One of the many messages in Thomas Disch's dystopian sci-fi novel is the danger of overpopulation. This was a major concern during the decade when the book was written—the 1970s—and is reflected in the book by the huge buildings of New York City 2021 that house a teeming population of people crammed into tiny apartments.
The governing social control and distribution agency, known as MODICUM, attempts to deal with the problem of chronic overpopulation by severely restricting the numbers of those who can have children. Only those deemed of sufficiently high levels of achievement in various fields—for example, military service, physical strength, and intelligence—are allowed to reproduce.
The overriding obsession with population control does strange things to the inhabitants of 334. A young woman called Shrimp becomes hopelessly addicted to getting pregnant, only for her babies to be taken away from her because she doesn't have a permit. She also happens to be addicted to movies, which provide her with some kind of therapy for her loss.
In the absence of children, people in this dystopian society have no choice but to form bonds of love and affection with fictitious characters on the silver screen. The overriding danger of overpopulation, then, seems not so much a scarcity of resources as an impoverishment of the soul.