Readers of mysteries, like those of romance novels, expect a certain type of ending. Romance readers expect the hero and heroine to be together in the end, living "happily ever after." In mysteries, readers expect justice to be served. The killer meets a fitting end, be it death itself, arrest and conviction, or some other suitable punishment for taking a human life. In that way, readers experience closure and its accompanying sense of satisfaction.
Closure is a psychological term. In the European Review of Social Psychology, cognitive closure was defined as "a desire for definite knowledge on some issue and the eschewal of confusion and ambiguity." So, people have an innate desire for a firm solution as opposed to enduring ambiguity. People's need for closure varies. Those with a high need for closure prefer order, clear rules, and predictability.
I know that I personally have a strong need for closure, and I suspect the same is true of most mystery readers. This need is what drives us to solve the puzzle of "whodunnit" along with the sleuth. We're struggling to close the gaps in our understanding of the situation by looking for clues, interpreting the behavior of suspects, etc.
Sometimes that driving need for closure can cause us to reach a conclusion too early that is erroneous, as described in this article in Psychology Today. And there are plenty of mystery authors that use that to their advantage in devising plots. They plant "twists in the tale" in their stories to drive readers into make one or more false conclusions before finally revealing some new information that leads to the real killer or the real explanation of what's going on.
But knowing "whodunnit" isn't enough for those of us who love reading mysteries. The murderer also has to be punished for their misdeed(s) so justice is served. That is true closure. We understand what happened and the characters receive their rewards and punishments as they deserve them. As I said before, sometimes a punishment is delivered within the legal system, and sometimes it occurs outside the legal system, but it's always fitting. And most mystery authors I know spend a lot of time devising an ending that is fitting and satisfying for their readers.
In my recent release, Deadly Currents, my protagonist, whitewater river ranger Mandy Tanner, is faced with a decision near the end. She has discovered who the killer is and has a choice on how justice should be served. She makes that choice based on her character--her upbringing, ethics, beliefs, and training. Her choice brought a satisfying closure to me, and I hope it does for my readers, too.
What about you? Do you have a strong need for closure, to not only understand what happened and who the killer was, but also to see that justice is served? Have you ever been disappointed in the ending of a mystery that didn't provide that satisfying closure to you?