How is naturalism used in A Doll's House?

"Naturalism" can be defined as either synonymous with "realism" or as a kind of subset of it. It usually refers to literature in which human behavior as it occurs in the real world is faithfully reproduced and in which ordinary, everyday events are depicted. Usually it also means a specific kind of realism in which unpleasant and harsh facts of life are presented, with an explicitness that includes subjects previously considered scandalous or offensive. Sexual matters, for example—things that had been thought outside the bounds of what was permissible in art—are often focused upon in naturalistic literature, rather than being discreetly glossed over. The same is true of violence and criminal behavior. In the late 1800's, an opening up of expression occurred, an expansion of what was proper for writers to discuss, regardless of the sensitivities of the reader or of the audience in the theater.
Most of these elements characteristic of naturalism appear in A Doll's House. The story centers around a blackmail attempt by Krogstad upon Nora, hinging upon the crime of forgery she has committed. Sexual matters are alluded to with a degree of frankness tame by our standards today, 140 years later, but shocking to many people in the 1800's. Abusive behavior by Nora's husband is shown with the kind of raw, stark quality that occurs in real life. A harsh, grim atmosphere pervades the story. Finally, Nora's walking out on Torvald represents a defiance of what was considered proper and allowable by the middle-class standards and by the religious codes of that time and later. All of this is characteristic of "naturalism."
One thing we do not see in A Doll's House that many commentators have recognized as a hallmark of naturalism is a depiction of poverty, of a working-class environment such as that shown in, for instance, Stephen Crane's Maggie, a Girl of the Streets. Nor do we see explicit violence such as occurs in Zola's novels like The Beast Within, or people who openly live outside the law as the courtesans in Zola's Nana do. Ibsen shows us what is, on the surface, a sedate bourgeois household, though the setting is a facade for emotions as violent and grim as anything in Zola or Crane.