When developmental psychologist Liz Gershoff began studying the effects of spanking and harsh parenting discipline in the 1990s, the topic was still the subject of intense debate in scientific circles: Was physical punishment actually harmful to kids?

In the decades since, a growing body of research has offered a clear and resounding answer. Spanking and other forms of severe discipline — such as verbally berating or humiliating a child — have been repeatedly linked to behavioral, emotional, psychological and academic problems, a conclusion that prompted the American Academy of Pediatrics to issue a new policy statement in 2018, strengthening its stance that parents should not use physical punishment.

But spanking is still prevalent in American families, and legal in all states. Though it appears to be steadily falling out of favor among younger generations, the 2018 General Social Survey — a long-running biennial national survey of American adults — found that 66 percent of Americans agreed that “a good, hard spanking” is sometimes necessary to discipline a child.

And one common argument in support of spanking has lingered: How can we be sure that a child’s lack of achievement or antisocial behavior can be traced back to physical punishment specifically, versus an innate or genetic factor?

Gershoff, a professor of human development and family sciences at the University of Texas at Austin, set out to settle the question of nature vs. nurture with her newest study, published in the March volume of Psychological Science. Gershoff and her research team analyzed more than a thousand pairs of twins — including more than 400 identical twins, who share the same DNA — many of whom were disciplined differently by their parents. The researchers found that the child who was hit or yelled at more often was consistently more likely to display delinquent or antisocial behavior.

“Identical-twin studies are sort of the classic way that psychologists have of differentiating what is innate behavior from what is learned behavior, so this study follows in a long tradition,” says Robert Sege, a pediatrician at Tufts Children’s Hospital and director of the Center for Community-Engaged Medicine at Tufts Medical Center who co-authored the American Academy of Pediatrics’s 2018 statement. “This is yet another, different way of looking at this, but all the data points to the same direction. As a scientist, when you see that, no matter how you do the experiment, no matter how you ask the question, you get the same result — that’s conclusive.”

Gershoff spoke with The Washington Post about her decades of research into physical discipline in parenting, and the implications of her newest study. The conversation has been edited for length and clarity.

Q: You've been studying the effects of spanking for a long time. How has the scientific understanding of this issue changed over your career?

A: When I began studying physical punishment, there was a lot of resistance to the idea that it might be harmful. I went into this with an open mind, thinking, ‘Well, maybe parents are right, maybe it does work, maybe that’s what the research shows.’ By then we had several-hundred studies that had been looking at it, but no one had taken an overview to say, ‘What did we find overall?’ So that’s what I did in 2002. There have been many, many more studies since then, and they have all continued to show that the more children are physically punished, the worse their behavior, the worse their mental health. Now we also have research showing they do worse in school, they have lower achievement.

Q: It seems like those conclusions are generally accepted among scientists and pediatricians, but what have you observed about our social and cultural perceptions of spanking?

A: It used to be that we only learned how to parent from our own parents, that was it, and maybe a couple friends. I think in the last 20 years we’ve seen more parents have access to parenting information from parenting books, from the Internet. Now we can talk to people across the world and find out how they’re parenting, what works for them. There has been more public discussion about physical punishment that we hadn’t really seen before — you started seeing figures like Oprah talk openly about how she was physically punished, and how harmful she thought that was. That was a big deal.

But there are still many parents who are under the impression that you have to hit children in some circumstances. That pattern cuts across cultural groups and racial groups and different areas of the country. So yes, there are still some parents to convince.

Q: Tell me about your new study, and why you focused on twins specifically.

A: Our first goal was really to address our colleagues who were skeptics about the data, the ones who said, “You can’t do experiments, so how do you know for sure it’s not genetic?” This is our attempt to answer, because — they’re right, I can’t do experiments. I can’t assign children to be randomly hit. But there are other ways to get at causal inference, and this study was really meant to get at that genetic question.

This was an ongoing study of twins in Michigan, including both identical twins who have the exact same genetic material, and also fraternal twins who share half their genetic material, following these twins since birth. My colleagues who are running the study said, “Could we look at the effects of harsh parenting, and can we look at it within twin families?” It’s a way to look at the genetic impact, to answer the question of, “Is this nature or nurture?”

What we did was we asked both of the twins and both of the parents to report how often parents physically hit their children, and also how often they yelled at them or said mean things to them. And then we looked at children’s antisocial behavior — like lying, stealing, getting into fights, destroying people’s things, that kind of stuff. We were looking to see: If you look within the same family, if you look at differences in how one twin is punished versus the other, does that predict differences in their antisocial behavior? If the twin who gets more harsh parenting is also the more antisocial one, then that would suggest that there’s some kind of environmental component, and it’s not genetic. That was the design of the study. And the vast majority of parents reported that they disciplined the kids differently.

Q: Wow — the parents treated their kids differently?

A: It basically shows that parents see twins as individuals, they’re not parenting them as a set. And that much is good, but it’s just too bad that one kid is getting singled out for the harsh parenting. We didn’t ask them about the circumstances, so we don’t know why. But what we found was that the twin who was spanked more or yelled at more within each pair, they were the ones who had more antisocial behavior. It was the same for the kids who were identical twins and for kids who were not identical twins. The amount of genetic material they shared didn’t matter. It came down to how much harsh parenting they received.

Q: Was that surprising, how clear the results were?

A: Oh, yeah. My colleagues who run this study look at twin differences all day long. They said this is one of the strongest patterns they’ve seen. When we study children’s behavior, oftentimes there’s a mix of both genetic and environmental factors, so we were expecting that when we looked at harsh parenting. But this one, it really was the parenting that explained the behavior.

Q: Do you think this study settled that question then, of whether a child's innate characteristics might be to blame for their behavior, rather than harsh discipline?

A: We are pretty convinced that this is definitive. There is just absolutely no evidence for a genetic component.

Q: You mentioned earlier that there are still clearly some parents left to convince. In light of these results, what would you most want those parents to know?

A: To me, it’s just further evidence that the way we parent our children really does impact them. And if parents are on the fence about physical punishment, this is just additional evidence that it’s not doing any good for the children, and in fact it seems to be making their behavior worse. And that in turn makes our jobs harder as parents — that’s the irony. People are trying to improve their children’s behavior when they’re using physical punishment, but they’re in fact making it worse.

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