“After” doesn’t always mean “because of”

Post hoc ergo propter hoc. Anyone up on their Latin?

It’s a logical fallacy meaning, “after therefore because of.” In other words, we often misinterpret “B” as the result of “A” simply because “B” occurs after “A.” So instead of identifying “A” and “B” as sequential, we claim “A” is causative of “B.”

Okay, all these letters are getting confusing and starting to remind me of high school calculus class. Let’s look at how this plays out and why it’s such an important fallacy to watch out for.

A week ago the American Heart Association ran a press release with the following headline,

“Study shows Voices for Healthy Kids increases policies to curb childhood obesity”

Now, I spent the first several years of my public health career in advocacy. I lobbied Members of Congress, United Nations Member States, and the World Health Organization. My job was to create as compelling a case as possible to push for a particular policy approach or funding.

At first glance, this headline reads like a slam dunk. Voices for Healthy Kids is a joint advocacy initiative of AHA and the Robert Wood Johnson Foundation that funds local coalitions trying to advance public policy in schools, communities, early childcare, and “out-of-school” time to address childhood obesity.

And the headline says that this advocacy initiative directly increased the number of policies that will in turn decrease the prevalence of childhood obesity.

Okay, let’s break this down, there’s a lot packed in here.

The first point is less to do with the crux of the headline, but nonetheless important. Implicit in this statement is the link between “policies” and “curb childhood obesity.” In other words, it’s assumed that policies – like school meal standards, minimum physical education requirements, menu labeling, and financing initiatives to increase grocery stores in underserved communities – either prevent or treat childhood obesity.

In many of these areas, the jury is still out. There’s a lot of contradictory evidence. And more recently, a large 3-year study from Alberta, Canada evaluated the impact of these kinds of policy-focused interventions that aim to facilitate access. In the case of this Healthy Alberta Communities initiative, it “included the expansion of community gardens, improved access to recreation and sport facilities, development of a healthy choice restaurant program, a program to provide subsidized local produce to food insecure households, and a linked trail system for active transportation.”

However, despite some positive trends related to blood pressure, “BMI and body weight were unchanged” and “there was a significant net increase in waist and hip circumference among 20–39 year olds within intervention communities.”

There’s obviously a lot more to this conversation (not the least of which relates to the fact that this study looked at adult outcomes and not kids), and the role of policy to improve health outcomes will continue to be a major focus of public health research. Nonetheless, the headline assumes more policy will equate to more “curbing” of childhood obesity. That’s a big assumption.

The second point is more a critique of the headline’s omission of contextual detail. Yes, I know headlines must be as concise, succinct, and punchy as possible to compete with the million others out there. But the headline’s general use of “policy” conveys a much larger mandate on the part of Voices for Healthy Kids. “Policy” is obviously a broad term and one can characterize the Farm Bill (a federal law) and the maternity leave policy at a major corporation both as “policy.” Both influence health, but in different and distinct ways. By generalizing “policy” one could infer that all policy influences childhood obesity equally, that is, the Farm Bill and maternal leave policies are comparable in how they influence health.

Now on to the crux of the headline. It posits that the Voices for Healthy Kids initiative increased said policies. In other words, Voices for Healthy Kids (A) caused the increase in policies (B). So because the increase in policies (B) occurred after the implementation of Voices for Healthy Kids (A), it was because of it. Post hoc ergo propter hoc.

What the headline confounds are sequential events versus causative ones. The press release (and the journal article) goes on to explain how “there was an increase in childhood obesity bills the year after the campaign began” and these increases were in states where organizations received grants from Voices for Healthy Kids.

But the increase in policies isn’t necessarily attributable to the grants. The increase in policies could’ve been because of a variety of other/additional reasons, not all of which are mutually exclusive: state or local health departments having more data available on childhood obesity; one or more influential policymakers are themselves overweight or obese and know firsthand about some of the associated consequences; policymakers view health and healthcare as key voter interests so they want to portray active involvement; and so on. I think you get the point.

So in an effort to attract attention to the study’s results with a catchy headline, the writer(s) draw a much broader conclusion than what the study actually found. Unfortunately, this type of overstepping is all too common when reporting on public health and advocacy initiatives. We (mistakenly) assume that just because “B” (increase in policies) occurred after “A” (implementation of Voices for Healthy Kids) that “A” caused “B.”

But the results aren’t the only important aspect of a study. If we’re going to accurately piece together the puzzle of complex public health issues like obesity, we must consider a study’s limitations as well. By doing so we avoid the all too common logical fallacies that perpetuate inaccuracies.


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