Deep Dive
What is the neurological basis for behaviors like sloth and gluttony?
Behaviors such as sloth and gluttony often have underlying neurological or medical causes. For example, sloth can be linked to sickness behaviors orchestrated by neurons that detect inflammation, while gluttony is influenced by the hypothalamus, which regulates appetite through hormones like leptin. These behaviors are not merely moral failings but are deeply rooted in brain function.
How does the brain regulate fatigue and physical tiredness?
Fatigue and physical tiredness are primarily mediated by the brain, not the muscles. Signals from the motor parts of the brain to sensory areas create the perception of fatigue. Studies show that blocking these signals with magnetic pulses can reduce fatigue, indicating that the brain plays a central role in regulating physical exhaustion.
What role does leptin play in regulating appetite and body weight?
Leptin, a hormone produced by fat cells, signals to the hypothalamus about the body's fat stores. Low leptin levels trigger hunger, while high levels suppress appetite. In humans and animals like grizzly bears, leptin levels can change seasonally or due to genetic mutations, significantly impacting appetite and weight regulation.
How do social networks influence obesity?
Obesity can spread through social networks, similar to a communicable disease. Studies show that individuals are more likely to gain weight if their friends or social peers are obese, regardless of household factors. This suggests that social influence plays a significant role in eating behaviors and weight gain.
What is the difference between benign envy and malicious envy?
Benign envy is a positive emotion where one admires another's success and uses it as motivation. Malicious envy, on the other hand, involves a desire to deprive others of their success. Pathological jealousy, an extreme form of malicious envy, can lead to violence and is considered highly hazardous.
What genetic factors contribute to aggression and violence?
The MAOA gene, often called the 'warrior gene,' influences aggression by regulating neurotransmitters involved in anger. Mutations in this gene can lead to extreme aggression and criminal behavior. Additionally, testosterone exposure during fetal development, indicated by finger length ratios, correlates with adult aggression levels.
How does childhood adversity affect anger regulation?
Childhood adversity, such as trauma, bullying, or abuse, can fundamentally alter brain areas responsible for generating and regulating anger. These experiences shape how individuals respond to stress and conflict, often leading to heightened aggression or difficulty managing anger in adulthood.
Are humans morally responsible for behaviors influenced by brain function?
The concept of free will is debated, but changes in brain structure or function can significantly influence behavior. Some individuals may have little control over their actions due to neurological or genetic factors, while others may have more autonomy. Determining responsibility is complex and depends on where an individual falls on the spectrum of brain function.
- Sloth can be a symptom of neurological disorders.
- Tiredness originates in the brain, not muscles.
- Sickness behaviors are orchestrated by the brain to divert resources for healing.
Shownotes Transcript
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Over the festive period, you might have indulged in a sin or two. Being envious of your cousin's new jumper, fighting over a board game, spending all day on the sofa eating chocolates, locking lips with someone inappropriate on the dance floor on New Year's Eve. We all have our particular transgressions, even professors and consultant neurologists.
I've come to the conclusion that I'm actually quite intrinsically lazy. I dislike physical exercise and I will, if given the opportunity, always take the easiest path to a solution. Across his practice, Guy Leschziner has seen the neurological roots of our failings and how we're hardwired to commit them.
We very frequently encounter individuals whose sins, if you will, are manifest in a very obvious way, but there is often an underlying neurological disorder or some other medical disorder, which really raises the question, if these sins can arise from a change in your brain, then that very strongly suggests that these sins are of the brain, and that's the case for all of us.
So today, the biology behind the seven deadly sins. I'm the Guardian Science Editor, Ian Sample, and this is Science Weekly.
Guy Leschziner, you're a professor of neurology and sleep medicine at King's College London and a consultant neurologist. You've recently written a book about the seven deadly sins from a medical perspective. I'd like to start with a sin most of us will have committed over the last few weeks and the one you said you were most prone to, sloth.
