Nature or nurture: can genes make us behave ‘badly’?

Summary

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Science Weekly – "Nature or nurture: can genes shape our behaviour?"

Date: June 25, 2026
Host: Ian Sample
Guest: Katherine Paige Harden, Behavioral Geneticist

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Overview

In this episode of Science Weekly, Ian Sample speaks with behavioral geneticist Katherine Paige Harden about the complex interplay between genetics (“nature”) and environment (“nurture”) in shaping human behavior—particularly behaviors society deems punishable or “bad.” The discussion is rooted in Harden’s new book Original Sin, sparked by a moving letter from an incarcerated man. Together, they examine the latest research on genetic influences, the challenges of separating genes from environment, the use—and misuse—of polygenic scores, and the ethical dilemmas posed by genetic technology, especially in areas like criminal justice and embryo selection.

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Main Discussion Points & Insights

The Inspiration: A Letter from Prison (01:56–03:08)

• Harden recounts receiving a letter from a prisoner who committed a violent crime as a teenager, asking, “What makes a child go bad? Nature or nurture?”
• This personal connection catalyzed Harden’s exploration into the genetics of wrongdoing and society’s search for scientific absolution.
• Quote:

  "Why should it matter if it's nature or nurture? Why do we look for answers for why we do the things we do? And when we look to science, are we also sort of looking for absolution in those explanations?"
  — Katherine Paige Harden [03:17]

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The Science: Are We Genetically Predisposed to “Bad Behavior”? (04:18–05:44)

• Harden explains her lab’s focus on “externalizing spectrum behaviors”—actions that conflict with societal norms, like substance abuse, aggression, and risk-taking.
  • These behaviors aren't always negative; the unifying factor is acting against some social or moral expectation.
• The research identifies certain genetic patterns (polygenic influences) linked to a higher propensity for these behaviors.

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Shifting Perspectives: Genes, Environment, and Causation (05:44–10:17)

• Traits once believed to be purely environmental (schizophrenia, obesity, even religiousness) are now understood to have significant genetic components.
  • Societal intuition about what is "influenced by genes" has repeatedly been wrong.
• Most gene variants have very small individual effects; behaviors are typically massively polygenic—shaped by thousands of genetic differences, each with minuscule impact.
  • Height is used as an analogy: many small gene effects combine to produce visible traits.
• The word “cause” is contentious; genes act as probabilistic risk factors, not deterministic agents.
• Quote:

  "It's not nature or nurture, genes or environment. It's always genes and the prenatal environment and the social structures that you're being raised in and your access to material resources and poverty and lead exposure."
  — Katherine Paige Harden [09:09]

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Nature vs Nurture Is a False Dichotomy (10:17–11:28)

• While science can measure “genes” and “environment,” the interactions between them blur any meaningful line.
  • Poverty can affect gene expression; children expressing certain genes may evoke different environmental responses.
• Quote:

  "That line between nature and nurture gets real fuzzy real fast."
  — Katherine Paige Harden [10:43]

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What’s the Point of Polygenic Scores? (11:28–13:45)

• Polygenic scores (aggregates of many small gene effects) are not destiny but offer statistical insight.
• Value in research: as control variables to build better developmental models.
• In law: helps humanize those accused by emphasizing no one chooses their genes, akin to how environment is considered in sentencing.
• Quote:

  "No one chose the body and the brain that they have by the time they're an adult from a legal perspective."
  — Katherine Paige Harden [12:50]

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Real-World Risks: Polygenic Scores and Embryo Selection (15:44–18:42)

• Fertility clinics already offer embryo selection for reduced cancer risk, “IQ,” and psychiatric traits—raising ethical alarms about genetics drifting into eugenics.

• Genes rarely map neatly onto single traits; risk factors for one issue (e.g., schizophrenia) may correlate with positive outcomes (e.g., higher education).

• Traits like risk-taking can lead to both social success or criminality, depending on context.

• Quotes:

  "Genes don't have a one to one relationship with behavior... Genes that are associated with an elevated risk for schizophrenia are also associated with higher chances of going to college. Is that a good gene or a bad gene? It's neither."
  — Katherine Paige Harden [16:23]

  "Is risk taking always a bad thing? Is lack of conformity to society's norms always a bad thing? Absolutely not."
  — Katherine Paige Harden [17:16]

• American society currently faces a “wild west” of regulation and debate on these technologies.

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Context-Dependence of Genes: Risk, Success, and Society (18:11–19:41)

• The same genetic predisposition (like risk-taking) might manifest as entrepreneurship in a wealthy environment; in a deprived context, as criminality.
• Society needs variation—deviation is crucial to progress and human flourishing.
• Quote:

  "There's no evolution without mutation, and there's no dynamism in a society without deviation from those norms."
  — Katherine Paige Harden [19:24]

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Genes and Accountability: How Should We Judge? (19:41–22:09)

• Public responses to genetic explanations for crime fluctuate—sometimes seeing genes as mitigating (less blame), sometimes essentializing (more blame, “bad seed”).
• The science of genetics is new, but society’s narratives about inherited “sinfulness” are ancient.
• On a personal note, Harden says her work has made her less punitive, appreciating the contingency and shared fallibility of human life.
• Quote:

  "I don't think of people as inherently bad or inherently good. I'm just continually reminded of our common, fallible humanity. When I think about genetics."
  — Katherine Paige Harden [21:19]

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Notable Quotes by Segment

• On causation:
  "The idea that we're going to find the smoking gun, the thing that is the cause of behavior, just isn't realistic." — Katherine Paige Harden [09:09]

• On the blurry line between genetics and environment:
  "That line between nature and nurture gets real fuzzy real fast." — Katherine Paige Harden [10:43]

• On the ethical ambiguity of embryo selection:
  "The core of eugenics is projecting our normative judgments about good and bad down into the genome." — Katherine Paige Harden [16:23]

• On variation in society:
  "Variety really is necessary to human flourishing." — Katherine Paige Harden [19:41]

• On personal impact:
  "I would say that it has made me much less punitive... I'm just continually reminded of our common, fallible humanity." — Katherine Paige Harden [21:19]

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Key Segments & Timestamps

• Prison letter, origins of the book: [01:56–03:08]
• Defining and finding genetic markers for “externalizing behaviors”: [04:18–05:44]
• Why the nature vs nurture dichotomy fails: [10:17–11:28]
• The value of polygenic scores: [11:28–13:45]
• Polygenic technology and eugenics debate: [15:44–18:11]
• Risk-taking gene in different environments: [18:11–19:24]
• Genetics and criminal accountability: [19:41–22:09]

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Conclusion

This conversation moves beyond simplistic debates, emphasizing that genetic influence on behavior is real but deeply entwined with environment, context, and values. Harden advocates for humility, compassion, and democratic debate as science gives us new tools—and new ethical dilemmas—about accountability, blame, and who we are.

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Transcript

[Katherine Paige Harden]

This is the Guardian.

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[Katherine Paige Harden]

I work at a university in Texas, which means my university mailing address is just the address of the Department of Psychology at the University of Texas. And so I sometimes get mail from people I don't know.

[Ian Sample]

Catherine Paige Harden is a behavioral geneticist. She studies how our genes impact who we are and the choices we make. She's used to her work attracting interest, but among her post one morning was something jaw dropping.

[Katherine Paige Harden]

One day I received a letter from a man who is in prison in one of Texas's oldest and largest prisons. And he said that he committed a crime as a teenager. He kidnapped a woman and sexually assaulted her, left her naked on the side of a road. And I believe he was 15 or 16 at the time he committed this crime.

[Ian Sample]

The man had read about her work in a local magazine and he had some questions.

[Katherine Paige Harden]

So he wrote me and he said I have 10 questions for you. And one of them was what makes a child go bad? Nature or nurture? And I was just really stunned by the letter.

[Ian Sample]

It became the jumping off point for her new book, Original Sin, on the genetics of wrongdoing, the problem of blame, and the future of forgiveness.

[Katherine Paige Harden]

Why should it matter if it's nature or nurture? Why do we look for answers for why we do the things we do? And when we look to science, are we also sort of looking for absolution in those explanations?

