In early October 2012, the media started running stories about how eating many servings of fruits and vegetables is linked with happiness. The study in question was an observational study of the eating habits of 80,000 Britons and did find that, controlling for other socio-economic variables, high levels of happiness were associated with eating 7-8 servings (2.8oz) of fruits and vegetables per day (Blanchflower, Oswald, & Stewart-Brown, 2012). The study made it clear (in both the abstract and text) that because of its observational nature causality could not be determined:
- "Reverse causality and problems of confounding remain possible."
- "This implies that, as in some other parts of the well-being literature, we cannot draw firm inferences about causality."
- "... with caveats about the lack here of clinching causal evidence..."
- "... it is sensible to emphasize, first, the need for extreme caution in the interpretation of this study’s findings..."
The authors repeatedly made appropriate statements about the interpretability of the study and the potential for future controlled studies to determine causality — exactly what one should do. The importance of this work is not diminished because of its observational nature, and serves to fill a gap in the well-being literature and suggest areas for future research.
But then the media happened.
- Eating 7-8 Fruits And Vegetables Per Day Will Make You Happier [STUDY] (Business Insider)
- Causation is suggested right in the title.
- No such causation is explicit in the article, but no limitations of the study are listed
- Study: 7 Servings Of Fruits, Vegetables Per Day Helps With Happiness (CBS Connecticut)
- No statements that explicitly say that eating fruits and vegetables make one happier...
- ... but it seems to be implied, and there are no limitations of the study listed
- Fruits Plus Vegetables Equals Happiness? (WebMD)
- The article starts with "Grumpy and out of sorts? Grab an apple. Or a carrot. Or a banana. Repeat daily." This clearly suggests causation.
- The article does make it clear that causation wasn't shown, only a link...
- ... but not until page 2.
Of course, some sources got the story right:
- 7 Daily Servings of Fruits, Veggies Best for Happiness, Study Finds (U.S. National Library of Medicine (NIH))
- A fair report of the findings (not surprising considering the source is the National Institutes of Health).
- Pick a number – now it’s 7-a-day for happiness (NYR Natural News)
- Another fair reporting of the results (somewhat surprising considering alternative news sources often misconstrue results, at least in my experience).