1. The story could actually happen.
Look at the facts. It's about a mental hospital with a prisoner on the loose. Does this seem pretty legit? You also have the government coming in to help, typical, and there are other problems with the case, such as add-ins,or plot twists, like Andrew Laeddis, which according to Teddy, was a patient at Shutter Island. "Andrew Laeddis, he said, was the maintenance man in the apartment building where my wife and I lived. Okay. He was also a firebug. Chuck took that in, studied Teddy's face. So... Andrew Laeddis, Teddy said, lit the match that caused the fire-- Holy F***--that killed my wife." (Lehane 247). So yes, things can come up in cases like this one, but the fact that Teddy later revealed that the main reason he was here was for Laeddis, does look into how real people have their mindsets, like whether to do their job, or to seek redemption, or in some cases, revenge.
2. The book uses Historical Accounts.
You can probably find this in a lot of fictional books, but what Shutter Island does that not a lot of other fictional books do, is that they go in-depth about the experience. For example, Teddy says that before he was a U.S. Marshal, he served in WWII. He constantly talks about his experience from Dachau Concentration Camp, how life was, what happened, etc. “At Dachau, the SS guards surrendered to us. Five hundred of them. Now there were reporters there, but they’d seen all the bodies piled up at the train station too. They could smell exactly what we were smelling. They looked at us and they wanted us to do what we did. And we sure as hell wanted to do it. So we executed every one of those f***ing Krauts.”(Lehane 290-291). This excerpt also reveals Teddy's character during the war, and even during present time in the book, at Shutter Island.
3. There are mentally insane people in the book and in-real-life.
You can't argue with me on this one at all. Think about all of those crazy, ill-minded people in our lives, or people that are in the world in general, people or groups like ISIS, Osama Bin-Laden when he was alive, Kim Jong-Un, etc. These people, and more, have a shared trait with characters like Rachel Solando and Andrew Laeddis from Shutter Island, they are mentally insane. Like who would test nukes in the range of other countries? Even though they know what happens if they accidentally hit a country? (Kim Jong-Un) or creating a scheme so terrorizing that it has changed every American's life? (Bin-Laden) , or even, terrorizing people to the point of just pure evil. (ISIS). Here's an example of this trait from Shutter Island: "You rapist! You cruel f***ing rapist! My husband will come and cut your throat open! Your hear me? He will cut your f***ing head off and we'll drink the blood! We'll bathe ourselves in it, you sick f***ing b**tard!" (Lehane 348). That excerpt clearly shows that those people that are locked up in those mental hospitals are there for a very good reason. This connects to real-life with mental, or psychiatric hospitals that the United States has.
4. The Government always gets involved.
Marshals Teddy Daniels and Chuck Aule didn't go to Shutter Island on a High School get-together, or on a vacation, they went because the government sent them to find the missing patient. On their trip to Shutter Island, they didn't even know much about the place, other than the fact that its a mental hospital for the criminally insane. "Doctor, Teddy said, it wasn't a request. We are here by the order of the government. This is a federal facility from which a dangerous prisoner--Patient, A dangerous patient, Teddy said, keeping his voice as even as possible, has escaped. If you refuse to aid two U.S. Marshals, Doctor, in the apprehension of the patient you are, unfortunately, Chuck? Chuck said, Obstructing justice, Doctor." (Lehane 106). Alse, when it takes U.S. Marshals to help out on a case, you know, its one serious case. Daniels said it himself, they were there by the order of the government, which connects the book to real-life when the government steps in to help criminal cases.