Mortimer’s loyalty to his family is constantly tested throughout the play. He grapples with the choice between his birth family, the Brewsters, or his future family with Elaine. In moving forward with his engagement, he will be abandoning the women who raised him. Conversely, in staunchly defending his aunts, he is rejecting the possibility of a new family with Elaine. Mortimer’s character arc in the play requires him to define how he will choose to conceptualize his family and deciding whether he can shed the metaphorical weight of the Brewster family name in order to save himself.

Kesselring’s play hinges on characterizations of mental illness that were typical in 1941; psychiatric conditions are wrongly conflated with criminal behavior or insensitively portrayed for comedic effect. This outdated conception of psychiatric conditions is essential to understanding Mortimer’s relationship to his family. Throughout the play, various characters discuss the violence, peculiarity, and “insanity” that appear in many of the Brewsters in various forms. Mortimer fears that his status as a Brewster means that he will also inevitably develop these tendencies. In one of the more popular quotes from Arsenic and Old Lace, Mortimer tells Elaine, “You see, insanity runs in my family. It practically gallops” (55).