Laughing Buddha

Fat and Happy and Fuzzy About Causality

In 1886, JAMA advised its readers that, with proper feeding, an infant grows fat and happy. Many cultures for many years have linked fatness with happiness. Now, in Social Science & Medicine, Shuanglong Lia and colleagues tell us they have empirical evidence. Among urban Chinese adults, they claim that happiness brings a higher BMI.

Causality from Observational Data

Their data comes from the the Chinese General Social Survey in 2010 through 2013. Their abstract tells us:

In this study, we present the first empirical evidence on the effects of SWB [subjective well-being] on BMI among adults, using data combined from four waves of the Chinese General Social Survey (CGSS) (2010–2013). We find that, among urban Chinese adults, those who have a higher level of happiness tend to have higher BMI, after extensively controlling for a constellation of individual sociodemographic and health attributes. Further analyses using the instrumental variable method and propensity score matching provided similar results.

It’s a strong claim, right up front in the title of their paper: Happiness affects body mass index. However, it’s a bit of an overreach. Even with their statistical methods, their evidence is inconclusive. What they have is a suggestion that the relationship might be one of cause and effect.

As matter of fact, the authors concede this point. But they bury that concession at the end of their paper. “Our study can be taken as a starting point, inspiring more contextually relevant hypotheses for future research,” they say. Fair enough.

Confusing Presumptions with Facts

Unfortunately, the claim of cause and effect is what stands out. In the title and the abstract, we have no qualifiers. No caution. Just the bold claim. Yet again, we have presumptions confused with facts about obesity. Each little presumption might seem small. Unfortunately, those presumptions add up. Because obesity is counter-intuitive, presumptions can be especially misleading.

Does happiness cause obesity? Maybe. But we need to know more. Is this tied to culture? Is it a complex and coincidental relationship? Or is it a simple matter of cause and effect? Does subjective well-being inevitably put people at risk for obesity?

Presumptions aren’t good enough. We need facts.

Click here for the study.

Laughing Buddha, photograph © Bernhard Scheid / flickr

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June 1, 2018

2 Responses to “Fat and Happy and Fuzzy About Causality”

  1. June 01, 2018 at 10:15 am, Kate said:

    This is a really common error, but “epidemiology” does not equal “observational study.” Modern-day RCT methodology, from which we confidently can identify causal effects, was introduced by epidemiologist Austin Bradford Hill.

    https://www.nejm.org/doi/full/10.1056/NEJMp1604635

    Epidemiology as a field is commonly thrown under the bus when people want to highlight the limitations of observational studies.

  2. June 01, 2018 at 11:34 am, Ted said:

    Good point, Kate.