Our society assess quantities ad nauseum on very broad scales in all respects - people, economic activity, etc.
Quality of life studies are done on an ad hoc basis and limited to very specific questions and / or limited geography. For example, many reports were written of the drought in Sudan, including all of the people who were suffering, with a focus on the children who died horribly from dehydration and starvation. That drought lasted a few years. The most terrible aspect was the fact that many of the children who died were babies conceived AFTER the drought started.
Any attempt to give you a specific answer will be ad hoc because that is the way the symptoms of overpopulation are collected, and there is no working definition of overpopulation.
There is no clear statement of what overpopulation is, nor for what reasons would a region be deemed over populated, or at what point would it be determined the region is over populated.
Those are very subjective questions - the evidence is often subtle and the symptoms evolve very slowly. We find ways to accommodate the worsening symptoms but all we accomplish is to make the degradation of life more orderly.
For example, there are water shortages in many areas of the U.S. Our response is to mandate water conservation programs. The water tables have dropped by the consumption demands of our society even when we were only 180,000,000± (circa 1960) but we are well over 300,000,000 and still growing.
We have massive immigration from countries that have much more severe overpopulation than we do, and we have vehement public reactions from both conservatives and liberals regarding the distribution and use of contraceptive practices that could help constrain the growth.
Of course, water shortages are only one of many aspects. Studies of ocean reefs suggest as many as 10% have been badly damaged or have died over the past (what, 50 years?). The point is that the pollution levels were much lower years ago than they are now. What can we expect in the future given the current levels of pollution.
Mankind is obsessed with Quantity on a global scale. Our most knowledgeable brains study economic growth ad infinitum but they never discuss how we should equate growth to progress.
For that matter, there is no definition of progress, without thinking about it most people work with the conclusion that growth IS progress, therefore progress must be growth. Worse, there is a prevailing opinion that population growth is hard wired to economic progress. If we have 3% economic growth that is good, no comment if it comes in combination with 4% population growth.
There are generally two concepts within the definition of “overpopulation”. The various dictionaries and Wikipedia do a poor job of differentiating between the two, and that confusion is reflected in the general population and with experts too. One definition can be seen from thefreedictionary: “Situation in which the number of individuals of a given species exceeds the number that its environment can sustain.” The second definition can be see on Merriam-Webster “the condition of having a population so dense as to cause environmental deterioration, an impaired quality of life, or a population crash”
Overpopulation - consuming faster than renewal
Regarding the first, the only way the number of individuals of a given species can exceed the number that the environment can sustain is if they are consuming resources faster than they renew, and obviously humans are consuming resources faster than they renew. We consume oil, coal, natural gas, drain aquifers, and mow down forests, to name just a few, faster than those renew in order to feed the current population. It does not matter that these resources are not directly eaten, what matters is that without them, we could not feed all humans that are alive. This is the situation in every country, therefore every country is currently overpopulated.
We might be able to figure out ways to provide for the current population levels using only renewable means, and there are plenty of theoretical solutions involving wind, solar, and wave energy. If we manage to do this, we can proudly claim we have transitioned from overpopulated to not overpopulated.
The important message is that if renewable means are not found, the population will be forced to drop to sustainable levels.
Overpopulation - suffering the effects of
The second definition of overpopulation is really used to describe “suffering the effects of overpopulation”, or one of being at the population limit. We know from above that at the population limit, the childhood death rate is forced to rise to cancel out the births above an average of two. We also know that humans have generally always been at the limit because the average has always been above two.
From this we can conclude that countries that are suffering a childhood death rate that is above the minimums that have been achieved in the best countries, are suffering the effects of overpopulation. Generally these are considered the developing countries. Note however, that generally these countries are also growing in numbers, which means that their population limit has been continually expanding at the same time. Thus typical developing countries are overpopulated because they are consuming resources faster than they renew, they are at the limit of what the environment can provide because their childhood death rates are miserable and they do not have control of their birth rate, and they also are happening to be improving their skills at providing for their numbers.
The developed countries are not suffering the effects of overpopulation because their childhood death rates are near zero. However, we know that unless an average of two or below is maintained in each country, all countries are destined to again suffer the effects of overpopulation, even though some are enjoying a respite now. We know that every country must figure out how to provide without using resources faster than they renew and clearly fewer people makes that easier to accomplish before the nonrenewable resources run out.