Intended for healthcare professionals


Detecting implausible social network effects in acne, height, and headaches: longitudinal analysis

BMJ 2008; 337 doi: (Published 05 December 2008) Cite this as: BMJ 2008;337:a2533
  1. Ethan Cohen-Cole, financial economist1,
  2. Jason M Fletcher, assistant professor2
  1. 1Federal Reserve Bank of Boston, 600 Atlantic Avenue, Boston, MA 02210, USA
  2. 2Yale University, School of Public Health, 60 College Street, New Haven, CT 06510
  1. Correspondence to: J Fletcher jason.fletcher{at}
  • Accepted 3 November 2008


Objective To investigate whether “network effects” can be detected for health outcomes that are unlikely to be subject to network phenomena.

Design Statistical analysis common in network studies, such as logistic regression analysis, controlled for own and friend’s lagged health status. Analyses controlled for environmental confounders.

Setting Subsamples of the National Longitudinal Study of Adolescent Health (Add Health).

Participants 4300 to 5400 male and female adolescents who nominated a friend in the dataset and who were both longitudinally surveyed.

Measurements Health outcomes, including headache severity, acne severity, and height self reported by respondents in 1994-5, 1995-6, and 2000-1.

Results Significant network effects were observed in the acquisition of acne, headaches, and height. A friend’s acne problems increased an individual’s odds of acne problems (odds ratio 1.62, 95% confidence interval 0.91 to 2.89). The likelihood that an individual had headaches also increased with the presence of a friend with headaches (1.47, 0.93 to 2.33); and an individual’s height increased by 20% of his or her friend’s height (0.18, 0.15 to 0.26). Each of these results was estimated by using standard methods found in several publications. After adjustment for environmental confounders, however, the results become uniformly smaller and insignificant.

Conclusions Researchers should be cautious in attributing correlations in health outcomes of close friends to social network effects, especially when environmental confounders are not adequately controlled for in the analysis.


  • We thank David Paltiel and the reviewers and editorial board for helpful comments that substantially improved the paper.

  • Contributors: JMF acquired and analysed the data and is guarantor. Both authors interpreted the findings and drafted and revised the manuscript.

  • Funding: No external funding was used to support this research.

  • Competing interests: None declared.

  • Ethical approval: The Yale HIC exempted this research.

  • Provenance and peer review: Not commissioned; externally peer reviewed.

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-commercial License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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