There is much more that can be said about what may well be our species's "innate tendency to solve problems with violence." For one thing, it often appears to be territorial. Human beings, nomadic or otherwise, stake out territory and then defend it. This is obviously similar to what certain other primates, close to us on the evolutionary tree, do and so it is reasonable to assume an evolutionary derivation for this behavior.
As societies developed -- got larger and more complex -- efforts arose to control destructive behavior within in-groups. These took the form of the laws referred to by Steven Pinker as well as the present authors. However, sometimes the data seems to belie this claim. For instance, why should the medieval period be so much more violent than the Paleolithic if societal institutions and laws were so much more developed at that later time? There might be extenuating circumstances to explain this, but the glitch does suggest that an overall answer to why the rates of lethal human violence go up and down is complicated and multifaceted.
And, what can we say of the modern era, which is supposed to be humankind's least murderous epoch? If the statistics are correct -- which seems counterintuitive -- we should be reassured. However, less reassuring is the fact that our technological know-how has also supplied modern mankind with nuclear weapons and thus the ability to wipe out our species, and most all the others too.
Part IV -- Dreaming of a Single, Secure In-Group
There may be a glimmer of hope for a more peaceful future if indeed our violent inclinations are tied to acquisition of territory, and within those territories we usually make efforts to minimize intra-group violence. Under those circumstances one can speculate that the development of ever larger states (culminating in a world state) with ever larger in-groups (culminating in humanity as a single in-group) seems the way to go. Then, in theory, law and order within these expanding categories would make for a more peaceful world.
Just to interpose this part of the analysis into today's U.S. politics, we can note that the Republican presidential candidate, Donald Trump, wants to make the country's collective in-group smaller by deporting hundreds of thousands and closing the borders to thousands more. Such a policy can only make the United States more insular and subject to the paranoia of a heightened us-versus-them worldview. On the other hand, the Democratic candidate, Hillary Clinton, seems to be advocating a hawkish foreign policy that emphasizes the need to control foreign territory directly or by proxy but with no inclination to increase the in-group. This too can only make the world a more dangerous place. The view of one candidate or the other being a "lesser evil" might depend on whether you are focused on domestic or foreign policy.
Whatever the optimistic claim of the Nature study about today's comparative level of lethal violence, it seems pretty clear that our laws are not doing well enough to supply the peaceful future most of us hope for. For instance, international human rights laws are so infrequently enforced as to be of minimal effect. And, as current migrant crises around the world make clear, the prospects for ever larger in-groups is but a dream.
All of this only gives added credence to the notion that our willingness to slaughter each other is innate -- an adaptive habit of a long evolutionary history. This conclusion is offered as an explanation rather than an excuse. For, as the Nature study authors recognize, culture can impact such behavior -- tamping it down at least within a designated in-group. Yet it is hard to shake the feeling that our addiction to lethal violence is our evolutionary fate, and that it hangs there, like a sword of Damocles. always ready to impose itself should the delicate strand of law snap.