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I think it is now pretty well established that the Human Condition, throughout what we know of recorded history, and from a biological perspective, on average and most of the time and in equilibrium, has these features:
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1. The average human reared no children to adulthood - the modal average reproductive success was zero - only a small minority of the population were responsible for producing almost all the adults of the next generation.
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2. The average destiny of a human was to die in early childhood - often around the time of birth. Many children were born, many people had children (almost all women) - but very few of these children lived to reproductive maturity. Therefore successful reproduction was mostly a matter of keeping your children alive.
ie. 'Everybody' was fertile; and between-individual reproductive success were mostly an outcome of differential mortality rates.
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3. The average adult came from a social class above the one he ended up inhabiting - so in Medieval England the average adult was a peasant, but his parents would have been craftspeople, merchants, clerks or somebody higher up the social scale than himself. In other societies the average adult would be a peasant but his parents might have been royalty (especially in highly polygamous societies), or successful warriors, hunters.
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4. In other words, the average adult was of lower status and less successful than his or her parents. High status positions were numerically fixed, thus it was a society of downward-mobility; in which lower class people did not rear children, and lower class social positions (niches - especially peasants and their wives) were mostly filled by the children of the upper classes.
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5. Historical societies were therefore far more (biologically) competitive than modern societies - and the punishment for failure was reproductive death (or actual death).
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6. because of this intense competition, and the small proportion of humans who produced a large proportion of the next generation; historical societies were therefore biologically set-up in such as way as to achieve a mutation-selection balance - especially among men, of whom very few had any adult aged offspring.
http://charltonteaching.blogspot.co.uk/2013/06/ma-woodleys-treadmill-metaphor-to.html
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7. 'Modern' societies - ie. those since about 1750 in England, with expanding populations due (mainly) to reduced childhood mortality rates, have been for many generations in a profoundly abnormal state, with respect to reproduction.
In modern societies, more than a half/ the vast majority/ almost-all babies who are born will survive to adulthood, a high proportion of the population produce a high proportion of the next generation. This must have a very significant effect on increasing the population load of deleterious genetic mutations.
i.e. Reproductive success is almost wholly due to differential fertility - because childhood mortality rates are so low.
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8. In conclusion, in biological terms the Human Condition has bee transformed in the past couple of hundred years everywhere in the world.
The situation is unprecedented in human history. The average human can now expect to become an adult; and can expect that any children he may have will all survive to the next generation.
The basic biological, selectional situation of the human species has been radically changed, for up to ten generations (in England).
Biological competition is minimal; the situation of mutation-selection balance has been severely weakened.
This must have had a profound effect on the make-up of the average human genome over the past 200 years or so.
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