In the book, when you describe Sloth, you start with one of your most incredible clinical cases. Tell me about him. Yeah, this is a chap that is so noteworthy that I've actually written about him before. He was a man who has referred to me
for excessive sleepiness to the extent where he would be asleep for over 20 hours a day. This had started shortly after the birth of his child and his wife had been responsible for looking after the child but also looking after him because he was completely unable to function. He'd been seen at numerous different hospital services and no clear cause had been found.
But when we admitted him and recorded his sleep, we saw that actually whilst he was apparently asleep and lying in bed for 20 hours a day, in fact, he was actually only asleep for about eight hours. And for the rest of the time, he was lying in bed awake, but with his eyes closed.
When it was suggested that this might not have an underlying biological basis, we arranged for him to be admitted into a neuropsychiatric unit. And the weekend before he was due to be admitted, he suddenly was cured of his illness.
So that's quite an extreme example of sloth. But of course, sloth manifests in many different ways and can be a feature of normal life. In fact, one could argue that sloth is absolutely crucial to normal life and to our survival. Wow. To go that far to get out of childcare is quite incredible.
But Guy, you say in most normal circumstances, sloth can be critical to our survival. What do you mean? For every action that we undertake, what we're constantly doing is we are weighing up whether it is worth it. What you don't want to be doing is you don't want to be running around burning through all the calories for no good reason.
We know that sloth is part and parcel of normal life, but it's also part and parcel of illness. So everybody will have experienced that sensation, perhaps when they had flu or COVID, when for some time they were completely unable to function. And these kinds of behaviours are called sickness behaviours. Now, in the past, we used to think, well, that's just a sort of toxic effect of whatever organism is invading us. But actually,
But actually what we now know is that there are a small cluster of neurons that detect whether or not we have inflammation within our bodies and actually orchestrate these sickness behaviors very carefully. It's a case of actually diverting resources from day-to-day living in order to heal, in order to recover. What is...
sloth though in terms of what's happening in our bodies how would you describe what physical tiredness is? Sloth I think encompasses a really wide range of symptoms when it comes to physical fatigue well it's very intuitive to think well if I exercise my legs a lot I'm
then my muscles will detect when they're tired. And that's what gives me the perception of tiredness. But actually, more recent evidence suggests that actually, tiredness or fatigue comes from the brain rather than from our muscles. In fact, if you give people an epidural anesthetic, whereby those sensations from our legs are blocked, that
that increases fatigue, not decreases it. So when the motor parts of our brains fire, yes, they send signals down to the legs, but they also send signals to the sensory areas of the brain. And it's that feedback from the motor to the sensory parts of the brain that are the primary mediators of fatigue. In fact, in a series of remarkable studies, they actually blocked those signals using a very strong magnetic pulse.
over the skull and found that that dramatically reduced fatigue as a result of physical exertion.
Okay, so let's get on to the sin I indulged in most over the Christmas break, gluttony. In the book, you tell the story of one of the most successful yo-yo dieters in the world, the grizzly bear, which, oddly enough, can tell us a lot about human appetite and where it can go wrong. So tell me about that. What we know is that in bears, as in humans...
there is an area of the brain that is fundamental to driving our appetite. And that's an area of the brain called the hypothalamus. So our body fat, rather than being a passive store of energy, is also an active organ. It produces chemicals, it produces hormones, and it signals to the rest of the body about how much fat we're actually storing.
One of the chemicals that peripheral fat produces is called leptin. And leptin feeds back on the hypothalamus saying, okay, your body stores are low because your levels of leptin are low and therefore you should be eating more.
What happens in bears is that as hibernation approaches, the thermostat that detects leptin and triggers the drive to eat fundamentally changes within their brains. And so as autumn approaches, their calorie intake absolutely hugely increases to the extent that they put on absolutely massive amounts of body weight and
with a very clear function, which is that over the hibernation period, they may not eat or drink for up to seven months at a time. So what does this tell us about humans? What it very clearly tells us is that in mammals, the level of leptin that is required in order to switch on the drive to eat can change.
