Is being ambitious just about getting ahead, or a sign of an impaired brain? What is the neurological basis of gluttony? Is apathy down to our frontal lobes? Is it all in the genes, or determined from the womb? Is pride a moral failing, or the product of our brains being moulded in childhood?

The answer to those questions could be yes, but also no, and mainly maybe; or so explains Dr Guy Leschziner, whose new book Seven Deadly Sins: The Biology of Being Human delves into the scientific debates behind these notorious vices.

This is Dr Leschziner’s third book, and follows two acclaimed explorations of sleep and our senses. It is a dense read, punctuated by colourful case studies and with a provocative question at its core: do human beings have free will?

“I’m still very much on the fence,” admits Dr Leschziner, who lives in south west London and works as a consultant neurologist at Guy’s and St Thomas’ Hospital, as well as being Professor of Neurology and Sleep Medicine at King’s. “For all of us there are factors beyond our individual control. I suspect that depending on our underlying brain basis, there are individuals who don’t have free will. But if you have the right genetics, the right upbringing, all the other environmental factors that favour your ability to function normally and if free will means that in any situation we can make two or more choices, the majority probably do have it. But there remains a significant proportion of individuals in whom decisions are predetermined.”

As the grandson of a Buchenwald survivor, he admits this is problematic. Did the perpetrators of the Shoah behave as they did because of the way they were wired? “Emotionally, I’m not sure I can say that that’s the case,” he agrees.

The irony, of course, and which is not lost on him, is that the way he thinks about these things is defined by his own brain function and structure. “You have underlying biology and complicate it further through ideology and politics and religion.”

Understanding how the brain influences behaviour might not help him “rationalise the Holocaust”, as he puts it, but he sees it as helpful nonetheless. “At least understanding the building blocks of these events gives us some hope for moulding, for improving society.”

Our brains also change as age. “Some factors are there from the moment we are conceived. Some are defined by our environment.”

A Jewish atheist, he was interested in how religion deals with “sin” and the how the idea of morality underpins decision-making. “From a religious perspective, anger and evidence of decay of the soul.”

He examines vices scientifically, arguing that you can define the acts that follow certain behaviours as good or bad but that it’s more complicated to define a single person thus. “I’m certainly not against a moral framework, but without understanding that some factors are beyond all of our control, it’s difficult to view some with compassion.”

In fact, the book came about from wanting to challenge perceptions of his most difficult patients. “Anybody who works in brain health will see individuals who exhibit a dramatic change in behaviour as a result of some sort of brain disorder or neurodegenerative condition, such as a stroke or tumour, events that can trigger apathy or aggression.”

What about parenting? When you praise your child are you instituting high self-esteem or setting them up to be dangerously over-confident?

“Make sure that you are parenting with warmth but giving realistic feedback, and not simply praising your child unnecessarily at every given opportunity” he says. “There’s quite good evidence that doing this will encourage self-esteem rather than narcissism.”

And what’s the neuroscientific verdict on the popular perception that gluttony is a vice? Can we help being obese and, if we can’t, can Ozempic and the other skinny jabs help?  “They target specific biological mechanisms and what’s becoming clear is that when people stop taking them, they tend to put weight back on. Many factors define our appetite. There are bariatric surgery patients who’ve also had extensive psychological work and who still fail to keep weight off.”

And though he not necessarily against antidepressants the evidence suggests that they too are a stopgap. “Depression and obesity are multifactorial conditions that are as a result of our psychology, brain function and societal factors. Without addressing all of those your chances of long-term resolution are going to be slim.

“As with all things, it’s a real spectrum,” says Dr Leschziner. “There are some genetic conditions you will never overcome. But for most, we have a genetic contribution, but also contributions from other things.

All of which has ramifications for how society approaches criminal justice, he says. “We have decide at some point whether or not somebody’s brain is fixable or if they have a huge genetic burden, that can never be overcome.”

Of course, criminal justice is also about safeguarding people and dispensing  punishment, not least because our brains react to this. But overall, his research suggests the value of a more rehabilitative approach. “I don’t think anybody working within the criminal justice system thinks we spend enough on rehabilitation.”

With modern medicine, at least, things are moving in the right direction. “Historically there has been division between the psychological, the physiological  and the neurological. Certainly in neuroscience we are very much beginning to appreciate that they are essentially the same thing.”

He cites patients who were once written off as hysterical or whose pain was deemed psychosomatic. “We now understand there are fundamental changes to the way that their brain works. Whether that’s related to a physical disease or to psychological factors is redundant. The fact is these individuals have a brain that isn’t working as it normally would.”

And there is much more to learn about them. “Although we know a lot about the brain, there are things we don’t even know we don’t know.”

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‘Seven Deadly Sins: The Biology of Being Human’ by Guy Leschziner is out now​