What is the theme in Arsenic and Old Lace by Joseph Kesselring?

Insanity/Madness are themes in the book. At the time that this play was written – 1941 – many people believed that if one member of a family was “mad”, there was an excellent chance that the rest of the family would be mad as well. This play uses this theme extensively, and takes it to the extreme by portraying every member of the Brewster family (and referring to previous generations) as being insane on one level or another. The single exception to this is Mortimer who, although he actually acts “crazier” in several scenes than any of the other characters in the play, is revealed at the end to be adopted. The audience is left with the idea that as he is not a Brewster, he is not insane.