I just came across another very interesting source by a Canadian scientist, Joseph Henrich, who defends the superiority of the benefits of monogamy on society in a court case defending against an appeal to legalize polygamy.
http://www.vancouversun.com/pdf/affidavit.pdf
He explains the benefits of enforced monogamy on society and why monogamous societies have an advantage. I restrict my comments to one main aspect: One important reason is that monogamy spares society the bad effects of those young men, who become nasty and aggressive, because they cannot have mates, when polygynous men have usurped most of the women for themselves.
As natural selection works on both levels, the fitness of the individual and the fitness of groups, then the puzzling question is, why polygyny did ever evolve in individuals at all?
According to the plausible theory of Kazanawa, males in the tribes in the Pleistocene fought for dominance and the fittest ones usurped all the women. Therefore there were surplus men raiding for women outside their own tribe. They either succeeded to acquire a mate or they got killed and the equilibrium of procreative possibilities was restored.
But there could be another explanation too. In those days, life for men was very dangerous. Hunting huge animals like mammoth with simple weapons was a hazard, but there were bears, wolves, lions, that the group needed to be defended against. Maybe the weakest and unfittest of young men got killed when hunting and defending against dangers. Maybe outgroup raids were attempted for resources like good hunting grounds, and the captured men from the other tribes were mutually just eaten. There are archaeological findings indicating prehistoric cannibalism.
So polygyny could with equal plausibility explained as the cause or as the consequence of the men reacting to the scarcity of women. That means that in those days polygyny solved a procreative problem of male scarcity, and therefore became a part of the genetic heritage as adaptive, while under different circumstances it now in fact leads to disruption by the surplus nasty young men.
But this is more of an explanation of the disadvantages of polygyny than why monogamy could become an option for personal choice.
According to Henrich, monogamy was first made compulsory in Greece about 2500 years ago. This means, the first historical evidence is available for this time. It does not mean, that there could not previously have been a slow development leading towards monogamy during a considerable time span.
It seems interesting to see this turning point on the background of other developments. It is estimated that for a long time until 12.000 years ago, the earth's entire population was about 1 million. 4.000 years ago, it is estimated to have grown to 27 million, 3.000 years ago to 50 millions and 2.000 years ago to 200 million.
This can only explained by the general progress in better adaptation to the environment, as
in this phase of the human history, important skills have been developed, building big dwelling from stone, life in large cities, farming, pottery, the use of metals. All this is well explained by Savanna-IQ Interaction Hypothesis.
But why did the people, who already 35.000 years ago painted animals on the walls of caves, not yet start to farm or build stone houses or write philosophical texts on the walls?
As far as I know, they mainly painted animals on the cave walls, but never any pictures of loving couples.
I suspect, that couples or polygynous families in those days were formed entirely based upon the urge to procreate, that they had not yet evolved the capacity to have any emotion of the kind of what we nowadays can feel as romantic love between a couple, which is based upon the feeling of emotional and intellectual bonds, independent of procreation.
I assume, that the evolution of this kind of love was the reason, that people started to choose to institutionalize monogamy 2500 years ago.
Written texts can be traced back nearly 5000 years, and the Greek philosophy started with the Milesian school roughly contemporary with the institutionalised monogamy.
The same growing ability for abstract thinking, that had enabled philosophy and science to emerge, obviously also has lead to more awareness for the individual needs of personal happiness, and allowed people to consider monogamy as more rewarding for individuals and for the society as a whole. Rationality allowed to consider the instinctive urges from a mental distance.
Such a better adaptation itself can only be explained by a rapid evolution not only of intelligence and rationality, but also of human consciousness, abstract emotions and emotional intelligence.
I only can see one plausible hypothesis as a more general explanation: The evolution of the species homo sapiens as an animal with instincts to survive and procreate under the conditions of the savanna is independent of the evolution of the specific human abilities of the brain, which I am as a simplification have been and am calling rationality as the counterforce to instinctivity. But it really means a lot more, like cognition, abstract thinking, intelligence, self-awareness, emotional intelligence and communication.
The development of instinctive survival and mating strategies like polygyny or monogamy in groups and in adaptation to external factors of the environment are in no way specifically human, and have become part of the genetically based instincts of animals during many millions of years. Logically, they are very persistent and not easily changed in a fairly short time.
It seems that during the last about 15.000 years, there had been a rapid evolution of rationality, that seems to have been accelerating until our present time, and this evolution of rationality has not modified or influenced the persistent instinctive urges. It seems much more, that instinctivity and rationality are two independent forces competing to determine behavior.
If instinctivity is a gas pedal and rationality is a brake, then a human is like a car driven at full speed, and rationality is a brake also pressed to max. The gas pedal is fixed to its maximum position, because this is the result of the long history of animal evolution.
In the beginning of the evolution of rationality, the brake was very weak, and could hardly slow the speed, but with further evolution, the brake got stronger, and the influence of the instincts could be more and more reduced. Now in some people, the brake has evolved so far, that it stops the car entirely. No more instincts drive us childfree people to breed, our rationality is stronger. While the gas pedal is fixed, not all cars without a brake would run at the same speed, because there is a wide variety of the strength of the engines, so a weaker brake can already stop a car from moving, when the engine also is weaker.
As I had mentioned already before, rationality can either control instinctivity or not, depending on the relative strength of both.
