We live in frightening times.

Acts of terror carried out by the individual are becoming more frequent and more inventive. Whenever I read about the latest atrocity I find myself asking the same two questions:

What drives a person to be both capable of, and willing to, direct a gun (or a vehicle) at a group of defenceless people?

And what would my reaction be if I were there to see it?

These are just the sort of questions Robert Sapolsky sets out to address in Behave: the Biology of Humans at our Best and Worst. In it, he tracks human behaviour from the moment it occurs, back in time, through the personal, generational and evolutional influences that affect everything we do.

I’ll admit that I approached the book with trepidation. Sapolsky is a neuroendocrinologist and I have concerns about explanations of behaviour that rely too heavily on descriptions of neurotransmitters and the release of ions. Purely biological explanations for our ugliest impulses tempt us to distance ourselves from responsibility for our actions. Reducing anger to a coloured dot on an MRI brain scan really says nothing at all about the source of that anger.

Fortunately, my trepidation was unwarranted. Sapolsky’s account of the science of what drives our worst impulses turned out to be a truly all-encompassing one.

Sapolsky quite rightly starts by saying that all behavioural responses are biological. Of course they are. There is nothing a person can think or feel that does not come with the activation of brain circuitry. He is also quick to add that understanding those processes does not have to exclude the wider influencers of behaviour. Psychological factors are mediated by biological factors so both must be understood together.

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The early focus of the book is on the brain’s immediate response to threat. Central to the story is a small almond-shaped region of the brain – the amygdala. 21st-century imaging is able to demonstrate that the amygdala bursts into action during acts of aggression. But no brain region is exclusive. The amygdala is also linked to the experience of fear, anxiety and depression. So if the amygdala is associated with both fight and flight, how do we decide which to choose?

As sophisticated as the human organism may be, in our immediate reactions to threat we secrete the same hormones as a frightened fish. Yet our potential array of responses are far more nuanced than that of any animal, gill-bearing or otherwise. How do we over-ride our animalistic primitive reactions? The answer, we learn, lies in our frontal lobes. Human frontal lobes are larger and far more complexly wired than those of even the most intelligent of other primates. It is the frontal lobes that temper and refine the amygdala’s instantaneous and ill-considered response to fear.

Behave then takes us through the intricate interplay between amygdala and frontal lobes by looking at studies of human and animal behaviour in all its aspects. The studies cited in this detailed, but still accessible, account are fascinating. In one, subjects are asked to imagine that they are in an out-of-control vehicle; if they pull the brake, they will certainly kill one person, but save 20 others.

Would they do it? Most would.

Then the scenario is changed. The subjects are faced with the same runaway vehicle, but this time the only way to stop it is to push somebody into its path. The person they pushed would certainly die, but the 20 on board would be saved.

Would they do that? Most wouldn’t.

Sapolsky is frank in letting us know that understanding these sorts of responses is a messy business. Emotional reactions are subject to great variability, both within a person and across individuals. The activation of a particular cortical region is not associated with any one function: place a person in an MRI scanner and ask them to contemplate either caring for somebody who is injured or shooting an alien invader, and the same part of the brain is seen to be engaged in both scenarios. Nor does the release of any one hormone produce a single prescriptive response: testosterone could make one person aggressive and the next magnanimous. So what are the factors that determine these differences? To answer this question, Salposky pulls the camera back so that we see more of the person and their environment.

Some influencers of behaviour are purely physiological. Our frontal lobes are not fully matured until we are in our 20s. Since we rely on them to censor our behaviour, it is not surprising that adolescents are prone to poor judgment: an underdeveloped sense of responsibility is innate. Or consider oxytocin, the hormone produced after childbirth, which makes a woman less aggressive and more socially attuned. Or remember that stress overloads our frontal lobes and makes it hard to make a decision.

Other influences are external. That the frontal cortices are still developing into early adulthood means that they are vulnerable to change. The care we receive in infancy will affect our hormone levels, and they in turn will affect brain development and thus learning and memory. Orphans who grew up in Ceausescu-era Romania suffered both emotional and physical neglect, and their brains have been shown to be relatively small in size compared to other adults, while their crucial amygdala is relatively enlarged.

But it would be a mistake to think that extreme deprivation is required to affect how our brains develop. We are all influenced by our childhood experience. What’s more, even the genetic determinates of behaviour are not set in stone. Our genes have “on” and “off” switches. During development, the pattern of genes activated depends on stress hormones and diet and on all aspects of our internal and external environments. On the never-ending nature-nurture debate, Sapolsky points out that, while genes do have a strong influence over behavioural traits, their effect depends entirely on context: “Stimulating environments, harsh parents, good neighbourhoods, uninspiring teachers, optimal diets – all alter the genes in the brain.”

Our frontal lobes help us to navigate social rules, but it is our external influences that teach our frontal lobes what those rules are. Cultures do not need to be worlds apart for differences to become noticeable. In a study comparing men from the south and north of the United States, those from the south were more easily provoked to aggression in response to a personal insult than those from the north. On the other hand, southerners have been shown to be generally more hospitable and chivalrous. As Sapolsky says, “brains shape cultures which shapes brains.”

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It is our interpretation of our own culture that helps us to decide who it is OK to kill: an animal, an enemy soldier, a convict, somebody with an unfamiliar ideology.

Through a truly comprehensive amalgamation of research, Behave draws together all the possible factors that might help us understand the biology of violence, aggression and competition. Skinner’s ideas of behaviourism and Bowlby’s attachment theory sit comfortably alongside a clear explanation of the genetics of the dopamine receptor and how it may have impacted migration.

Behave is also a humorous book, told in a conversational style. Tonally, this did threaten to alienate me: case in point the terms “pissy” and “horny”. But once I had found a way to put aside my own cultural influencers and tolerate the occasional “bad-ass” to get at the nuggets of science that litter this book, it was worth it. Sapolsky is unafraid. He moves from religion to politics to race, describing how our brains have an instantaneous reaction to people of a different colour. Whatever our upbringing, education and beliefs, we are designed to divide the world into “them” and “us”, even if we don’t want or mean to.

This book brings together disparate brain research in a way that is easy to read. Do not be put off by the sheer size of it. It can be dipped into. Each chapter could be understood by itself. Want to know the difference between the brains of people whose politics are right and left wing? Try chapter 12. (Hint: this may not be your favourite section if you are right of centre.) You will never be short of dinner party conversation again.

Did you know that male monkeys instinctively prefer to play with “masculine toys” like trains, while the females prefer dolls? And that the reward hormones in our brains are at their happiest when we are in pursuit of a goal rather than when we accomplish it?

We behave the way we do because our genes and brains are pliable. They have been shaped by our personal experiences but also by those of our ancestors and by evolution.

Genes aren’t about inevitabilities; they are about potentials and vulnerabilities. A biological explanation of behaviour does not obviate our personal responsibility. When faced with the ideological differences in the world, our genes will never mean that the choices of forgiveness and gentleness are closed to us.

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This article has be republished from Sydney Morning Herald, but is originally from The Telegraph, London.