his money/time). He may know what "all the people on Amazon say" but he doesn't know all the people on Amazon, so he has to add some other layer of rationality to their reviews in order to respect them in the first place. On the other hand, he knows me better than anyone on Amazon, and yet seems to complicate that "rational purity" by superimposing another layer of rationality concerning mass psychology and his own tastes as a socially formed reader.
What splits the decision in terms of ideal rationality? Personal testimony or mass appraisal?