Mental Disorders in The Mousetrap

In this story, it is made known to the reader that the murderer has schizophrenia, something discovered during his military days by the army psychiatrist. When Trotter (the murderer) snaps and draws his revolver on Mollie, he reverts back to a childlike personality. This switch is attributed to his mental disorder.

In fact, Agatha Christie uses mental disorders as plot devices in a few of her works. This could be due to the fact that mental illness was not well understood during the time period she was alive, and is not even understood fully today. Unfortunately, mental illness as the explanation for the twisted actions of a character has become a trope since Christie's lifetime, and it stigmatizes mental illness, and those who suffer from it.

There is also some debate as to the medical accuracy of the disorders depicted in fiction. The murderer in The Mousetrap is said to have schizophrenia, but he suffers from multiple personalities. His symptoms are more inline with dissociative identity disorder, which actually accounts for split personalities. Schizophrenia gives people delusions, hallucinations, disorganized thinking, or symptoms that mimic sociopathy, but it does not give people multiple personalities.

But, if we take Christie at her word and leave the discrepancy in Trotter's symptoms up to factors we don't understand, we need to determine how he developed this disorder. Christie's writing suggests that it was completely environmental; a product of Trotter's traumatic childhood. But schizophrenia is a combination of environmental and genetic factors, so if Trotter was at risk for this disorder, it follows that his siblings would have been too. If that is the case, why didn't Miss Casewell develop schizophrenia or some other mental disorder as well? She endured a significant amount of trauma, just like her brothers. Of course, the events of their childhood would have affected them both differently, but it is clear that these events still haunt Miss Casewell, even though she is mentally sound.

Ultimately, it seems that giving Trotter a mental illness was simply convenient for Christie. By designating Trotter an 'other', separate from the norm, she saves the reader from discomfort because they can't identify with the murderer. She also saves them from difficult questions of human nature.