Crowded Earth

This Crowded Earth is one of the early entries in the overpopulation subgenre.

It begins with Harry Collins in that far-future year of 1997, and follows him through various adventures to 2065.

Harry is one of the growing number of people who can't stand the over-crowded cities.

He lives in a cramped bachelor's apartment -- he could upgrade to a larger one if he'd get married, but he'd also have to add an hour to his commute each way, bringing the total to six hours.

One day he suffers a mental breakdown and is sent to a bucolic clinic in the country, where he soon enters a relationship with a nurse. Or so he thinks. A doctor at the clinic approaches him with the truth -- the "e;nurse"e; is another patient who was instructed to deceive him in order to start a tryst.

They're both, in fact, part of an experiment to solve the over-population problem.

The doctor is apprehended by the Powers That Be at the clinic, and Harry is told the man's a nut.

But soon his "e;nurse"e; disappears and he ends up in a relationship with another ... and then another ... and another. Bloch plays coy about what exactly the experiments are about, but we eventually learn that they're part of a project to end overcrowding by breeding midget babies. (Goodreads)