Definition 4 in the Newbery Award Terms and Criteria states that a book need not have a single author in order to be eligible for the award. It can be the product of co-authors (as well, unlike for instance, the Nobel Prize, as someone who is deceased, so long as the work hasn't been previously published). I was thinking about this because of the appearance of Patrick Ness's A Monster Calls on several Newbery shortlists, even though that's not a co-authored novel in the usual sense of things, given that, according to the jacket, it was only "inspiried by an idea from" Siobhan Dowd. (Perhaps a good thing for the book's award eligibility, given that although for seven years in the middle of her life, Dowd did in fact live in NYC, she was a British writer who spent the vast majority of her life living there.)
But regardless of how one categorizes Ness's book, it's fascinating to me that, even though 90 Newbery awards have been handed out so far, not one has ever been given to a book with more than a single author. In fact, only seven co-authored books have ever even received an Honor, and three of those were by the same two people: Mary & Conrad Buff, who got an Honor for Big Tree in 1947, The Apple and the Arrow in 1952, and Magic Maize in 1954, but never took home the big prize. Even that hasn't occurred in almost four decades, since James Lincoln Collier & Christopher Collier's My Brother Sam Is Dead snagged an Honor in 1975. (The other three co-authored Honors, for you trivia buffs out there, were Song of the Pines, by Walter & Marion Havighurst in 1950; Mr. Popper's Penguins, by Richard & Florence Atwater in 1939; and the immortal Ood-Le-Uk the Wanderer, by Alice Lide & Margaret Johansen in 1931.)
I don't really know what the reason for this is -- whether people who write the kind of books the Newbery is looking for tend to write alone, the publishing industry isn't friendly to co-authored books, or some other thing or things that are escaping me. I'd be very curious to know. But I find it fascinating that there's a specific line in the criteria for something that seems not unreasonable, but has never, ever actually happened.