From WD Hamilton, Innate social aptitudes of man. In Robin Fox (ed.). ASA Studies 4 : Biosocial Anthropology. Malaby Press: London, 1975. pp133-53.
The incursions of barbaric pastoralists seem to do civilisations less harmin the long run than one might expect...
It may even be suggested that certain genes or traditions of the pastoralists revitalize the conquered people with an ingredient of progress that tends to die-out in a large panmictic [randomly mating - i.e. not restrictive or assortative] population...
I have in mind altruism itself, or the part of altruism which is perhaps better described as self-sacrificial daring. By the time of the Renaissance it may be that the mixing of genes and culture (...) has continued long enough to bring the old mercantile thoughtfulness and the infused daring into conjunction in a few individuals when then find courage for all kinds of inventive innovation against the resistance of established thought and practice.
Often, however, the cost in fitness of such altruism and sublimated pugnacity to the individuals concerned is by no means metaphorical, and the benefits to fitness,such as they are, go to a mass of individuals whose genetic correlation with the innovator must be slight indeed.
Thus civilization probably slowly reduces its altruism of all kinds, including the kinds needed for cultural creativity.