The question of "optimum" global human population is an interesting one.

Here's "Optimum Human Population Size" - GC Daily, AH Ehrlich, PR Ehrlich , from '94: (full text: http://dieoff.org/page99.htm )

> All optima must lie between the minimum viable population size, MVP [...] and the biophysical carrying capacity of the planet. At the lower end, 50-100 people in each of several groups, for a total of about 500, might constitute an MVP.

> Above the minimum viable level and within biophysical constraints, the problem becomes a matter of social preference. [...] Human population sizes have never, and will never, automatically equilibrate at some level. There is no feedback mechanism that will lead to perfectly maintained, identical crude birth and death rates. [...]

> 1. An optimum population size is not the same as the maximum number of people that could be packed onto Earth at one time. The maximum would have to be housed and nurtured by methods analogous to those used to raise [battery] chickens, and the process would inevitably reduce the planet's longterm carrying capacity. [...]

> 2. An optimum population size should be small enough to guarantee the minimal physical ingredients of a decent life to everyone [...], even in the face of an inequitable distribution of wealth and resources and the uncertainty regarding rates of longterm, sustainable resource extraction and environmental impacts. We agree with Nathan Keyfitz (1991): "If we have one point of empirically backed knowledge, it is that bad policies are widespread and persistent. Social science has to take account of them." [...]

> 3. Basic human rights in the social sphere [...] should be secure from problems generated by the existence of too many people. Everyone should have access to education, health care, sanitary living conditions, and economic opportunities; but these fundamental rights are difficult to assure in large populations, especially rapidly growing ones. [...] Democracy seems to work best when populations are small relative to resource bases [...]

> 4. We think an optimum population size should be large enough to sustain viable populations in geographically dispersed parts of the world to preserve and foster cultural diversity. [...]

> 5. An optimum population size would be sufficiently large to provide a "critical mass" in each of a variety of densely populated areas where intellectual, artistic, and technological creativity would be stimulated. [...]

> 6. An optimum population size would also be small enough to ensure the preservation of biodiversity. [...]

Ignoring the question of what an "optimal" population size might be, it seems reasonable to attempt to clearly state what criteria we'd use to try to make some kind of decision.

Some other criteria that are not included in the above list could include:

* individuals prefer to have freedom about if/when/by-how-much they reproduce. On the one hand many people don't want to have individual freedoms limited by society or government, although these freedoms might be incompatible with other objectives such as assuring a minimum level of human rights for the whole population. On the other hand, some individuals do have access to additional freedoms or options provided by society: access to contraception, IVF, etc.

* some countries with growing populations & economies are run like some kind of demographic pyramid scheme. Regardless of what an "optimal" population level might be there will be a lot of push back from people in positions in power who don't want to change the status quo.