Themes are the fundamental and often universal ideas explored in a literary work.

Human Regression by Industrialization

The resounding theme of The Hairy Ape is the effect of industrialization and technological progress on the worker. Industrialization has reduced the human worker into a machine. The men are programmed to do one task, are turned on and off by whistles, and are not required to think independently. Today, the job of the coal stoker is actually done by a machine. Workers are thus forced into jobs that require nothing but grunt work and physical labor, which has, in turn, caused a general deterioration of the worker into a Neanderthal or Ape- like state. This is made clear by O'Neill's stage direction, which indicates that the Firemen actually look like Neanderthals and one of the oldest workers, Paddy, as "extremely monkey-like." The longer the Firemen work, the further back they fall on the human evolutionary path—thus Paddy, one of the oldest, is especially "monkey-like." As a whole, the play is a close investigation of this regressive pattern through the character Yank—the play marks his regression from a Neanderthal on the ship to an actual ape at the zoo.

The Frustration of Class

Mildred and Yank are representative of the highest and lowest societal classes—as Long would term it, the bourgeois and the proletariat. However, while Mildred and Yank's lifestyles are extremely different, they share similar complaints about class. Mildred describes herself as the "waste product" of her father's steel company. She has reaped the financial benefits of the company, but has felt none of the vigor or passion that created it. Mildred yearns to find passion—to touch "life" beyond her cushioned, bourgeois world. Yank, on the other hand, has felt too much of the "life" Mildred describes. Yank desires to topple the class structure by re-inscribing the importance and necessity of the working class. Yank defines importance as "who belongs."

Class limits and determines both Mildred and Yank's financial resources, educational opportunities, outlook on life, and culture. The Hairy Ape reveals how deeply and rigidly class is inscribed into American Culture and the cultural and financial boundaries it erects.