I gave a talk last night to Dr. Tali Walters’ class on forensic psychology at Tufts University. Dr. Walters is an eminent forensic psychologist who has interviewed some of the most notorious serial killers of our time. She asked me to describe Joseph Vacher, the villain in my book, talk about his crimes and see what diagnosis the students came up with. Almost to a person they decided that he was legally sane and fit to stand trial. In doing so they agreed with the diagnosis of almost every other serial killer, whether in past times or present — that the act takes such cunning and planning that the perpetrators must be legally sane.

I’m struck by this diagnosis, for it illustrates the gray area between psychiatry, the law and common sense.  In any trial involving the insanity defense, psychiatry and the law are used for protection — of  the suspect (if he’s insane) and society (if he’s not).

But amid all the diagnosis and definition, our common sense is tempted to rebel.  Can someone who compulsively kills his fellow humans ever  truly be thought of as sane?

A defense attorney made that point more than a century ago at a murder trial in France. The defendant had killed two children whom he just happened to walk past in a park.

“A crime without motive?” pled the attorney. “Are you not, members of the jury, struck by all that these words mean; a crime without motive! And what a crime! The murder of two children! But who is he who does not immediately respond: ‘This man is mad.’”