Memes push humans into close proximity to facilitate their own reproduction - and to do this, the humans need not to bristle with hostility. So the cultural elements manipulate the humans into exhibiting low levels of hostility.
This is much the same process that has resulted in human ultrasociality - though there, since memes are (on average) beneficial, such traits are gradually getting transferred into the genome via genetic assimilation. This process is currently far from complete.
However, Pinker has - fairly recently - publicly paraded his ignorance of cultural evolution (see also his views in "How The Mind Works" and "The Darwin Debate"). Pinker has previously described some aspects of cultural evolution as "one of the domains that we shouldn't look to evolutionary biology to explain" - and he apparently endorses the idea that "the most interesting things about humans are precisely those that evolutionary theory can't explain". So: how come he is writing a book about a topic which requires explanation in terms of ideas which he doesn't seem to know very much about?
Pinker apparently endorses Robert Wright's argument in NonZero - although NonZero is full of cultural evolution and memes.
According to a recent review by Gintis:
Perhaps the most surprising, and welcome, aspect of Pinker's new work is that it is implicitly a devastating nail-in-the-coffin critique of the brand of evolutionary psychology with which Pinker has identified for many years....and goes on to say that Pinker offers a gene-culture coevolution theory. If so, that might be the beginning of quite a turnaround!
This video has Pinker making his case:
Pinker mostly seems to just assume technological and social development, without presuming to explain how or why that development happens.