I believe,  to prevent civilization collapse and environmental ruin and to establish 400 years of sustainability (est. life of hydro dams) at developed world lifestyles, the earth's population should be about 50 million.

I propose humankind implements a limitation of 500,000 births per year. A lottery issues 500,000 birth permits. Universal, but reversible, sterility allows permit effectiveness. Lottery for permits -- implemented by civil law created by majority vote of global population (Mutual coercion, mutually agreed upon-- G.Hardin.)

Today, with 140 million births expected this year, 500,000 permits would allow only 1 birth for every 280 women that normally giving birth. But because at least half of these births are unwanted the restriction would inhibit 139 of every 140 women who wanted children. Only 1 woman would produce a baby for every 140 who wished to do so.

This is very painful. However, this pain does not last very long only 30 to 40 years. Every year, a cohort of 50 million women is replaced by cohort of only 1/4 of million women, after the last large cohort stops producing babies, only the 35 cohorts of 1/4 million are left in the pool to be abused by the lottery.

What is interesting is that those now in the fertility pool are not abused. If you follow the calculations below these women can have the children they want.

If:
      by 2113
      when people probably will live to 100
      the total global population would be 50 million.
                   (100 cohorts of 500,000)

Then:
      the 500,000 birth permits
            would allow every woman (now 25 million total) to each have two children.

To maintain the population,
      all 500,000 birth permits would have to be used.
            or population would drop below 50 million

This means that:
      if:
            about 1/3 of the woman would have no children by choice or health
                  (another woman would have to have these 2)
            about 1/3 of the women would have one child by choice or health
                  (another woman would have to have this 1)
      then:
            the remaining 1/3 would have to have an average of 5 children
                  (their 2, plus the 3 from the above two groups)
                        (that is some have 2 children and some 8)

By contrast, if in an alternative plan allowed every woman to have only one child.
         
      a) the population would not decline fast enough to prevent
            civilization collapse,
             human die-off, and
            environmental ruin, and
      b) Also that once the one child per family became the norm of a culture, when the sustainable population was reached where births would have to equal deaths the one child family would not maintain it and it decline to extinction.

I conclude the genetic bias to have children should not be altered. and civilization law should be enacted to initiated the lottery to attain the flow of births to create a sustainable population.