Juvenile & Children`s Books/Middle Grade
I am striving to become an children's author and I am currently researching for several possible fiction books, which have plots that are fitting for middle grade (ages 9-11, or 12).
But, in your opinion, could middle grade readers handle a psychological-type book, as in the book Shutter Island, but, of course, keeping in mind my intended audience it won't be as dark as that book.
Your question is a good one, and calls for careful thought. As someone who read a great variety of literature while in elementary and middle school, I can answer from experience that I do believe middle-grade readers can handle junior novels with the type of complex psychological aspects you describe. Elaine Marie Alphin is one good example of an author who has dealt with surprisingly dark psychological matter, and done so primarily for a middle-grade audience. Ultimately, I believe middle-grade readers can handle quite a bit, and as you work with your agent and editor to create and refine a story that could sell, you'll find the right level of psychological challenge with which to confront your young readers.
Good luck on your quest to become a published author. I know the road isn't always an easy one, but it's eminently worth the time spent to travel down it.
Thanks for the question, and if you have any followups, or would like a more detailed response, please don't hesitate to write me again.