China, and the rest of the world, would be better served by a choice-based marketable birth license plan, or ‘birth credits,’ that could stop or reverse population growth on a dime… Each person would be issued half of a birth credit, which he or she can combine with a partner to have one child, or a person can sell his or her (half) credit at the going market rate. Each additional child costs one more credit. Noncompliance would bring a fine greater than the cost of the credit, and there would be sanctions for non-compliant countries (such as migration restrictions).
Birth credits would allow any woman to have as many children as she wants, as long as she buys a license for any children beyond an average allotment that would result in zero population growth. If that allotment was determined to be one child, for example, then the first child would be free, and the market would determine the cost of the license for each additional child. The incentive to society is the prevention of an overpopulation-related tragedy of the commons, including an immediate reduction in unwanted children… As with traffic laws, enforcement of birth credits could be through fines, tax levies, or loss of privileges.
The development of a long-term sterilizing capsule that could be implanted under the skin and removed when pregnancy is desired opens additional possibilities for coercive fertility control. The capsule could be implanted at puberty and might be removable, with official permission, for a limited number of births. No capsule that would last that long (30 years or more) has yet been developed, but it is technically within the realm of possibility. (Pg. 786-7) [emphasis added]
Now, thanks to the one-child policy – to which there are many exceptions, by the way – China’s ageing population will probably not grow much more from now on, as long as they don’t remove the restrictions.
Zero population growth is the minimum we should aim for, but negative population growth would help prepare for the time in the near future when people will live indefinitely long.