Unhealthful News 117 – Exercise is good for six-year-olds, but not because of this study result

A new study had researchers looking into the back of the eyeballs of over a thousand Australian six-year-olds and found that those who watch more television or engage in less outdoor sports have narrower capillaries there.  The alarming conclusion that was reported is that since narrower blood vessels are associated with cardiovascular disease in adults, that this means that watching more TV or doing less exercise for six-year-olds means that they will be at greater CVD risk.

If the conclusion is simply that less exercise and more television viewing are probably associated with poorer cardiovascular health, then it is undoubtedly right.  But we kinda knew that already.  If the conclusion is something more specific about the meaning of this particular study, then we have a really serious problem of epistemology.

I will set aside the question of how well habits of six-year-olds predict future behavior and focus on some other problems with the reasoning of the study authors and the reporters who blindly transcribed their claims.  The most important of these is that correlation does not necessarily mean causation.

You have no doubt read that observation before.  Unfortunately, most of the time you read it, it is just being used as weasel words by someone who does not like what a study suggests about causation (e.g., cigarette companies forty years ago protesting that the overwhelming correlation between smoking and lung cancer, which was pretty obviously causal, does not mean that smoking causes cancer because it is just correlation).  Occasionally the statement is used by a study’s authors themselves if they are looking for an excuse to not really stand up for what their study shows.  Other times it is used by observers who are not intent on discrediting the study, but are trying to make themselves seem like knowledgeable reviewers of the science by pretending there is some simplistic bright-line hierarchy of study types such that “more research of the right type is needed” to show causation (regular readers will be aware of the fact that this mostly just shows their lack of knowledge).

But correct statements are not made incorrect because the ignorant or venal misinterpret them.  In many cases we observe correlations and have good reason to believe they are causal, and absent the presentation of any affirmative reason to believe otherwise, the causal conclusion is warranted.  In other cases, there are so many other compelling explanations for the correlation that a causal conclusion without further information is foolish.  Sometimes we are somewhere in between.  For example, if we observe that roofers have an elevated risk for skin cancer, it seems safe to conclude it is causal (with a causal pathway that passes through sun exposure).  If they have a very elevated rate of liver cancer, it is plausibly causal, and we should look into chemicals they are using.  If they have a higher rate of diabetes, we might consider looking at their dietary patterns or ethnicity.

The implicit causal claim in the present case is: (a) the behaviors cause narrow capillaries; (b) that narrowness is associated with CVD in adults so either (b1) the narrowing causes the CVD or (b2) whatever is causing it causes CVD and that “whatever” is being caused by the six-year-olds’ behaviors (and is, in turn, causing the narrowing that we observe).  I can think of twenty different stories that explain the observations without supporting that full causal pathway.  E.g., adult narrowing of blood vessels is caused by CVD, not the other way around; narrow blood vessels in adults cause CVD, but children who use their eyes on televisions just have effects in their eyes that  do not affect this; physically unhealthily people (or even just children) have narrower blood vessels and also eschew outdoor sports, which causes CVD via some other pathway; and so on.

The headlines mindlessly repeated the causal claims even though the same reporters would probably have mindlessly reported the claim “oh, but this is just correlation, not causation” in some other case where causation is the only compelling explanation.  USA Today reported “Couch-potato kids could be risking their hearts”, the New York Times reported it in their “Risks” series, and some more sensationalistic sources included the whole causal claim in their headline, like “TV Causes Heart Risks in Children”.  Funnier were the ones who reported headlines that told us nothing that was not already obvious, but still managed to imply the study showed results that it did not, like “Watching Television Could Be Harmful for Kids“.  That is just about as simultaneously obvious and misleading as the usual stating that correlation is not necessarily causation.

To further put the study’s naive claims in perspective, here is a pretty good analogy in terms of the many flaws in the causal conclusion:  Having darker skin is associated with an increased chance of imprisonment.  Kids who spend lots of hours in front of screen have lighter skin, while those who do more outdoor exercise have darker skin. 

Let’s break down the implications of the analogy:  There is no reason to assume that the mechanism that causes narrowing/darkening among screen users is the same one that causes the association with CVD/imprisonment.  Darkening from tanning has no relationship with the association of race and imprisonment; it is a completely different phenomenon though it still involves darker skin.  Even when race is a step along the causal pathway (or a proxy for one) between screen and outdoor activity and health outcomes – e.g., kids in poor urban neighborhoods have no place to safely play outdoors – the implied causal path is still wrong.

The point is, when causation seems like the only plausible explanation for a correlation, it is a reasonable conclusion.  When other explanations seem comparably compelling, it is not.  Instead, efforts should be made to identify the other explanations (well-educated epidemiologists know how to do this; >95% of those publishing epidemiology do not) and test them in ways that discriminate the different explanations (which is something that scientists know how to do).

There are a few obvious questions to ask.  Is there something special about looking at TV (the study looked at other screen use but did not find such interesting results) or do bookish six-year-olds have the same narrowing?  If the type of sedentary activity matters, the implications are different.  Similarly, does indoor exercise have the same effect as playing outdoors.  We are talking about activities that directly affect the eyes, after all, and a measurement in the eyes.  Maybe there is nothing important about that confluence and it is all about overall vascular health, but maybe not.  Do we even know if narrow blood vessels in kids predicts adult CVD?  In a field that was more serious than epidemiology or health reporting, these questions would have been at least mentioned.

In fairness to the professor who is the senior author of the study, he was quoted in the NYT as being cautious about interpreting the results.  Apparently he has not taught this wisdom to his student / advisee / employee who was the first author of the paper, though, who was quoted in the USA Today article basically making the full causal claim.

Finally, as a subtle technical point, one that I could pick up even without doing a careful analysis of the study, I noticed that the authors chose to divide the kids into thirds based on level of outdoor sporting activity, comparing the top third to the bottom, while they divided the TV watching into quarters, again comparing the top to the bottom.  This is always a sketchy methodology, since the extreme groups in population studies like this pick up all the extreme people (e.g., the kids who hardly ever go outside and watch TV all day, perhaps causing huge genuine health problems that only exist at the extreme, or perhaps caused by major non-lifestyle health problems that keep them from being able to play outside).  But even beyond that standard error(!), the change of what fraction the group was divided into is very suspicious and suggests that they made choices that gave them more dramatic results.  If this is really the case then something is very wrong since the less extreme comparison (top third to bottom third) would produce a less dramatic result than the more extreme (top quarter to bottom) if there were really a trend.  If the comparison that should produce a larger contrast does not, then we should be very suspicious of even the claimed correlation.

In summary, there is no doubt that exercise is better for cardiovascular health than being sedentary.  It is plausible that this manifests in vascular size in adults, though it is not clear to me that we know how way the causation runs (though it might be clear to those who both understand causal inference and are expert in the subject matter).  That the same thing happens in six-year-olds is plausible, though far from obviously true, and the new study might be seen as lending a bit of support to that claim though it has major limitations.  As for whether this effect in kids has any bearing on adult health, this is purely speculative at this point.

So exercise – both your body and your reasoning ability.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s