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Open access Research BMJ Open: first published as 10.1136/bmjopen-2018-026518 on 7 February 2019. Downloaded from Selective citation in the literature on the hygiene hypothesis: a citation analysis on the association between infections and rhinitis Bram Duyx,1 Miriam J E Urlings,1 Gerard M H Swaen,1 Lex M Bouter,2,3 Maurice P Zeegers1,4 To cite: Duyx B, Urlings MJE, ABSTRACT Strengths and limitations of this study Swaen GMH, et al. Selective Objective Our objective was to assess the occurrence and citation in the literature on determinants of selective citation in scientific publications ► The study assesses how evidence regarding the hy- the hygiene hypothesis: on Strachan’s original hygiene hypothesis. His hypothesis a citation analysis on the giene hypothesis propagates over time by analysing states that lack of exposure to infections in early childhood association between infections the likelihood of citation. increases the risk of rhinitis. and rhinitis. BMJ Open ► It investigates which characteristics of a publica- Setting Web of Science Core Collection. 2019;9:e026518. doi:10.1136/ tion—such as study outcome, journal impact factor, Participants We identified 110 publications in this bmjopen-2018-026518 author gender and affiliation, and authority within network, consisting of 5551 potential citations. the field—have an impact on citation. ► Prepublication history and Primary and secondary outcome measures Whether a additional material for this ► We check whether supportive studies are cited more citation occurs or not, measured and analysed according paper are available online. To often by other studies within the field. to the preregistered protocol. view these files, please visit ► Only articles related to the original hygiene hypothe- Results We found evidence for citation bias in this field: the journal online (http:// dx. doi. sis are included in this analysis. org/ 10. 1136/ bmjopen- 2018- publications supportive of the hypothesis were cited more 026518). often than non-supportive publications (OR adjusted for study design [adjOR] 2.2, 95% CI 1.6 to 3.1), and the same Received 7 September 2018 was the case for publications with mixed findings (adjOR The hygiene hypothesis was originally Revised 26 November 2018 3.1, 95% CI 2.2 to 4.5). Other relevant determinants for proposed to explain the rising prevalence of Accepted 28 November 2018 http://bmjopen.bmj.com/ citation were type of exposure, specificity, journal impact allergies, with up to 20%–40% of the popula- factor, authority and self-citation. Surprisingly, prospective tion in developed countries being affected.4 cohort studies were cited less often than other empirical Modern, urbanised life in developed coun- studies. tries generally shows higher levels of hygiene Conclusions There is clear evidence for selective citation than in previous times or in low/middle-in- in this research field, and particularly for citation bias. come countries. Hygiene limits exposure to infections. Exposure to infections, especially early in life, helps to develop and adapt BACKGROund the immune system to the environment in on September 27, 2021 by guest. Protected copyright. The hygiene hypothesis postulates that a high which we happen to live, in such a way that it degree of hygiene in early life will increase the learns to discriminate between harmless and risk of developing allergies later in life.1 2 The harmful intruders. According to the hygiene underlying mechanism has been the topic of hypothesis, it is this lack of exposure to rela- scientific debate. Over time, this debate led tively harmless intruders early in life that to several adaptations and extensions of the causes the immune system to malfunction hygiene hypothesis, which, as such, provides later in life, hence the rise in allergies. © Author(s) (or their a good example of how science progresses. The hygiene hypothesis has been amended employer(s)) 2019. Re-use Ideally, this progress should be based on all several times since its early days to give rise permitted under CC BY-NC. No commercial re-use. See rights existing evidence, but this is not always the to newer theories such as the ‘old friends 3 5 6 and permissions. Published by case. A citation analysis can help to reveal hypothesis’. According to this theory, it is BMJ. which part of the available evidence is taken not hygiene per se that is causing the rise in For numbered affiliations see into account, and which evidence is ignored. allergy prevalence, but the lack of exposure end of article. The current study does not concern the to some specific infections, and also to the Correspondence to validity of the hygiene hypothesis per se, but gut microbiome and to non-viable intruders Mr Bram Duyx; rather the citation relations within the scien- from the natural environment, such as endo- b. duyx@ maastrichtuniversity. nl tific literature on this hypothesis. toxins. Humans have been exposed to these Duyx B, et al. BMJ Open 2019;9:e026518. doi:10.1136/bmjopen-2018-026518 