. . and is instead cared for by an income generating couple, that is a cost benefit to the State.

Increasing population requires the state to spend more on infrastructure to support that population, and more on human services as well. Taxes have to be higher with a growing population than for steady state due to a larger part of the population being under wage earning age.

The environmental costs of overpopulation are too obvious to require a study (except for the benefit a study may provide to a person preparing a doctoral thesis).