Why do human beings hurt other human beings? That, says the barrister and sometime judge, Dexter Dias QC, is the most fundamental question in his book Ten Types of Human. In it, we meet sex traffickers, the sex-trafficked, a woman whose career was very nearly ended when she blew the whistle in Bosnia, a man whose life very nearly was, when he tried to stage a rescue. We go from the post-earthquake shanty towns of Haiti, where Hobbesian brutality meets human dignity, to the living room of a woman with locked-in-syndrome; we meet women whose features have been destroyed by acid attacks, whose lives have been changed by FGM, men whose minds have been rewired by violence, we meet people who don’t survive to the end of the book. But this isn’t a story about victimhood.

Dias tells the story of human behaviour through 10 tropes. The Kinsman will protect his or her own gene pool at the expense of any other. It’s illustrated in the first instance by a thought experiment in which there is a gunman in your child’s school (how many classmates are you prepared to sacrifice for the sake of your own? One of his colleagues got to, “all the other children in the world, except for one, for my child to play with) and moves into a detailed consideration of FGM as an iteration of parental love twisted by cultural norm.

There’s the Perceiver of Pain, who doesn’t just see but feels the pain of another, illustrated by numerous psychosocial experiments, but also by the gut-wrenching story of Michael and Anthony, two child slaves of Lake Volta. The Ostraciser exists from human societies all the way down to fish, with the specific example of the tiny, ugly goby fish, which monitor one another’s weight constantly, ostracising fish that gain too much, like in a girl’s school.

The Tamer of Terror is a type to describe how, being conscious of our mortality, we daily face down the fear of it, examining with a goldsmith’s detail the human pain and despair beneath high-profile euthanasia cases, as well as cases of paralysed people who don’t want to die. The Nurturer looks at the complicated impulses of parenting, from self-sacrifice to abandonment, from the foundling wheels of 17th-century Florence to the baby-black-market of modern Russia.

The Romancer is the generous, ingenious spirit we find when we are in love, or even just take a fancy to someone, told partly through the eyes of a hard-boiled Cameroonian. The Aggressor uses violence to ward off the stress of trauma – often the more florid and sadistic the violence, the more effective it is at shutting off the mind’s clamour. The Tribalist drives epic acts of violence, the Beholder responds to facial beauty but sometimes with brutality. The Rescuer is possibly the most uplifting type of all – the person in us who will save another not for a gene pool, nor out of love, but from an altruism so profound it gave Darwin a crisis of confidence.

The proposition is that the human brain, and the behaviour emanating from it, has been shaped by evolution

The proposition is that the human brain, and the behaviour emanating from it, has been, much like the human body, shaped by evolution. Acts of the most heinous violence, as well as acts of the most affecting altruism, are adaptive responses to the situations in which our societies and communities were built. The ultra-aggression of a child soldier is his way of inoculating himself against post-traumatic stress (and, in the moment, it works). Dias’s inquiry falls broadly into the field of moral cognition, “this incredible new discipline, putting together psychology, philosophy and neuroscience to map how the brain makes decisions,” he says, which he discovered recently at Harvard.

Evolutionary psychology is “highly contested,” Dias says. “There is a very persuasive and respectable group of scientists who are convinced that there are evolutionary modules of the brain, and we have these adaptations. There are others who cling to the idea of the brain as a more general computing processor. I don’t think that’s right, but this is an area of contest. It would be highly remiss of me to suggest that this is settled.”

In defining and describing his human types, he draws on lab experiments (volunteers who will pay to end the suffering of another, even if the suffering is pretty mild, and the other a total stranger), the animal kingdom, and all the literature you might expect from a modern behaviourist. Yet the main body of the book, the bits that transfixed and stayed with me, stopped me sleeping but got me up in the morning, were the human stories that Dias has travelled the world to find. His interviewees are always people, the most vivid flesh and blood, never case studies. And yet that’s not what’s so extraordinary about it: storytellers are rare, but not vanishingly so.

Rather, Dias’s work as a human rights lawyer gives him this instinctive, urgent belief that all injustices, all catastrophes, exploitations, acts of violence, all human misery can be taken on and overcome, somehow, if the right people try and there are enough of them. In plenty of these stories, there is no plan; in some, their protagonists have already died or disappeared. Yet his combination of rigour, idealism and pragmatism makes fertile terrain out of the most appalling battlefields. Even if we are hard-wired to kill one another in the vilest ways – “no, no, no” he corrects amenably, “if there are these adaptive mechanisms, it doesn’t mean it’s predetermined” – I emerged from this great slab of a book feeling better about almost everything.