And in humans, there are fundamental biological systems that regulate our appetite and influence our body weight, that this is not purely a function of
low moral fibre. And this predilection to eat more than we should isn't just about hormones then. I mean, there's complex genetics going on as well, right? So there are rare families who have mutations in their genes that result in this leptin system going awry.
So there are families out there who have a mutation that causes them not to produce leptin at all, or mutations in a gene that encodes a receptor for leptin. So either way, they cannot get that feedback from their body stores. And in those individuals, the degree of weight gain is really quite dramatic. So by the age of one,
children are often of the order of 10 to 20 kilos in weight and by the age of five they can reach 50 kilos which is really very very heavy indeed.
Now, those are rare cases, but we know that there are countless genetic variants for all of us that have some influence on our adult body weight. Another part of this, of course, is that we live in an obesogenic environment. We're surrounded by these terribly unhealthy but delicious treats and also by other people eating them.
How does that influence us? Well, there's a really fascinating study done in Harvard, I think, from around 2007, which actually demonstrated that obesity, to some extent, behaves very much like a communicable disease, that obesity spreads through social networks. It's not whether or not people are in the same household as you, but whether or not it's your friends, people who you see as similar to yourselves are also obese.
So there's some evidence that what we eat depends on who is around us, but there are lots of other environmental factors at play too, right? Both external and internal. So some viruses have been associated in animals at least.
causing significant weight gain. They can directly infect fat tissue and can cause fat cells to proliferate as well as affecting leptin production. And then there is our microbiome and the impact of the bacteria that we carry within our gut and how that influences the way that we process food, the way that we absorb calories and how much
weight we gain. And I hate to think what's happened to our microbiomes over Christmas with the sort of the onslaught it must have faced. But hopefully these things are fairly resilient. I'm not sure brandy sauce is particularly good for your microbiome, but I think there's probably many culprits. Guy, let's go on to the next sin we may have committed over the past few weeks as presents get unwrapped. Envy.
Now, you describe it in two different ways in the book, envy and benign envy. What do you mean by each? Benign envy is really the kind of envy that we see on social media. So when you are happy for somebody, but you very much utilise that envy in order to achieve more for yourself. But there's also malicious envy, which is the desire to deprive somebody of what it is
that they have, the success that you determine that they have. And obviously, that can be a very negative emotion. Now, at its most extreme, we talk about something called pathological jealousy. Jealousy is often about when you feel that your resources are under threat.
and often has a sexual component to it. And actually, of all the conditions that psychiatrists see, pathological jealousy is one of the ones that they fear most because it so often results in violence in terms of death or suicide. So this is a very, very hazardous emotion. It sounds like most of us will feel some benign envy at some point, and that might even motivate us to...
save up for some new clothing or work harder to get a promotion like that annoying cousin of ours. But when it comes to this more malicious kind of envy you're talking about, you argue in the book that there's a personality disorder that's particularly relevant. Tell me about that. We know that one of the aspects that often drives envy is
is a particular personality trait. And rather surprisingly, that personality trait is narcissism, which at its extreme is narcissistic personality disorder. Now, in narcissism, people often have a very heightened self-view of themselves. They often feel that they're
better than other people. And therefore, one could be excused for thinking, well, if you already think that you as an individual are superior, that you're better, then what have you got to be envious of? But actually, we do know that there are some individuals with narcissistic personality disorder who have a phenomenon called narcissistic vulnerability. So they share that sort of inflated self-importance and entitlement and have some
feelings of grandiosity, but associated with that, they're also very hypersensitive and they're very vulnerable to shame.
