The American Academy of Pediatrics recommends that children spend two hours or less at a screen—either computer or television—a day. And, to a large extent, that advice has been supported by studies that show screen time associated with both sedentary behavior and some emotional problems.

But since screen time is generally associated with sitting still, and most studies have relied on self-reported measures of physical activity and media consumption, teasing apart cause and effect has been challenging. Now, a new study indicates that screen time is associated with psychological issues; sitting isn't.

The authors recognize the challenges of performing this sort of study up front: self-reported measures of any activity, from exercise to TV viewing, tend to be biased, and there are a set of interconnected factors that influence childhood behavior that can be extremely difficult to untangle. However, the UK researchers designed their study to avoid as many of these as possible. The children, aged 10-11, were given a standard test that measures behavioral problems, and the confounding influence of the environment was controlled by using a measure of the economic wellbeing of a participant's postal code (the UK Index of Multiple Deprivation). BMI scores were also obtained, since obesity can influence wellbeing in a way that's distinct from inactivity.

Unfortunately, time spent on TVs and computers was still self-reported. But the authors got accurate measures of sedentary behavior by giving the kids a belt with an accelerometer in it, and had them wear it for a week. However, this didn't necessarily provide an exact measure, since there is a debate about precisely how much inactivity counts as "sedentary" or "active," so the authors repeated their analysis with several different cutoffs. Not every kid wore the accelerometer consistently, or returned it at the end of the study, either, and the ones who failed tended to have more behavioral problems than the group that ended up included in the study (1,013 of them), which may have biased the results.

Despite the remaining challenges, the accelerometers at least leave one source of bias out of the results, some of which were rather unexpected.

The connection between computer and TV use and sedentary time was significant, but only very weak, and there was no correlation, either positive or negative, between screen time and exercise-level activity. There was a weak-to-moderate association between the different types of screen time and, somewhat surprisingly, there was a correlation between time spent sedentary and the amount of exercise-level activity.

How does this all play out in terms of behavior? Greater time spent sedentary actually correlated with reduced behavioral problems. In fact, exercise, which has generally been considered to help reduce psychological issues, wasn't associated with better scores on the behavior test on its own; only when the connection between exercise and sedentary time was considered did the link become significant. The authors admit to being rather surprised by this result.

However, when all of these confounding factors are taken into account, children who went above two hours of screen time a day were more likely to have higher scores on the tests for behavioral issues (the odds ratios for TV and computers were nearly identical). This held true even if the kids who got lots of screen time were also very active. So, it's pretty clear that screen time, rather than the mere act of sitting around, is linked to behavioral problems.

This doesn't necessarily indicate a causal relationship. It's possible that the kids use their screens as a coping mechanism to escape from the behavioral issues they face at school or with friends and families. Nevertheless, it indicates that the guidelines for media consumption should be taken seriously, either as a way of preventing trouble, or an indication that trouble may already be present.

Still, the absence of an association between inactivity and behavioral issues is quite a surprise, and goes against some of the literature in the field. So, it's very likely that people are going to try to repeat or improve upon this study design and see whether the results hold up.

Pediatrics, 2010. DOI: 10.1542/peds.2010-1154 (About DOIs).

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