Although Betty Smith denied every consciously writing a novel with sociopolitical motives, A Tree Grows in Brooklyn cannot be separated from class issues. Nearly every anecdote, character, and chapter represents or addresses the problem of poverty in early twentieth-century America. Being poor means that the characters constantly must think of being poor—how they will buy the next loaf of bread, or what one's house or neighborhood looks like compared to another. Smith shows that poverty does not only imply the absence of food, heat, or comfort. Poverty results in Johnny's worthlessness and death, causes Uncle Flittman to run away, and means that Francie cannot attend high school. Every activity, game, action is planned around a li mited pool of resources. In addition to the Nolan's life, Smith presents an entire poor community, and shows the close connection between poverty and exploitation. Store proprietors take advantage of children's innocence to lure money out of them; piano t eachers beg for tea from their students. Smith's sympathetic treatment of her characters that poverty itself is the evil—not the people. Like the tree man, people just need to think first of their own families and children.
The author often juxtaposes the lower class with people of privilege to further develop this theme. Although having money makes for an easier life, in many cases, the most lovable characters in the book are impoverished, or come from a poor background. The rich doctor reigns as a villain in the book; the charity event is exposed as self-righteous and hurtful. By the end of the book, Neeley and Francie pity Laurie for growing up without any hardship, saying that she will never have as much fun as they did. Like the tree, the author seems to be saying, she "likes poor people."
Although Johnny, Katie, and Mary Rommely vary tremendously in personality, they all agree that education is the way out for the Nolan children. This theme goes along with the immigrant history of the Nolans and Rommelys. Mary Rommely does not even know when she moved to America that education was free; in fact, she still does not even know the word "education," but she rejoices over and over again that her daughters can read and write. Katie has an epiphany part way through the book when she looks at her children who believe that their life is good—who are happy with a Christmas tree and vulgar neighbors and a humble life. She knows that education will save them. This version of "education" is almost always synonymous with "schooling." Although the Nolans do think lessons in life are important, schooling will change their quality of life. Francie is said to have qualities of both her mother and father. Although education will be Francie's way out of poverty, it can also be ostracizing. She and Katie both reflect on different occasions how their relationship will change as Francie becomes more educated and refined; still, the novel privileges the discomfort this separation engenders over ignorance. In the same way, Francie cannot make friends because she talks like characters in Shakespeare, and books eventually replace friends for her.
Gender dictates many different social relations in this novel. Upon Francie's birth, Mary Rommely comments that to be born a woman is to be born into a humble life of pain. This abstract idea of life's pains is juxtaposed with the narrator's consistent reference to the pain of childbirth. At least in the context of this book, women of all faiths and all socioeconomic backgrounds are bound together by the pain of child labor. All in all, the women in the novel are more equipped to handle pain—both emotional and physical—than the men are. Oftentimes, strength is divided on gender lines: Katie is strong, while Johnny is weak; Evy is strong while Uncle Flittman is inadequate; and Katie even sends Neeley to school over Francie, because she is sure that Francie will find a way to go back. One may also think about this theme in terms of geographic spaces. Even in the first chapter, there are certain stores that Neeley enters, but Francie does not, and vice-versa. Women congregate in the kitchen sometimes without men, and the scene of childbirth is considered women's space. Men congregate at the barber or at McGarrity's. Setting and place play such a large role in this novel, that it behooves the reader to consider the way gender interacts with geographic spaces.
Although the novel may seem rather tame to modern readers, there are a surprising number of implicit references to sex and sexuality. The fact that one of the most warmhearted characters—Sissy—has a serious habit of promiscuity suggests that the author is challenging ideas about women and sex in this time period. Joanna's presence in Francie's neighborhood is a startling reminder of how the old world (the stoning women) conceive of women's sexuality. The book rectifies this perspective by showing Francie's reactions to Joanna. Francie sympathizes with her, offers her story for Joanna to read (symbolically offering her knowledge or education), and in general makes clear that the cruelty of the stoners will not be tolerated. Francie's youth implies that in the future women's sexuality will not be confined to narrow and hypocritical conceptions. The continual references to the pains of childbirth also makes visible the idea that women have few sexual health choices; a lack of sexual consciousness, birth control, and technological advances forces them into this experience of pain and suffering.
This theme relates closely to the American dream motif and the symbol of the tree. The strength of the Rommely women suggests that they can withstand any hardship. When Francie is born sickly, she perseveres like the tree; her mother never doubts her strength. Francie's sickness and persistence only foreshadows many other physical and emotional hardships. When Katie has children, she only becomes stronger, determined to make a better life for her children. Every time Katie could give up a little luxury, she puts the money she would have spent in the tin-can bank. This theme obviously relates more to the Rommely women than it does to the Nolan men.