"More than four hours of smart phone use a day harmful to teens."
During the past week, some version of this headline appeared on national news websites (Newsweek; Yahoo), health blogs (Healthday; Drugs.com), and social media (Reddit's r/science – 31.1 million subscribers).
The headline refers to a study of 50,000+ adolescents published last Wednesday. Over four hours of daily smartphone use was indeed linked to greater stress, depression, suicidal thinking, and other problems.
In this newsletter I'll talk about whether we can trust the study, and whether it was accurately portrayed in the media. My main concern is a practical one: Is there anything in the data that could actually help young people?
I will argue here that the study is not only flawed, but dangerously so. At the risk of sounding cranky, if not paranoid, the results are presented in a way that are more likely to cause harm than to help. At the end of the newsletter I'll touch on some more useful findings.
Why is this topic important?
1. Recent data on young Americans' mental health is troubling. For instance, the Youth Risk Behavior Survey (YRBS), conducted bienially by the CDC, shows a steady increase since 2011 in the percentages of high school students who experience persistent sadness, stress, suicidal thoughts, and other mental health problems. The figure below shows the percentages of students who responded "yes" to the question "During the past 12 months, did you ever feel so sad or hopeless almost every day for two weeks or more in a row that you stopped doing some usual activities?"
In figures like this you can see that the pandemic exacerbated an existing trend rather than creating a new one. You can also see one of the many gender differences that are causing alarm. Here, the trend is worst for girls. In studies of suicide, for instance, we see sharper upticks during the same time period for boys, and generally high rates for trans youth.
2. Recent studies link excessive smartphone use to poor mental health. The message is not that smartphone activities are the only source of mental health problems, but simply that they contribute.
3. According to media coverage of the new study, 4 hours of smartphone use per day marks an upper limit, beyond which greater use is associated with increasing harm. If we can trust that number, we should be worried, as 46% of Americans now report using their smartphones between 4 and 5 hours per day.
New study
This study, published last Wednesday in the journal PLoS One, was conducted by Jong Ho Cha and colleagues at Hanyang University in Seoul.
Particpants consisted of 54,948 randomly-selected students from 793 middle and high schools. Students completed a 103-question survey concerning their mental and physical health. Students were also asked one question about smartphone use: “In the last 7 days, how many hours per day did you use the smartphone on average?”
The journal (PLoS One), the university (Hanyang), and the survey (Korean Youth Risk Behavior Web-based Survey) are all prominent. Studies of this sort may or may not be credible, but they will almost surely be influential.
Main findings
1. A small amount of smartphone use appeared to be a good thing. Compared to zero use, students using their smartphones 0 to 2 hours per day reported better mental health (less stress, less dissatisfaction with sleep, less depression, etc.).
2. As smartphone use increased, mental health declined. According to the researchers, 4 hours per day is the inflection point, beyond which stress, dissatisfaction with sleep, depression etc. start to get significantly worse.
Here are graphs of some of the key findings:
The y-axes of these graphs are odds ratios. For each smartphone usage group (0-2 hours, 2-4 hours, etc.), the odds ratio tells you how much more likely that group experiences some problem to a greater degree of severity than the zero use group. (If you're a stats person, ordinal logistic regression was used for the main analyses, as the outcome variables were measured on 5-point Likert scales.)
Can we trust the data?
In a word, no.
Listing all the flaws with this study would be like shooting fish in a barrel. Here I'll just delve into two that potentially endanger adolescents.
1. The direction of causality problem was not acknowledged.
The data shows that beyond about 4 hours per day, greater smartphone use is associated with with poorer mental health. But it's not clear what causes what. Perhaps more frequent smartphone use causes poorer mental health. Or, perhaps students with poorer mental health spend more time on their smartphones.
(Recent research points to a combination of the two: Students who are already depressed, for instance, may spend more time on their smartphones, which in turn exacerbates their depression.)
This direction of causality problem is a well-known limitation in correlational studies. Researchers typically avoid causal language unless their design justifies it. These researchers were not cautious. They refer repeatedly to the "adverse effects" of smartphone use, though literally nothing in their data supports this interpretation, and they have much to say about imposing limits on smartphone use.
I find this irresponsible. If the direction of causality runs the other way – in other words, if adolescents with the poorest mental health rely heavily on their smart phones for comfort – then focusing on the amount of use isn't going to help them. In fact, it may hurt.
Think of a depressed, isolated teen who uses his smartphone a lot because he doesn't have many friends, and he finds solace lurking on social media sites or playing video games. This teenager needs support for his depression and social isolation. What's not going to help is nagging him about how much he uses his phone, or imposing a limit on his daily use.
In short, if the point of the research is to give parents, educators, health care professionals – and teens themselves – useful information, don't say that the findings show "adverse effects" of smartphone use. Just say, more accurately, that smartphone use and mental health are related.
(Practically speaking, one might question the importance of figuring out what the direction of causality tends to be. If excessive smartphone use is associated with poor mental health, either one should be a cause for concern. As for what to be concerned about, that depends on the individual teen. For some kids, excessive use might need to be addressed before it creates problems. For other kids, the focus should be on the psychological issues motivating excessive use.)
2. The 4 hour inflection point is not credible.
Let's assume for the moment that smartphone overuse tends to be a cause rather than a symptom of mental health problems. We might ask: How much is too much? How many hours per day begin to jeopardize mental health?
According to these researchers, there's a simple answer: 4 hours. That statistic made the headlines, as it's the one the researchers themselves emphasize repeatedly in their write-up.
Here are the main reasons you can't trust this number:
(a) The researchers didn't ask kids what they were doing on their smartphones.
Lots of studies tell us that what people do online influences their well-being more strongly than the exact number of hours they spend doing it. 4 hours (or any other specific value) is unlikely to mark the upper limit of safe use, independent of the way the smartphone is used.
It's easy to think of harmful ways to spend 4 hours on a smartphone, but they're not the only options. Completing assignments that teachers post online, discussing homework with a classmate, catching up with a family member or friend, reading about current events, communicating with co-workers or co-volunteers... these are time-consuming smartphone activities too, but they're not generally harmful.
(b) Measurement and analysis were crude.
Teens were asked to estimate how much they used their phones each day, on average, during the prior week. For purposes of analysis, the researchers then assigned each teen to a simple category (0-2 hours per day, 2-4 hours per day, etc.).
Studies show that people aren't very accurate at estimating the frequency or duration of their smartphone use. But even if the teens in this study had been accurate, their one-week estimates may not generalize to earlier points in time. Information was then lost in the creation and analysis of those usage categories. The 2-4 hour group lumps teens who use their smartphones 2.1 hours per day with those who report 3.9 hours of daily use. Meanwhile, teens who use their phones 3.9 hours per day vs. 4.1 hours per day get placed in separate categories.
In short, this study isn't set up to identify clear limits on safe use. The researchers misread their own data by identifying 4 hours as the cut-off. The actual cut-off, if the usage estimates had been treated as continuous variables, could've been as little as 3.4 or 3.7 hours, or as much as 5.3 or 5.6 hours. The data, as presented, can't rule out these possibilities. (I emailed the lead author to ask about this and other topics but haven't heard back.)
It's dangerous to get people talking about a 4-hour-per-day limit on smartphone use if that's not the right number. One kid who uses their phone 5 hours per day may be fine; another kid who only uses it 2 hours per day may not be fine at all. (Think of how badly those 2 hours could be spent: visiting conspiracy theory blogs, texting during class, etc.). It's dangerous in the first place to be focusing on the amount of usage if that's merely a symptom of a teen's underlying distress.
Media coverage
I've noted that the results of the study may be harmful. Did news and social media coverage mitigate those harms?
Yes and no.
1. Some journalists correctly noted that the study simply shows an association between smartphone use and mental health. Kudos to them for reducing the study's potential for harm. Others, following the researchers, used causal language, claiming that more than 4 hours of daily smartphone use undermines mental health.
You might say that the writers who described the main findings accurately (in the sense of hewing closely to the researchers' own language) actually got it wrong, while those who got it wrong (by using correlational language) were actually in the right.
2. Most of the coverage in both news and social media ran with that 4 hour statistic as the upper safe limit. This is misleading, for the reasons I discussed earlier, and because, on general principle, specific estimates derived from samples tend to be, well, reflective of samples. Add or subtract 20,000 students from this particular sample and the estimated upper limit might change.
On balance, the news and social media coverage seemed fairly accurate but didn't catch the study's main flaws, which include a few I didn't mention here. This is concerning. 95% of American teens own a smartphone, and in some cases it may be harmful if the adults closest to them are excessively concerned about the amount of use.
Final thoughts
What should we tell adolescents about their smartphones?
To start with, I wouldn't tell an adolescent anything. I'd listen to what they have to say.
1. I would listen because people at any age want to be heard. Sometimes being heard is all a person needs. If they need more, understanding them is a great place to start.
2. I would also listen because the science isn't clear yet about the relationship between smartphone use and mental health. Excessive smartphone use (however you might define that) could be a cause of future problems or merely a symptom of existing ones. I wouldn't talk to someone about their "excessive" smartphone use if the underlying problem turns out to be stress or depression.
What is clear from the research is that the studies all rely on aggregate data. This suggests a need to understand the particular adolescent in front of you, because they may not fit the general pattern. (Researchers don't agree yet on the nature of that pattern; whatever it happens to be, it's not going to apply to everyone.)
3. I would also listen because how you feel about your smartphone use turns out to be more important than how much time you spend using it.
A cleverly-designed 2020 study shows that the strongest predictor of anxiety, depression, and stress is neither how much people think they use their smart phones, nor how much they actually use them. Rather, it's how they feel about their own smart phone use, as measured by something called the Smartphone Addiction Scale (SAS). The SAS asks about topics such as whether a person feels dependent on their smartphone, whether usage distracts them from necessary tasks, whether they feel anxious without their phone, etc.
(You can take a short version of the SAS here. Responses are immediately scored, and you're shown your percentile compared to others of your age and nationality.)
The message from this study is that in most cases, we shouldn't be talking to adolescents about how many hours a day they use their smartphones. We should be discussing how they feel about the experience. If the teenager feels there's a problem, they're right, regardless of how many hours a day they're actually using the phone.
4. Finally, I would listen because, as I mentioned earlier, what people do with their smartphones is more important than the amount of time they spend doing it. Here I'll punt, because there's so much data on the risks and benefits of different kinds of digital activities. I've written about activities such as video games and Instagram, but this barely scratches the surface. The key point is that where the phone takes you tends to be more important than how much time you spend on it.
A particularly uncomfortable illustration of this point comes from research showing that teens get a lot of push notifications on their smartphones – in one study, more than 50 per day, on average, during school hours alone – and that notifications can be deeply distracting. Glancing at their phone during class may not register in a teen's estimate of how much they use their phone, but doing so may undermine their grades (and, consequently, their mental health). In short, fractional amounts of use can have a meaningful impact.
In the end, most of what might concern you about an adolescent's smartphone use are the same concerns you'd have about other electronic technologies: Harmful content. Passive consumption. Displacement of healthy activities. Distraction. The problem isn't really the phone. What's distinctive about a smartphone is merely its immediacy. It's right there in your hand.
More resources
For a fairly balanced depiction of how teens use their smartphones, see this 2023 "week in the life" report.
If you're concerned about a young person's "addiction" to their smartphone, this is a helpful, balanced article on fear-mongering around the concept.
If you're concerned about smartphone use and cancer, please don't be (see here or here). Although it's hard to prove the negative, studies have either shown no relationship between smartphone use and cancer, or small relationships that can be attributed to methodological flaws.
Here again is that free, 3-minute activity that invites you to explore how you feel about your smartphone, and find out how you compare to others of your age. Seems like a fun activity to do together with a teenager (if they're willing). Just don't take the results too literally. I think the wording of the measure creates a slight bias toward overestimating smartphone dependence.
I have to admit though that I'm biased myself. After all, I'm old enough to remember actually dialing a rotary phone and speaking with an operator. : )
Thanks for reading!