Best-selling author and Harvard biological anthropology professor Richard Wrangham says humans as individuals are becoming more peaceful individually, but as a group we create increasingly more devastating threats of violence.
Professor Wrangham is the author of The Goodness Paradox, which explores why we don't need to worry so much about day-to-day aggression, but do have to worry about nations and groups fighting.
He talked with RNZ's Sunday Morning, explaining that reactive aggression has been decreasing over time, while proactive aggression still looms threateningly from groups.
"Reactive aggression is when you get insulted by someone who calls your mother a bad name, and you want to fly off the handle and lash out at whoever is creating the problem for you. Proactive aggression is when you might want to be violent to somebody because they have insulted you and you plan to do so over several days - it's premeditated, it's often calm and cool and collected," he says.
"It's not a huge separation, there are regions of the brain that are engaged with both. But it's the particular neurons that are linking these that are activated in different ways. A male monkey might use proactive aggression to stalk and kill an infant in the group who has been fathered by a different male."
He says as our society evolves to become more complex, there's less natural selection for the bigger, stronger, more violent genes, and therefore less tendency toward reactive aggression.
He describes troupes of chimpanzees and bonobos in the Congo; the animals are similar physiologically and socially, and live in the same conditions, but on opposite sides of a river.
The defining difference for the two troupes is that gorillas live in the chimps territory but not the bonobos territory, making it safer for bonobo females to move about unescorted by males, and to form their own female coalitions not dependent on males, and to act as a group to oppose males they take exception to.
"The males that do best in terms of mating success are the males that kowtow to the females."
Whereas on the chimp side of the river where the gorillas pose a physical threat the female chimps statistically have offspring more often with the males that beat them up, rather than those that act benevolently toward them.
Wrangham says while humans aren't paragons of virtue in the way men act toward women, we have evolutionarily moved further from the chimp model and closer to the bonobo model.
"Men are much less aggressive than they used to be."
But there's more to it.
"Now it gets complicated in war, because you have leaders who are telling people who are not making their own decisions, what to do. In general proactive aggressors take great pains to avoid the costs that are incurred in a simple straight-on fight."
But "humans are a dangerous species; we are overconfident when we fight as groups or nations ... We seem to find lots of way of actually seeking trouble, and overconfidence would be a real worry in the context of today's weaponry."
We're warlike and violent in groups, partly because we're overconfident and emotional in groups, he says.
"There's this really remarkable tendency, even if you have methods for finding out what the other side is really like in terms of numbers of weapons they have, and how they're deployed, and their likely capacity. Nevertheless as you approach the time of a potential conflict then there is a documented tendency for each side to stop believing its own spies and start believing much more from an emotional basis, than from direct information.
"We become less rational, and frankly more like some fighting animals that rear up and attack each other in a way that risks tremendous damage to both.
"The point of all this is that if you take the view that many people have in the past that humans are basically a fundamentally good species and they are sometimes corrupted by societal ideologies, then you tend to have the optimistic perspective that humans will only get into really bad conflicts out of some kind of misfortune.
"But if you take what I think is the more biologically and politically reasonable assessment, and think that there is also a part of humans not just adapted for all sorts of good and tolerant cooperative relationships, but also for aggressive and violent competitive relationships, then you have to start thinking about how those adaptations work; because natural selection gives us emotions that ultimately are liable to make the conflict worse."
Wrangham says President John F Kennedy's decision to okay the failed Bay of Pigs invasion of Cuba 1961 as an example of this.
"It's this extraordinary tendency that happens as we approach major war decisions — they tend to overestimate the strength of their own side, and to underestimate the strength of the opposition. In retrospect, Kennedy said how stupid we were to make those assessments.
"Animals probably do the same things - if we could get inside their minds; it's much to do with the importance of confidence.
"Because it really pays to be supremely confident when you are fighting some well-matched individual or agent, because unless you're totally committed to the notion you can win, then part of your energy and mental resources are given to thinking 'well, if we lose, how can we escape here?' and you're not putting all your energies into trying to win — which is of course is the best strategy to be able to win."
He uses Mark Twain's quote: "Nations do not think, they only feel. They get their feelings at second hand through their temperaments, not their brains."
"Sometimes it's just a single leader, and sometimes it's a coalition of the leading politicians who work out a strategy; but it does seem that either way the strategies can be interpreted in terms of rather fundamental psychological attributes of individuals of the human species."
Also, he says the longer peace lasts the worse the casualties and violence once war eventually breaks out.
"That's true, it's just a careful statistical analysis, which of course is slightly gloomy for us right now since we're engaged in a relatively long peace. We'd hope that particular statistic doesn't apply to the end of this one."
Wrangham says while women tend to dominate men domestically, publicly we are still patriarchal.
"The kind of patriarchy humans have is different to the kind of patriarchy animals have, because of the ability of males to form coalitions in which they conspire to maintain their position of dominance. The ways in which modern society is ultimately oiled to make it operate smoothly is by having systems for controlling violent individuals, and the control of violence is ultimately through power.
"We have at the base of our society a coalition of males who are responsible for the ultimate punishments; no society in the world has got away from that - so one feature you can say is that there is no society that is matriarchal in the sense that coalitions of women run the place. The law is always ultimately made by men."
While women have had great freedoms or wealth in some societies, males have always wielded the mechanisms for power.
Wrangham says, while he's personally opposed to capital punishment, historically severe punishments like executions have been important in our evolution away from reactive aggression.
Through much of our development "we've had a capacity for people to get rid of incorrigibly aggressive men in only one way; execution. It so happened that what this led to was a genetic evolution of a new form of human.
"When men were being executed then their genes were being lost. So there was a genetic change in our ancestors, such that those who were typically very given to reactive aggression were the ones that were liable to be killed, and their genes stopped being represented at such high frequency in the population, and we became a calmer, gentler, nicer species.
"Nowadays, of course we don't think about the genetic evolution of the species, we think about whether it's morally appropriate to kill people. And we have alternative methods - we have prisons - those people living hundreds of thousands of years ago had no prisons and had no police, they just had to listen to their own interests, out in the bush."
Wrangham says there will always be competition for power among rival states, and the longer peace lasts after a major war then the greater the chance of conflict arising from shifts in the economic and military power balance between states.
"If the world continues as it is nowadays, then China will be a rising power and America might well be a falling power, and how do you foresee a relationship between China and the US in which China becomes dominant without a war to decide that?"
To hear more listen to the full interview: