Quality of life versus quantity of life
In any moral system, the big issues get top billing. Life, death, sex and children are logically the big issues for any conscious, self-replicating, and self-organizing collection of atoms. Does that mean that pursuing a longer life and having more children is always good? Or is there a more nuanced perspective on what is best?
Genes Versus Brains
In the pursuit of happiness and well-being, longevity and reproduction are the evolutionary ends, but the means is a brain and body that is capable of more nuance. What is it worth to extend life at all costs, if doing so only creates pain and hardship? This fundamental dichotomy between the goal of our genes versus the goal of our brains is at the heart of the biggest moral controversies of our time: abortion, euthanasia, war, overpopulation and the death penalty. It is also a driver of opinions regarding things like income inequality, sexual permissiveness, social safety nets, and work/life balance.
Applied to the Issues
Take the example of the Catholic Church which applies its moral absolutes consistently, "Thou shalt not kill" is given as a moral trump card. Therefore, abortion, euthanasia and the death penalty are all against their moral code, and war is generally frowned upon except for the occasional crusade or Hitler. In all of these cases, the common thread is that the greatest good, so great that no justification can ever be given, is the extension of life at all costs. In other words "quantity of life".
The opposing viewpoint is that in certain circumstances, happiness and well-being is enhanced by things like not having an incest baby at 13, or dying naturally instead of spending years in constant pain and round the clock medical attention. In these cases the common thread is that the impact on "quality of life" must be the most important consideration in these moral decisions.
While there are many evidence-based reasons to oppose the death penalty in all cases, and war is easy to oppose unilaterally until someone is shooting at you, in all of these cases the utilitarian calculus that prefers the quality of life implications without using the extension of life as a moral trump card will produce the greatest happiness and well-being for society.
Quality of Life and Overpopulation
Check out the population page for a discussion of overpopulation issues and their impact on quality of life. For a detailed analysis using utilitarian ethical calculus, read this article by Derek Parfit (PDF).
The Generating Equation for Progress
In fractal terms, the question of Quantity versus Quality is the fundamental difference in the generating equation between progressive and conservative modern culture. Our society is transitioning from one that had broad consensus around Quantity of Life for centuries, to one that has the general education levels to take on the more nuanced Quality of Life approach. Society will eventually reach consensus again, but only if the fundamental question is addressed directly.
Another way to look at it is that humanity is slowing working it's way through the Kohlberg stages of moral development where improving education causes each new generation to have a higher percentage of the population achieve the post-conventional stages, corresponding to the more nuanced Quality of Life philosophy.
We're Actually on the Same Side
Ultimately the Quantity of Life position is in service of Quality of Life. The only difference is the misapplication of the moral trump card, which puts infinite moral value on the lives of theoretical people. People on both sides of the issue are trying to create a society that maximizes happiness and differ only on the relative moral value of certain actions. Therefore, we can focus on demonstrating how the moral trump card is illogical, rather than subjective topics like "when does life begin?"
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