[Ian Sample]

So today I talked to Paige about what our genes can predict about our behavior and asked if our genes make us more likely to commit a crime and are we just as guilty? I'm the Guardian science editor, Ian Sample, and this is Science Weekly. Katherine Page Harden, welcome to Science Weekly. Now, the letter we just heard about came at an interesting time because it coincided with a paper your lab produced linking DNA differences with people's tendency to behave in ways that we, as a society, punish. So take me through that research. What were you looking at and what did you find?

[Katherine Paige Harden]

Yeah, so the technical term of what we're looking at is called externalizing spectrum behaviors, which is a mouthful. And it's basically the idea that sometimes people have problems that are internal, like depression and anxiety, and sometimes people have life problems that are more external, so they come into conflict with social norms, moral norms, legal norms. And externalizing behaviors can range from the very minor. So you smoke cigarettes even though you know that it's bad for you, you drink alcohol to the point that it's a problem in your life, to various serious harms like you aggress against another person. So what we were looking at is could we identify specific DNA sequences, differences in the DNA letters between people that are correlated with being more likely to engage in not just one form of externalizing, but all of them. So these are genes that make it more likely for you to smoke and. And also to drink and also to have sex with more people and also potentially to aggress against another person. And for some people listening, you might think, well, having sex with lots of people and aggressing against someone, those are totally different behaviors. One is not necessarily harmful in any way. And that's absolutely true. But what all of these behaviors have in common is that at least someone in society says, you shouldn't do them and you do them anyway. And so what we see is that there's genes that are in common across all of these things.

[Ian Sample]

And there's an idea in there, then that people can be genetically predisposed or more at risk of criminality. And that's obviously controversial and it's unsettling in some ways. But you make the case in the book that there are many other Things that we used to put down to sort of nurture environment that actually have turned out to have these genetic components. Things like obesity, things like religiosity, things even autism. People put that down to environmental factors not too many decades ago.

[Katherine Paige Harden]

Yes. So there's this idea that we can kind of pre intuit or pre know which behaviors are going to be influenced by genes and which aren't. But then if we look in our history, we can see obvious examples where we were wrong in the past and our intuitions have shifted in response to science. So schizophrenia used to be thought to be the result of a psychiatric schizophrenic mother. And we now know that it is very strongly heritable. We used to think of obesity in terms of sloth and gluttony. And now, especially in the age of ozempic, we can see that it's profoundly biological and can be manipulated by injecting a peptide in your body. So we can see over time that societies always kind of have a carve out where we think, well, this behavior is somehow floating free of our body. But that's almost never the case. All of our behavior is being produced by a body, by a brain, and that brain's going to be shaped by the genes we happen to inherit.

[Ian Sample]

How much difference does like each actual gene variant make?

[Katherine Paige Harden]

So ordinarily, for the vast majority of people and for the vast majority of genetic variants, a difference between us and our genes makes only the tiniest bit of difference to our probability of showing a behavior. I think the best analogy for helping people understand this is to think about height. So there's a few people who inherit genes for gigantism that make them very tall, or genes for dwarfism that make them very short. But most of the reasons why people are taller or shorter is because there's thousands of genes scattered throughout the genome that might make you a millimeter taller or a millimeter shorter. But even though each one of those genes only has a tiny effect, you could have many, many, many height increasing genes and then you're gonna end up on the tails. And it's the same sort of thing for behavior. So we're looking at things that are what are called massively polygenic. So poly meaning many genic, meaning obviously genes. So there's many, many genes involved. Each one of them is just like adding a little tiny hair to the scale. But we have a large DNA sequence and we have a lot of genetic variants and so they can add up so that it makes a large difference at the tails of the Distribution.

[Ian Sample]

And you talk about these polygenic scores which look at, you know, the contributions of lots of genes and how they correlate with like, risky antisocial behaviors. But even when you look at sort of groups of genes where these effects may add up, is it possible to attribute causation at like an individual level? Or, you know, are there other factors you have to consider? And obviously the things that come to mind are like cycles of deprivation, abuse, racism, you know, these other factors that are gonna be non genetic.

[Katherine Paige Harden]

So I think you've just said the most controversial word of the podcast, and that's the word cause, which is a word that not even philosophers of science agree on and not even different branches of science agree on. So I'm trained as a psychologist, as a social scient. So we tend to think of causes as probabilistic difference makers. So if you inherited something, it makes a difference to the probability of showing a behavior. And our best evidence that these genes are actually causal is that we see that they make a difference even within families. So if the genes are associated with behavior, even when comparing between siblings or even when controlling for your parents genes, that's pretty good evidence that those genes are operating causally. But everything we're talking about with human behavior doesn't have just. So it's not nature or nurture, genes or environment. It's always genes and the prenatal environment and the social structures that you're being raised in and your access to material resources and poverty and lead exposure. The idea that we're going to find the smoking gun, the thing that is the cause of behavior, just isn't realistic.

[Ian Sample]

There's something I want to ask a bit more about, because in the book you say there's no way to meaningfully distinguish between nature and nurture, that our genes and our environment combine in ways that are just impossible to pick apart. But I wonder if you could unpack that, because clearly you're saying, and sometimes drawing on these twin studies, you're saying that you can isolate genetic components and you can say that genes do affect the chances of someone doing something.

[Katherine Paige Harden]

So you can measure different factors and you can loosely categorize them as I'm measuring your DNA, so I'm measuring your genes, or I'm measuring the amount of income that your family makes. And so I'm measuring your environment. But as soon as we shift our attention from measuring how people differ to trying to understand why those differences matter, we inevitably end up in the world of gene environment interaction. So we know that poverty gets under the Skin to change how your genes are expressed. We know that children with genetic differences experience different environments because adults respond to them differently. They. That line between nature and nurture gets real fuzzy real fast.

[Ian Sample]

So the influence of these gene variants in the average person is still nowhere near like, deterministic. We are not. We're getting away from the idea of genes being deterministic, thank goodness. So if that's the case, though, what is the value of the kinds of polygenic scores that you are interested in? If they're not gonna say, yes, this person is destined to be a wrongan, what is the value of those scores?

[Katherine Paige Harden]

Oh, I'm gonna use the word wrongan. I'm gonna try to put that accent and take that from here. So I think it's helpful to think about how we use other variables that are not deterministic but do make a difference. So we know, for example, that children who are raised in poverty are more likely to develop substance use problems. That's not deterministic. If you're raised in poverty, you're not destined to have an addiction, but it does raise your vulnerability for that. So how do we use that information? One way we use that information is just. It's another variable to control for when we're trying to develop these models of development. We wouldn't compare a kid from a rich family and a kid from a poor family without taking that variable into account. The other thing is that it serves to humanize people when we're confronted with their bad behavior. I don't know how it is here, but in the American criminal legal system, it's very common in the sentencing phase for the defendant's representation to talk about, well, how has his or her life been shaped by these environmental factors? And again, it's not that if you were maltreated or you were raised in a poor home, you were destined to commit a crime. But it does help the jurors and the judge perhaps understand how might he have come to this place in their life? So I think genetic information is very similar. It's another variable that's probabilistically related, and it can serve to be a control for research that's kind of the least sexy application, but the one that scientists are most interested in. It can serve to humanize people, to remind us that no one chose the body and the brain that they have by the time they're an adult from a legal perspective. And it can help us see who's being left behind.

[Ian Sample]

Coming up from the lab to the fertility clinic, the troubling real World applications of polygenic scores.

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[Ian Sample]

We were talking about how some of this work can feel unsettling. And we know that fertility clinics are already offering embryo selection using polygenic scores and that they are selling, you know, selection for lowering risk of certain cancers. But even iq, whatever they mean by iq and psychiatric conditions like bipolar and schizophrenia. Which leads you to the question of whether the kind of work that you're doing will make people wonder, oh well, does this open the door to selecting for well behaved people which obviously starts getting into, you know, we get into the realm of eugenics, it has to come up.