1 Open access BMJ Open: first published as 10.1136/bmjopen-2018-026518 on 7 February 2019. Downloaded from ‘old friends’ for many centuries and our immune system has co-evolved in their presence. As a result, our immune system has become dependent on the presence of these old friends in order to develop and function properly. This adapted hygiene hypothesis states that lack of expo- sure to these old friends may give rise not only to aller- gies, but to autoimmune diseases as well. The original hygiene hypothesis and its later adapta- tions have a lot in common, and much of the evidence that is supportive for one hypothesis is equally supportive for the others. However, this is not always the case. In our project, publications are classified as either supportive or non-supportive with regard to a hypothesis. For that reason it is important to precisely define the investi- gated hypothesis. In our citation network, we focus on the hygiene hypothesis as it was originally stated by Stra- chan, and not on later modifications.1 2 This allows us to investigate the development of this hypothesis from the start. Concretely, this means that we focus on the impact of infections and the number of siblings on the develop- ment of rhinitis, like in Strachan’s original study.2 The number of publications in the research on the hygiene hypothesis is large. It is therefore not feasible for authors to cite every relevant publication in the network and some kind of selection needs to take place. If this selection is based on study outcome, we speak of citation bias.3 7 The consequences of citation bias can be similar to those of publication bias and reporting bias: disregard of counterevidence leading to unfounded consensus8 or polarisation,9 ill-advised research programmes and research waste,8 10 distorted information in the media11 and misguided medical decisions.12 Citation bias has been studied in many disciplines. Our systematic review Figure 1 Flow diagram of article selection process. http://bmjopen.bmj.com/ gives an overview of these studies.13 Many of these studies showed evidence for citation bias in their field, with supportive publications being cited about twice as often described above. We will make use of the claim-specific 8 as non-supportive ones. methodology developed by Greenberg, but with a modi- Factors other than study outcome may also have an fication of the statistical analysis that allows us to test the impact on citation, as was recently shown by Onodera and impact of multiple factors, adjust for study design and Yoshikane.14 Measures for journal status (impact factor), take into account the variation in publication time. author status (number of citations, country of affilia- on September 27, 2021 by guest. Protected copyright. tion) and collaboration (number of authors, number of affiliations) were often found to be related to citation METHOD count. The same was consistently found for the number Prior to performing the citation network analysis, we of references of the cited publication. Furthermore, the described our methods in a study protocol and stored it at reporting15 and source16 17 of funding were shown to be an online repository.27 (Protocol deviations are described related to citation, but the impact of author’s affiliation18 in the online supplementary text S1.) In brief, we applied and gender19–21 is less clear. On the other hand, sample a search strategy to the Web of Science Core Collection size and study design—both markers of study quality, and (WoSCC), identified relevant literature, downloaded as such legitimate reasons to base a citation on—often these records with their reference lists, extracted data for seem unrelated to citation.17 18 22–24 In our previous cita- each publication, built a dataset with potential citation tion networks, we also found associations with self-citation paths and used specialised software to determine which and the specificity of a publication, but not with the title citations had occurred. These steps will be explained in of a publication.25 26 more detail below. Article selection (first based on title, In our study, we aimed (1) to assess the occurrence of then on abstract and finally full text; figure 1) and data citation bias in the scientific literature on the original extraction were performed independently by MJEU and hygiene hypothesis and (2) test for other signs of selec- BD. Results were compared after each step, and disagree- tive citation by assessing the impact of the other factors ments were resolved in consensus meetings. 2 Duyx B, et al. BMJ Open 2019;9:e026518. doi:10.1136/bmjopen-2018-026518 Open access BMJ Open: first published as 10.1136/bmjopen-2018-026518 on 7 February 2019. Downloaded from For clarification: a publication in our network can both that only investigates the association between number cite and be cited by other publications in the network, of siblings and rhinitis would be classified as ‘3’; if it leading to a multitude of citation paths.

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