How does the setting of "Desiree's Baby" affect the story?
The setting of the story helps to illuminate what a terrible hypocrite Armand Aubigny is. Since the story seems to be set in the American south prior to the liberation of slaves, race places a key role in the plot. Most obviously, we know that he casts his wife, the beautiful Desiree, out because it seems she is not "white" (i.e., she doesn't have a completely Caucasian heritage). Their child has a skin color that calls his mother's heritage into question, and it seems that people automatically assume that it is Desiree's parentage that must be responsible for the child's coloring since she was a foundling. Thus, Desiree's husband tells her in no uncertain terms, "'Yes, I want you to go.'" He abandons her either because he is so repulsed by what he believes to be her black heritage or because he knows the baby's coloring is the result of his own parentage and does not want anyone to guess.
However, there are several clues that Armand is having a relationship with another woman who is not white. At one point, Desiree tells her mother that their baby's cry is quite loud, so loud that "Armand heard him the other day as far away as La Blanche's cabin." This implies that they were having an affair. Moreover, one day, when "One of La Blanche's little quadroon boys . . . stood fanning the child" that Desiree seems to realize how similar her child and this boy are. A "quadroon" is an archaic word that described a person with one-quarter black heritage. We learn later that Desiree's baby does have one grandparent who is black: Armand's mother; it is possible, then, that their skin colors could be quite similar. Further, "La Blanche" means the white one in French, and so she must have very light skin, perhaps because she has one black parent and one white (it was not uncommon for plantation owners to rape their slaves, producing children of mixed race). This "quadroon" boy's father would have to be white, then, and—of course—Desiree believes that Armand is white. He looks white. It seems possible, then, that this child is actually the offspring of Armand and La Blanche (and would really be light-skinned, like his parents, who are both half white and half black). These are clues that Armand has relations with his slaves—and yet he casts his wife out for allegedly being of mixed race.
If, however, Armand is actually aware of his own mixed racial heritage, then he also seems like a total monster for condemning his wife. Either way, he is revealed to be an absolutely awful character who lacks integrity and compassion for others, and the concerns about race that are a part of this time period play a role in revealing it.
Although Kate Chopin does not directly state that the story is set in the pre-Civil War South, several details from the story support this setting, such as the story's theme and the inclusion of plantations, slaves and slave owners, and French vernacular.
The story's themes of the oppression of women, male hypocrisy, and prejudice relate directly to the South before the Civil War. Desiree fortunately gets to marry for love, but because of the time period, even love is not enough for her husband to overlook his and others' prejudice toward a possibly mulatto wife. Similarly, Desiree has no choice but to leave her husband's home with their baby because men dominated her society, leaving her powerless. She cannot simply ease into single motherhood; instead, she has to write her mother to see if she can come back home. As a 19th-century woman, her whole life is dependent upon her husband's decisions.
The story's time setting also plays a huge role in Armand's actions. Because he is a slaveowner, he must set a precedent with his slaves in order for them to "obey" him, and decides to expel Desiree and the baby from the home so that no one challenges his authority (if he is married to a mixed woman).
Finally, the story's setting in the French part of Louisiana makes Armand's heritage and his lack of knowledge about it believable. For his mother to have lived in a more forward-thinking France during this time period is not far-fetched, and Armand would have been brought up thinking that his French mother simply preferred Paris to Louisiana.