In my fictional nation, the government has figured out how to provide sufficient nutrition to the entire population indiscriminately. This leaves most citizens time to explore and develop technologies and artisanal crafts. This is similar to what has happened in real life. However, due to the increased life expectancy, there has been an enormous jump in human populations.
If I am looking to avoid this scenario, how can I achieve this using hard science?
Background: My fictional nation lives in a mixed temperate grasslands and forests biome on a planet with average wind speeds of 20 mph where they live. Basic food and water are provided for every citizen, but harder-working citizens earn better supplies. Disease is unfeasible since these humans have an immune system similar to bats. I want to create a civilization that has a limited population at all times. This can be in the form of long-living humans or a high death rate. Systemic purging and relocation are not feasible options. I'm looking for biological changes, like something about fertility or decreasing life expectancy. This is not a governmental issue, I purely want to prevent ecological and societal damage from humans over the years on this planet. A stable population is preferable, just at low numbers.
Question: So, what can be done to avoid a growing population in this fictional utopia?
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