And it seems that those vulnerable narcissists have very heightened levels of envy and they enjoy, for example, schadenfreude. They enjoy seeing other people belittled. I think we've probably all experienced that overlap between benign envy where you adulate somebody, you celebrate their success. But at the same time, there is probably a little bit that thinks, oh, you know,
It would be nice if they weren't quite so successful. I think that that's a very common human emotion.
Now, I'd like to ask about wrath. I mean, hopefully none of our listeners felt too wrathful over Christmas, but there's always a risk when you're playing board games with the family. But what makes one person prone to anger and violence while another can keep a calm head? It never enters into their world to be violent. Yeah, I mean, there are a huge range of factors that influence how wrathful, how aggressive we are. Genes are important.
There is one particular gene called MAOA, which has been given the moniker of the warrior gene, or in slightly less polite circles, the psycho gene, because this gene influences the activity of an enzyme that breaks down many of the neurotransmitters within our brain that are invoked
in anger, in aggression. So mutations in that gene can result in a really quite dramatic picture that is inherited, whereby males, and this is an X-linked disorder, so it only affects males, experience really quite significant aggression and anger to such an extent that individuals with this syndrome, which is called Brunner syndrome, frequently undertake criminal acts, arson, rape, murder.
But the reason why that's relevant to us all is because actually there are variants in that gene that are very clearly correlated with levels of aggression, levels of violence, to such an extent that actually the genetic sequence of that gene has been used as a mitigating factor in criminal cases for murder or attempted murder in various countries around the world, sometimes successfully.
So in extreme cases, mutations in that gene or variants of it can lead to aggression and associated criminality. What about other factors? Violence is something that is particularly associated with being male. It's associated with the effects of testosterone on our brains.
And one of the markers which has always struck me as being very surprising for how much testosterone we've been exposed to within our mother's womb during the time of our development is the ratio of the length of our ring finger compared to our index finger.
the longer your ring finger is in relation to your index finger, the higher the levels of testosterone that you've been exposed to within your mother's womb. And if one compares that finger length ratio to levels of aggression or anger during, for example, psychological studies,
You can see correlations between those finger length ratios and how aggressive you are in your adult life. Equally, one of the really important factors is the experience of childhood adversity, like psychological trauma or physical trauma or sexual trauma, but also things like bullying at school.
And it seems that those sorts of childhood experiences fundamentally shape the areas of our brain that are responsible in terms of generating and regulating anger.
Finally, Guy, after writing this book, delving into each of these deadly sins and the sort of biological underpinnings of them, what's your view on the responsibilities we have? I mean, are we morally responsible when we act in these ways? I think this concept of free will, which is ultimately what it boils down to, is...
something that has been argued about for millennia by philosophers and more latterly scientists. And I think that really is a lack of consensus over whether or not we have free will at all. What is very, very clear is that changes within the structure or the function of our brains can influence how we behave. And many of these factors are entirely out of our control.
I suspect that in reality, there is an entire spectrum. And there are some individuals in whom their brain structure or brain function is so altered that they have no free will whatsoever. And there are other individuals who have the individualism
environmental, the genetic, the anatomical basis to enable them to have more say-so. But the big problem is to define where one particular individual sits within that spectrum. Who is the arbiter of where the physiological or the normal becomes the pathological? You know, is it the doctor? Is it the neuroscientist? Is it the priest? Or is it the judge?
A big thanks to Guy Leschziner. His book is Seven Deadly Sins, The Biology of Being Human. And you can order a copy at guardianbookshop.com.
The Science Weekly team is planning an episode all about how to get healthy in 2025. And for that, we need you to send us your questions. It could be anything you've always wondered, from whether intermittent fasting really helps us stay young, how to get started with running, or whether that protein powder sitting on your shelf will really help you build muscle. We'll choose our favourites and bring you answers backed up by science. Just email scienceweekly at theguardian.com.
And that's it for today. This episode was produced by Madeline Finlay. It was sound designed by Joe Cox and the executive producer is Ellie Burey. We'll be back on Thursday. See you then.
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