Ah, theme. Theme is a gnarly thing to discuss because so many people think it is a moral lesson. Bah! I spit on moral lessons. Theme, in the Gauthier world view, is an abstract idea about how people live their lives around which a writer constructs a concrete story. That abstract idea doesn't necessarily involve telling readers how to live. It may just raise questions about how we live.
Some themes occur more frequently in books written for specific age groups. Common YA themes, for instance, will often involve: How we separate ourselves from our families; how we are like/different from our families/peer groups; what will we do with our lives; what do we believe in; what will become of us--Pretty much anything that relates to setting out on our lives and moving toward adulthood without actually being adults.
So adult books that include but are not necessarily limited to YA themes may be of interest to YA readers. At least, that's my argument.
Determining theme, of course, is more of an art than a science. For instance, in one of this week's study subjects, The Dead Father's Club, a possible theme could be how we determine a correct course of action, since young Philip isn't really all that keen on offing Uncle Alan but feels he ought to because Dad's ghost is insisting upon it. That definitely fits into the YA theme scheme of determing what we will do with our lives. But another theme could certainly be children's responsibilities toward parents. When is enough enough? Again, this would fit in with YA themes relating to how we separate ourselves from our families.
Yesterday I was talking about Mary Russell in The Beekeeper's Apprentice, who is a nonYA narrator because she is, technically, an old woman recalling her late adolescence, with adult knowledge of what is going to happen. Though Russell has a great voice, it's not the YA voice teen readers are accustomed to. I suggested this might not be a deal breaker because of theme.
In The Beekeeper's Apprentice, Mary Russell's family is dead. This is what you might call the ultimate separation from family. She accepts a new family in the form of her chosen father, Sherlock Holmes. She "chooses" a father (or falls over him on the first page of the book) who is her intellectual equal. Thus we're dealing with a character who is working out how she is like her "family." As Holmes' protege and an Oxford student she is determining what she will become and moving toward what she will do with her life. At the same time, as a theology student and a Jew who embraces her culture, she differentiates herself from chosen dad. Then, of course, since The Beekeeper's Apprentice is a true mystery (The Dead Father's Club isn't), one of its themes deals with the restoration of order, a love of which crosses over between young and adult readers.
Does theme trump voice when considering crossover potential for young readers of adult books? My guess is that it will depend on the reader.
Off the subject note: It has occurred to me that an adult reading this book who couldn't care less about Young Adult literature, might see themes relating to accepting parental responsibilities, parental love enhancing the parent's life, etc. As I said, determining theme is an art, not a science, and themes might be like communists in the 1950s--under every bush.