dear Frederic,
the reason for these flurry of claims may be 1), the book by Steven Pinker, the Better Nature of our Angels but 2) a more recent extensive review on all possible historical data about violence (including cemetary counts, tribal burial sites etc ..)
the study found that human killing is pretty much in line with our primate cousins, but with spikes, and that in general, the trend is downward
I agree with Bob that such figures may not show the difference that matrilineal societies may bring,
but I repeat a very important argument, see the site of the Institute of Psycho-History for details, that a initial phase of 'maternal' attachment parenting was counter-balanced by male initiation and de-sensitizing during puberty, to prepare for the warrior and hunting role.
Also, biologically, I have seen studies showed how the love hormone is linked to violence; the key here is an ability to 'extend' the feeling of love to those outside the bonded kinship family; to my mind, that is not at all a given.
some links: