Last week, the children’s commissioner, Anne Longfield, launched a campaign to help parents regulate internet and smartphone use at home. She suggested that the overconsumption of social media was a problem akin to that of junk-food diets. “None of us, as parents, would want our children to eat junk food all the time – double cheeseburger, chips, every day, every meal,” she said. “For those same reasons, we shouldn’t want our children to do the same with their online time.”

A few days later, former GCHQ spy agency chief Robert Hannigan responded to the campaign. “The assumption that time online or in front of a screen is life wasted needs challenging. It is driven by fear,” he said. “The best thing we can do is to focus less on the time they spend on screens at home and more on the nature of the activity.”

This exchange is just one more example of how children’s screentime has become an emotive, contested issue. Last December, more than 40 educationalists, psychologists and scientists signed a letter in the Guardian calling for action on children’s “screen-based lifestyles”. A few days later, another 40-odd academics described the fears as “moral panic” and said that any guidelines needed to build on evidence rather than “scaremongering”.

Faced with these conflicting expert views, how should concerned parents proceed? Into this maelstrom comes the American psychologist Jean Twenge, who has written a book entitled iGen: Why Today’s Super-Connected Kids Are Growing Up Less Rebellious, More Tolerant, Less Happy – and Completely Unprepared for Adulthood – and What That Means for the Rest of Us.

If the book’s title didn’t make her view clear enough, last weekend an excerpt was published in the American magazine the Atlantic with the emotive headline “Have smartphones destroyed a generation?” It quickly generated differing reactions that were played out on social media – these could be broadly characterised as praise from parents and criticism from scientists. In a phone interview and follow-up emails, Twenge explained her conclusions about the downsides of the connected world for teens, and answered some of her critics.

The Atlantic excerpt from your book was headlined “Have smartphones destroyed a generation?” Is that an accurate reflection of what you think?

Well, keep in mind that I didn’t write the headline. It’s obviously much more nuanced than that.

So why did you write this book?

I’ve been researching generations for a long time now, since I was an undergraduate, almost 25 years. The databases I draw from are large national surveys of high school and college students, and one of adults. In 2013-14 I started to see some really sudden changes and at first I thought maybe these were just blips, but the trends kept going.

I’d never seen anything like it in all my years of looking at differences among generations. So I wondered what was going on.

What were these sudden changes for teens?

Loneliness and depressive symptoms started to go up, while happiness and life satisfaction started to go down. The other thing that I really noticed was the accelerated decline in seeing friends in person – it falls off a cliff. It’s an absolutely stunning pattern – I’d never seen anything like that. I really started to wonder, what is going on here? What happened around 2011-2012 [the survey data is a year or two behind] that would cause such sudden changes?

And you concluded these changes were being brought about by increased time spent online?

The high-school data detailed how much time teens spend online on social media and games and I noticed how that correlated with some of these indicators in terms of happiness, depression and so on.

I was curious not just what the correlations were between these screen activities, mental health and wellbeing, but what were the links with non-screen activities, like spending time with friends in person, playing sports, going to religious services, doing homework, all these other things that teens do?

And for happiness in particular, the pattern was so stark. Of the non-screen activities that were measured, they all correlated with greater happiness. All the screen activities correlated with lower happiness.

You’ve called these post-millennials the iGeneration. What are their characteristics?

I’m defining iGen as those born between 1995 and 2012 – that latter date could change based on future data. I’m reasonably certain about 1995, given the sudden changes in the trends. It also happens that 1995 was the year the internet was commercialised [Amazon launched that year, Yahoo in 1994 and Google in 1996], so if you were born in that year you have not known a time without the internet.

But the introduction of the smartphone, exemplified by the iPhone, which was launched in 2007, is key?

There are a lot of differences – some are large, some are subtle, some are sudden and some had been building for a while – but if I had to identify what really characterises them, the first influence is the smartphone.

iGen is the first generation to spend their entire adolescence with the smartphone. This has led to many ripple effects for their wellbeing, their social interactions and the way they think about the world.

Psychology professor Jean Twenge. Photograph: Gregory Bull/AP

Why are you convinced they are unhappy because of social media, rather than it being a case of the unhappy kids being heavier users of social media?

That is very unlikely to be true because of very good research on that very question. There is one experiment and two longitudinal studies that show the arrow goes from social media to lower wellbeing and not the other way around. For example, an experiment where people gave up Facebook for a week and had better wellbeing than those who had not.

The other thing to keep in mind is that if you are spending eight hours a day with a screen you have less time to spend interacting with friends and family in person and we know definitively from decades of research that spending time with other people is one of the keys to emotional wellbeing; if you’re doing that less, that’s a very bad sign.

A professor at Oxford University tweeted that your work is a “non-systematic review of sloppy social science as a tool for lazy intergenerational shaming” – how do you respond?

It is odd to equate documenting teens’ mental health issues with “intergenerational shaming”. I’m not shaming anyone and the data I analyse is from teens, not older people criticising them.

This comment is especially strange because this researcher’s best-known paper, about what he calls the “Goldilocks theory”, shows the same thing I find – lower wellbeing after more hours of screen time. We’re basically replicating each other’s research across two different countries, which is usually considered a good thing. So I am confused.

Your arguments also seem to have been drawn on by the conservative right as ammunition for claims that technology is leading to the moral degradation of the young. Are you comfortable about that?

My analyses look at what young people are saying about themselves and how they are feeling, so I don’t think this idea of “older people love to whine about the young” is relevant. I didn’t look at what older people have to say about young people. I looked at what young people are saying about their own experiences and their own lives, compared to young people 10, 20, or 30 years ago.

Nor is it fair or accurate to characterise this as “youth-bashing”. Teens are saying they are suffering and documenting that should help them, not hurt them. I wrote the book because I wanted to give a voice to iGen and their experiences, through the 11 million who filled out national surveys, to the 200 plus who answered open-ended questions for me, to the 23 I talked to for up to two hours. It had absolutely nothing to do with older people and their “complaints” about youth.

Many of us have a nagging feeling that social media is bad for our wellbeing, but we all suffer from a fear of missing out.

Teens feel that very intensely, which is one reason why they are so addicted to their phones. Yet, ironically, the teens who spend more time on social media are actually more likely to report feeling left out.

But is this confined to iGeners? One could go to a child’s birthday party where the parents are glued to their smartphones and not talking to each other too.

It is important to consider that while this trend also affects adults, it is particularly worrisome for teens because their brain development is ongoing and adolescence is a crucial time for developing social skills.

Many tech-connected people in Silicon Valley restrict their own children’s screen use – so they know its effects.

You say teens might know the right emoji but in real life might not know the right facial expression.

There is very little research on that question. There is one study that looked at the effects of screens on social skills among 11- to 12-year-olds, half of whom used screens at their normal level and half went to a five-day screen-free camp.

Those who attended the camp improved their social skills – reading emotions on faces was what they measured. That makes sense – that’s the social skill you would expect to suffer if you weren’t getting much in-person social interaction.

So is it up to regulators or parents to improve the situation? Leaving this problem for parents to fix is a big challenge.

Yes it is. I have three kids and my oldest is 10, but in her class about half have a phone, so many of them are on social media already. Parents have a tough job, because there are temptations on the screen constantly.

What advice would you give parents?

Put off getting your child a phone for as long as possible and, when you do, start with one that doesn’t have internet access so they don’t have the internet in their pocket all the time.

But when your child says, “but all my friends have got one”, how do you reply?

Maybe with my parents’ line – “If your friends all jumped in the lake, would you do it too?” Although at that age the answer is usually yes, which I understand. But you can do social media on a desktop computer for a limited time each day. When we looked at the data, we found that an hour a day of electronic device use doesn’t have any negative effects on mental health – two hours a day or more is when you get the problems.

The majority of teens are on screens a lot more than that. So if they want to use Instagram, Snapchat or Facebook to keep up with their friends’ activities, they can do that from a desktop computer.

That sounds hard to enforce.

We need to be more understanding of the effects of smartphones. In many ways, parents are worried about the wrong things – they’re worried about their kids driving and going out. They don’t worry about their kids sitting by themselves in a room with their phone and they should.

Lots of social media features such as notifications or Snapchat’s Snapstreak feature are engineered to keep us glued to our phones. Should these types of features be outlawed?

Oh man. Parents can put an app [such as Kidslox or Screentime] on their kid’s phone to limit the amount of time they spend on it. Do that right away. In terms of the bigger solutions, I think that’s above my pay grade to figure out.

You’ve been accused by another psychologist of cherry-picking your data. Of ignoring, say, studies that suggest active social media use is associated with positive outcomes such as resilience. Did you collect data to fit a theory?

It’s impossible to judge that claim – she does not provide citations to these studies. I found a few studies finding no effects or positive effects, but they were all older, before smartphones were on the scene. She says in order to prove smartphones are responsible for these trends we need a large study randomly assigning teens to not use smartphones or use them. If we wait for this kind of study, we will wait for ever – that type of study is just about impossible to conduct.

She concludes by saying: “My suspicion is that the kids are gonna be OK.” However, it is not OK that 50% more teens suffer from major depression now versus just six years ago and three times as many girls aged 12 to 14 take their own lives. It is not OK that more teens say that they are lonely and feel hopeless. It is not OK that teens aren’t seeing their friends in person as much. If we twiddle our thumbs waiting for the perfect experiment, we are taking a big risk and I for one am not willing to do that.

Are you expecting anyone from Silicon Valley to say: “How can we help?”

No, but what I think is interesting is many tech-connected people in Silicon Valley restrict their own children’s screen use, so they know. They’re living off of it but they know its effects. It indicates that pointing out the effects of smartphones doesn’t make you a luddite.

iGen: Why Today’s Super-Connected Kids Are Growing Up Less Rebellious, More Tolerant, Less Happy –and Completely Unprepared for Adulthood – and What That Means for the Rest of Us by Jean Twenge is published by Simon & Schuster US ($27) on 22 August