What if, in a future generation, human fertility has dwindled to the point that fertility is a rare gift?
Every species has a natural drive to survive and reproduce. It is one of our most basic instincts, and let's face it, without that instinct, humankind wouldn't exist today. We fight for survival, and we love to reproduce.
Indeed, in many sectors of human society, couples who choose to remain childless are thought to be failing in their responsibility. This attitude, however dubious, is driven by primal instinct, not reason.
In recent years, studies have revealed some sobering facts. It seems that every generation is, on average, less fertile than its predecessor. We may be growing taller and bigger, but we're not as good at making babies.
The news reports which highlight these studies focus on the generation to generation decline in fertility, but do little to speculate on where it will lead. The reports generally measure fertility in terms of how many children are produced per couple. If the average is less than two then the population is declining.
We have to look a bit harder to find reports which talk about the biological ability of a couple to reproduce, rather than the choices they make. It seems that every generation is less biologically capable of reproduction than the last. According to Skakkebæk et al, “... data on semen quality collected systematically from reports published world wide indicate clearly that sperm density has declined appreciably during 1938-90, although we cannot conclude whether or not this decline is continuing.” .
That's pretty scary. If we were to take this to its logical conclusion, the end result could be catastrophic.
So, why is this happening? According to the Infertility Centre of St Louis, “The human male is known to have the worst sperm count of any mammalian species, with the exception of the gorilla, possibly because the fragile location of these sperm production genes lies on the Y chromosome. The process of recombination "repairs" chromosomes. Since genes on the Y do not recombine, the chromosome degenerates. Thus, the Y chromosome — which makes the male a male — deteriorates with each succeeding generation. It is not a very safe place for sperm production genes.” 
There's plenty of evidence, also, that lifestyle choices of parents, such as obesity and smoking, can have a serious adverse effect the offspring's fertility.
In today's society, most people are fertile, and the infertile are the unlucky ones, but will that change? Fifty generations from now, will we be frantically trying to find ways to secure the future of the human race?
What will society look like if fertility becomes a rarity? Will fertile people still be considered lucky when the rest of society depends on them to propagate the species? In such a scenario, would a fertile person be allowed to choose not to have children? How would genetic diversity be maintained to avoid the birth defects which result from inbreeding? How would the population be maintained?
In today's society most of us consider ourselves to be free, but are we really? We are subject to a profusion of laws which take up an impressively large room full of bookcases, most of which we don't know about and wouldn't understand if we did. More than this, we are subject to social pressures and taboos which greatly influence every aspect of our lives. Still, most of us are born 'free', and spend most of our lives believing that we live so.
Certainly there exists a slave trade, and a trade in unwilling prostitutes, but it is not exactly rare to be human or female. We're not going to be born with any biological features which might make us such a rarity that people would treat us as a resource; a commodity. Something to trade in.
But what if that weren't true? If fertility were rare, would those lucky few be the victims or the benefactors of their accident of birth?
Would they be treated as childbearing royalty or breeding slaves? Would they be able to chose a partner because they love them, or would they be forced into breeding with carefully selected, genetically compatible partners? Would their lives be dominated by social pressures or statutory rules? Would surrogacy be a choice, a lucrative contract between a fertile people and their clients, or would it be state controlled?
Would there be a whole black market; a criminal underworld, trading in the fertility of the few? Would supporting industries spring up? How about services for fertile people who wish to escape the bonds of their condition?
These questions are not easy to answer. How society might respond to such changes is no more than speculation. Nonetheless, if and when it comes, we will have to respond, one way or another.
However we set about tackling the problems posed by increasing infertility, conflict is bound to arise. Different people will have differing beliefs about how to deal with the problem. Whatever solution is chosen, there will be those who don't accept it.
Would that lead to social unrest? Quite possibly. Large scale differences of public opinion on emotive subjects often do.
Will it change the face of politics? The world of politics is there to serve the greater social interest (or so it should be), and what greater interest than the survival of the species?
We don't really know how long it will take for dwindling fertility to become a threat to the future of humankind, or indeed, whether it ever will, but one thing is clear. The trend towards diminished fertility raises some difficult social questions.
For now, we have to rely on works of speculative fiction, such as An Accident Of Birth, for answers to these questions.
 Carlsen, E., A Giwercman, N Keiding, N Skakkebæk. 1992. Evidence for Decreasing Quality of Semen During Past 50 Years. British Medical Journal 305:609-613.