[Katherine Paige Harden]

I think we're already in the realm of eugenics. There was ads in the New York City subway a few months ago saying have your best baby. And it was an ad for a direct to consumer polygenic testing company for embryos. And this technology raises really urgent questions about our values and the tensions between those values. I think what's important for people to understand is not only are these genes not deterministic, but genes don't have a one to one relationship with behavior. Genes do lots of things in our body because what they do is they code for proteins, and their proteins are involved in lots of different processes. So, for instance, genes that are associated with an elevated risk for schizophrenia are also associated with higher chances of going to college. Is that a good gene or a bad gene? It's neither. And the core of eugenics is projecting our normative judgments about good and bad down into the genome. When it comes to the specific genes that we're looking at in relation to aggression and antisocial behavior, we also see that they're associated with positive risk taking. We also see that the people who are most likely to be entrepreneurs by the time they're 30 or 35 have a history of delinquency and adolescence. So is risk taking always a bad thing? Is lack of conformity to society's norms always a bad thing? Absolutely not. I think America is a real wild west right now in terms of lack of regulation, but also just lack of democratic participation and debate about how do we want to use this technology and how do we not want to use this technology? And I don't think it should be one person or one company's vision for how this technology is deployed.

[Ian Sample]

And it's interesting again how you go through in the book where risk taking can have different outcomes depending on the environment that that risk taking person is born into. And so if you're risk taking a new to be born into an extremely wealthy family on the west coast and you get into the tech world, then maybe your trajectory is one that's praised by society. But maybe if you are born into a different family under different circumstances, that risk taking might end up with you just stealing loads of cars and being aggressive and ending up in prison.

[Katherine Paige Harden]

Exactly. I was having this conversation with a friend of mine and she said, well, wouldn't everyone, if they had the chance, select their child to be, you know, the least likely to be addicted to drugs, the least likely to have adhd? And I said to her, didn't you have a child with a fighter pilot like you mated with someone whose entire job depends on taking risks and the ability to manage those sensations, even at the, you know, real negative physical consequences of crashing this plane into the ocean? So I think we have this idea that, you know, less risk taking is always good, but our behaviors often belie that statement.

[Ian Sample]

And we probably do want that variation in society, right?

[Katherine Paige Harden]

Exactly. There's no evolution without mutation, and there's no dynamism in a society without deviation from those norms. I mean, I've loved being in London this week in part because you walk down the street and you see such variety, and variety really is necessary to human flourishing.

[Ian Sample]

And coming back to the. The premise of the book, this question of whether we might feel differently about someone's crimes if we knew there was a genetic element to them. What do you feel the evidence shows at the moment?

[Katherine Paige Harden]

I think the evidence shows that our intuitions are wildly all over the place, depending on the person, depending on the person that's being judged, and depending on the crime. So sometimes genes tend to have a humanizing effect or a mitigating effect. And that happens when there's this idea that there's a person and then the genes are constraining that person's agency in some way. My genes made me do it. I couldn't have done otherwise. That's a more determinist story about genes. But it's very easy to slip into a more essentialist framing of genes, which really sees the genes as a reflection of who a person really is. They're bad to the bone. They're a bad seed. And that tends to make us more punitive, or at least less likely to trust that there's hope for rehabilitation and change the future. Genetics is a very, very new science, and our ideas about heredity and sinfulness and can we inherit something that makes us blameworthy are much, much, much older. And so the new science is kind of being grafted onto these much older debates that we have.

[Ian Sample]

And so, finally, Katherine, I want to know where you come down on all of this. Or maybe you're, you know, delaying coming down on one side, but how do you think that the knowledge you and others are gaining should inform our approach to accountability to punishment?

[Katherine Paige Harden]

So I won't say how I think everyone should approach this, but I will tell you how I have ended up approaching it. As someone who has spent most of her adult life thinking about how are genes connected to the bad things or the things that are called bad that people do? And I would say that it has made me much less punitive. And I live in one of the most punitive states and one of the most punitive countries in the world, because I see how my life could have gone differently not just if I had been born into a different family or different environmental circumstances, but if there had been a different combination of DNA letters when, you know, the egg or the sperm that made me happened. I don't think of people as inherently bad or inherently good. I'm just continually reminded of our common, fallible humanity. When I think about genetics, I'm sure

[Ian Sample]

you've got more letters that will be coming into your mailbox. Paige, thank you so much.

[Katherine Paige Harden]

Thank you for having me.

[Ian Sample]

Thanks again to Katherine Page Hardin. To support the Guardian, you can order the book original sin via guardianbookshop.com and that's all from us. The sound design was by Ross Burns and the executive producer was Ellie Burey. We'll be back on Thursday. See you then.

[Katherine Paige Harden]

This is the Guardian.

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