Since you're working with the numbers from the
Ehrlich paper, I'll start by deconstructing some of the problems within it.
Ehrlich et al. 1994
An optimum population size should be small enough to guarantee the minimal physical ingredients of a decent life to everyone (e.g., Ehrlich et al., 1993), 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." The grossly inequitable distribution of wealth and basic resources prevailing today is highly destabilizing and disruptive. While it is in nearly everyone's selfish best interest to narrow the rich-poor gap, we are skeptical that the incentives driving social and economic inequalities can ever be fully overcome. We therefore think a global optimum should be determined with humanity's characteristic selfishness and myopia in mind. A further downward adjustment in the optimum should be made to insure both against natural and human-induced declines in the sustainable flow of resources from the environment into the economy and against increases in anthropogenic flows of wastes, broadly defined, in the opposite direction.
In a nutshell, this is saying, "The 1% is never going to give up the status quo, so we have to plan on numbers that let them keep it."
Problem #1 is right here - part of our problem with "optimum population" is that we're over-consumers. Even the high end of the lower classes in America are over-consumers. They buy things they don't need and can't afford because of the selfishness, myopia, and bad policies mentioned in that section of the paper. Furthermore, point four of their paper actually reinforces that same problem rather than presenting a separate issue:
Ehrlich et all 1994
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. It is by no means obvious that the dominant and spreading "western" culture has all the secrets of longterm survival (Ehrlich, 1980)—to say nothing of cornering the market on other values. We believe that cultural diversity is an important feature of our species in and of itself. Unfortunately, many cultures borne by small groups of people are in danger of being swamped by the dominant culture with its advanced technologies and seductive media, or worse, of being destroyed deliberately because of social intolerance or conflicts over resources.
This paper's authors describe a "decent life" in the context of our Western civilizations, complete with the gross wealth disparity and disregard for others characteristic of it, and then turn around and say "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" - in other words, "We're pretending we think those people are valuable, but we definitely want them to stay over there, on their part of the world."
One of the reasons "optimum population" becomes an issue is because local populations always have been and continue today to be distrustful of immigrants. The strain on local resources automatically makes the local population target immigrants as the problem, as has happened in both the US and UK.
What is the UK's Optimum Population? BBC News, 2012
My husband, daughter and I are currently living with my husband's parents, and my husband's sister lives with us part-time and motel/couch surfs part-time while she's waiting for her new apartment to come available in two weeks. There is enough room in their house to accommodate us; she stays in her old bedroom, my husband, daughter and I share his old bedroom, his parents still have their bedroom, and we have shared access to three bathrooms, a kitchen, a dining room, a living room, a den, a basement living area, a storage room, a two-car garage, a gazebo, and two storage sheds. And yet, there are plenty of Americans who think we don't have a "decent life" because my husband, daughter and I don't have all of that of our own PLUS his sister having all of that of her own PLUS his parents still having all of that of their own.
THAT'S where the bullshit train needs to stop, and THAT'S the "solution" to the "population problem."
Want to stop the abject poverty and criminal activity that runs rampant in dense, urban areas? Start with education - free, comprehensive, complete, and mandatory. If someone's "not getting it," they need to stay in school until they
do get it. Bullying cannot be accepted and schools cannot accept "boys will be boys" or "kids will be kids" as excuses for insensitive, discriminatory, violent, and destructive behavior. This goes for the adult world, too - children found living in those homes need to be moved into boarding schools supervised by adults who are trained in mindfulness and loving kindness approaches to problem solving. Adults who use the same anti-social behaviors are also removed from the general population until they can learn better coping skills, and that goes for politicians, corporate CEOs, high-rollers, and community leaders. We need to stop rewarding sociopaths for making it to the top of their businesses by being "tougher" than the rest or for "kicking down" all their competition. Caps need to be put in place on all industries to prevent those whose only goals are personal gain from attempting the top. An "optimum population" cannot sustain dysfunctional behavior, period. Obviously, this runs contrary to Daily, Ehrlich, and Ehrlich's assertion that we can't change those behaviors and so need to accommodate them.
Want to stop the poor from having a ton of kids? Can't figure out why the lowest castes in India or a starving family in rural Uganda have 15 kids? Provide free and accessible clean water and health care to them. Of those 15 kids, five will die in infancy, one will be stillborn, and the other nine will struggle to make it to age 35, when the surviving child or children (usually eldest son, unless all the sons are dead) will then become responsible for the care of elderly parents who are no longer able to care for themselves.
Think taking the burden off of surviving children to care for the elderly (and thus remove the incentive to have children to take care of them from parents) will reduce the number of children a couple will have? Possibly. In that case, you'll also need to provide free, sustainable, and quality elder care.
Kids and young adults roam in packs called "gangs" because they don't feel any meaningful connections to their blood family or the community at large. They stake out "turf" because of our compulsion toward feeling ownership over places and resources we inhabit and fight - often violently, with fatal results - to protect members of the pack and their turf. This is common in primates, and especially among male-dominated populations. The only time it is not the default is when a population has
evolved without severe competition for restricted resources,
are matriarchal (female-led), and/or
have a cultural change toward more peaceful behavior introduced into the males of the species.
We over-eat due to a myriad of mental and emotional health issues and throw away millions of pounds of food a year because it's not satisfying us. We wear a dress or tuxedo to prom once and wear a different dress or tuxedo to our wedding once, and only a fraction of those clothes will end up being worn by another person. We wear clothes made cheaply in Bangladesh that wear through in a month and throw them away to buy new ones. We create electronics and software that are not backward-compatible so even the few hippies who want to hold out and not over-pay for the newest gadget are eventually forced to throw away a perfectly good item and get a new one. These are not problems of the "smaller" populations of which you speak, not because of "comparative plenty," but because those smaller populations typically are closer-knit communities of people and have learned to maintain both their personal and social emotional health as well as relatively sustainable practices toward their environment. They live in isolation from the "seductive media" Daily, Ehrlich, and Ehrlich describe, and their technology develops with progressive need, not to fill emotional emptiness.
Bottom line (TL;DR) : If you want to find the "optimum population," stop pushing Western culture as the ideal, or even as a "good," culture.