Dias was doing perfectly socially beneficial work as a barrister, over a decade ago, when he was presented with the case of Gareth Myatt, a child who was already dead, having been killed by officers trying to restrain him in a young offenders’ institution.

I’d been practising at the bar for almost 20 years and I didn’t know what we were doing to children in our custody

The heartbreaking detail is that they were trying to remove a piece of paper with his mother’s phone number on it, when he started to resist them. There began the state-sanctioned process that would end a 15-year-old’s life. “It had been two years since he’d been killed [in 2004] when Pam, his mother, came to see me. She was all excited, saying, ‘The inquest is about to start, we’re going to get to the truth.’ And I said, look, Pam, you’ve got to brace yourself. It’s going to be a disappointment. My experience is that when someone dies in the care and custody of the state, all the doors slam shut.

“In fact, Gareth was one of the most difficult, the most acrimonious cases I’ve ever done. The narrative verdict from the jury was very, very damning, failures all the way up to government for instituting a system of restraint of children that was unsafe.”

Legally, he was pleased with the victory, but “I’d been practising at the bar for getting on for 20 years and I didn’t know what we were doing to children in our custody. So I had to make a decision, do I just forget about this, or do I do something about it?”

He had only just been made a QC when he left to go to the Cambridge institute of criminology, to study this precise and rather niche area, not even children in custody, but child restraint in custody. However, this led inexorably to broader questions – “how these practices are produced and reproduced, how they become part of a brutalising culture in a prison”. All that he describes pretty briskly, never shy about sharing a victory: “On a meta level, we cracked it. But I was really aware that, at a more granular level, the intimate relationships, what goes on between a child and a prison officer, I didn’t have a clue.” This ultimately led him to Harvard, to study psychology, where he fetched up on a corridor opposite the moral cognition lab.

These are waypoints on his intellectual journey – professionally, he was all the while making legal headway, notably on FGM, where he took part in a parliamentary enquiry, in which he wanted to “critically evaluate the UK’s protective mechanisms, and whether they met our undertakings under international law. Amazingly – this was after 20 years as a human rights lawyer when no one listens to anything you say – they agreed: a lot of what we suggested was accepted by the government.” FGM is a subject to which he returns often, in the context of the Nurturer – what is it in the way we care for our children that makes parents continue this practice, which they find so appalling to think about that they very often can’t stay in the house when it takes place?

Many of the drivers of this behaviour are not pretty: in our urge to belong, we will overlook the pain of others; to stay sane, we will torture; to protect the group, we will attack those outside it; to overcome our fear of death or the pain of empathising with another, we will think all kinds of nonsense (yet, on the flipside, chase exquisitely noble dreams, as in Dawn’s story, which starts with a catastrophic stroke and ends with her being accepted by York University to do a PhD). Precisely because he won’t sugarcoat the worst of impulses, Dias is deeply credible when he talks about the best. “The science tells you that self-interest is wrong: look at the research into the Rescuer [this is the observed phenomenon of people helping others whom they have nothing to do with, no stake in the survival of]. It’s absolutely fundamental to human beings, I see it all the time, especially sitting as a judge.

Any social group that is not aware of defectors is ripe for exploitation. This is very, very deeply embedded in us

“Jury service can be inconvenient, people would rather not be there. What I see is that, for all these jurors, the case becomes the most important thing in their lives. I just did a case with someone accused of very serious sexual offences. The case had gone over, as ever. Two weeks before, this was just a pain. When they came back to give the verdict, half of them, including the men, were in tears. Why does that happen?

“Look at the altruistic punishment experiment: you have two people playing a game, and a paid observer. One starts to cheat. If you offer the observer the chance to punish the cheat, at a cost to themselves of $2, they’ll do it. Why? This profound sense of justice is an evolutionary adaptation. For us and our communities to have developed, we had to become acutely aware of what are called defectors, people who aren’t playing by the rules. Any social group that is not aware of defectors is ripe for exploitation. This is very, very deeply embedded in us, I see it time and again.”

Will moral cognition become the defining academic inquiry of our age, the one by which we get to the heart of what makes us what we are? I find it credible but am no arbiter. The ideas in the Ten Types of Human are a layperson’s opening into a fascinating academic field: but more than that, a mosaic of faces building into this extraordinary portrait of our species.

The Ten Types of Human by Dexter Dias is published by William Heinemann (£25). To order a copy for £21.25 go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99