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Externalities in knowledge production: evidence from a randomized field experiment


Are there positive or negative externalities in knowledge production? We analyze whether current contributions to knowledge production increase or decrease the future growth of knowledge. To assess this, we use a randomized field experiment that added content to some pages in Wikipedia while leaving similar pages unchanged. We compare subsequent content growth over the next 4 years between the treatment and control groups. Our estimates allow us to rule out effects on 4-year growth of content length larger than twelve percent. We can also rule out effects on 4-year growth of content quality larger than four points, which is less than one-fifth of the size of the treatment itself. The treatment increased editing activity in the first 2 years, but most of these edits only modified the text added by the treatment. Our results have implications for information seeding and incentivizing contributions. They imply that additional content may inspire future contributions in the short- and medium-term but do not generate large externalities in the long term.

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  1. 1.

    Traditional channels of personal knowledge transmission require a double-coincidence demand and supply of knowledge. The “knowledge-seeker” and the “knowledge-holder” have to meet in person or at least at the same time. The elimination of such double-coincidences has been modeled to understand the advantage of monetary over barter-economies. Kiyotaki and Wright (1989). These features give such systems a drastic competitive advantage that may affect the education sector and other traditional channels of knowledge transmission. The sector of encyclopedic knowledge is one of the most salient examples of the new technology’s potential.

  2. 2.

    Nagaraj (2019) describes how such policies have been used by Wikipedia (seeding articles on more than 30,000 US cities from US Census Bureau data), OpenStreetMap (US Census maps), and Reddit (fake user accounts).

  3. 3.

    A comprehensive description of the experiment is provided in Hinnosaar et al. (2021b), who studied the impact of this treatment on real-world outcomes.

  4. 4.

    Other studies on Wikipedia have analyzed biases in Wikipedia’s content (Greenstein & Zhu, 2012, 2018; Hinnosaar, 2019) and the impact of Wikipedia on market outcomes (Hinnosaar et al., 2021b; Xu & Zhang, 2013) and science (Thompson & Hanley, 2018).

  5. 5.

    More generally, the literature suggests strong effects of social influence on individual choices related to savings (Duflo & Saez, 2002, 2003), education (De Giorgi et al., 2010; Hanushek et al., 2003), entertainment (Salganik et al., 2006), etc.

  6. 6.

    For further details of the randomization, see Hinnosaar et al. (2021b).

  7. 7.

    Kane and Ransbotham (2016) provide some evidence that the effect could be larger for less developed content. They find that in the case of less developed articles, 1%-increase in length implies 0.03–0.04 more monthly contributors. In our case, the treatment was on average 23% of the page length, which would imply 0.7–0.9 more users.

  8. 8.

    The minimum detectable effect size is calculated at 5%-significance level and 80% power.

  9. 9.

    A revision (or an edit) is a version of a Wikipedia article saved at a specific moment by a particular user. All revisions with the corresponding metadata, including full text, user, and timestamp, are preserved by Wikipedia and publicly available.

  10. 10.

    The drop in both the treatment and control groups in early 2013 comes from technical changes in Wikipedia: Addbot removed about 2000 characters from each page with an explanation similar to “Migrating 77 interwiki links, now provided by Wikidata”.

  11. 11.

    By August 2016, the page of Cordoba in French Wikipedia was relatively long, with 19,426 characters (at the time 93% of the pages in our sample were shorter than that). During August 2016, this user increased the page length to 100,702 characters, which is almost twice the length of the longest page at the time (57,076 characters). Our conclusions do not change if we exclude this page.

  12. 12.

    As we show below, the articles about Spanish cities in English-language Wikipedia are sometimes quite incomplete, so ideally we would have preferred to use Spanish Wikipedia as a comparison. Because the combination of necessary language skills is not common, it would have been prohibitively costly.

  13. 13.

    Similar correlations between quantity and quality of content have been found previously, for example by Chen et al. (2019).

  14. 14.

    Many Wikipedia editors save many revisions to the same page in a short period of time, for example, generating a new revision after each sentence they write. This is partly motivated by the fact that someone else might edit the page at the same time.

  15. 15.

    Edit distance is widely used in computational linguistics and computer science to measure the similarity of strings. It is a generic term that allows any weights of insert, delete, and substitution operations. Common variants put weight 1 to addition and deletions, and weight substitutions either by 1 (called Levenshtein distance) or by 2 (the measure we use). For each edit, we calculate the edit distance using PHP FineDiff class at the granularity level of a character.

  16. 16.

    Both indexes are used to measure the similarity between two documents. The Jaccard index is also known as the Intersection over Union and sometimes called the Tanimoto similarity. Another similarity measure used in earlier works in economics (Chen et al., 2019; Thompson & Hanley, 2018) is cosine similarity. It would be preferable if we want to capture the similarity of two pages not only in terms of content covered but the language and tone. As we aim to capture completeness of the page compared to a benchmark, we found the Tversky index most appropriate for the task.

  17. 17.

    For more information, see

  18. 18.

    For example, pronouns (“it”, “their”) and prepositions (“on”, “before”). The full list is in B.7.

  19. 19.

    Terms such as “america”, “archipelago”, “area”, “arona”; about 250–500 terms for each article.

  20. 20.

    The three lowest grades in the Wikipedia content assessment are: Stub—a very basic description of the topic; Start—developing but still quite incomplete; Class C—substantial but is still missing important content or contains much irrelevant material. Source:

  21. 21.

    Note that these articles, on average, are also not very important according to the English Wikipedia article importance scheme. The scheme uses ratings from Low, Mid, High, to Top. Only 7 of the pages are rated as High importance, 15 as Mid, and 9 as Low importance, 29 of the articles have not been assigned an importance rating, which probably also implies that those articles are not highly important.

  22. 22.

    To make the pages comparable, we subtract the length of text added by the treatment from the length of pages in the treatment group after treatment (both in 2014 and in 2018). We do that because the outcome variable measures the percentage change, and hence, without subtracting the length of the treatment text, the treatment group would have a higher base when calculating the percentage, then the same increase in characters would give a smaller percentage for the treatment group.

  23. 23.

    Table B.2 in online appendix shows that the results are robust to alternative control variables. Table B.3 shows that the results are also robust when including the Dutch pages either in the control group or estimating intention-to-treat.

  24. 24.

    For expositional clarity, we interpret the coefficient in column 1 as measuring a percentage change in length. This is a logarithmic approximation that performs well when changes are small which is the case in our sample.

  25. 25.

    To make the pages comparable, we subtract the length of text added by the treatment from the length of pages in the treatment group after treatment. Hence, the estimates should be interpreted as the effect of treatment on page length after removing the mechanical increase created by the treatment.

  26. 26.

    For the long-term (4-year) effect, the ex-post minimum detectable effect size is 0.11 users.

  27. 27.


  28. 28.

    Without externalities, our model is similar to the model in Chen et al. (2019), with two differences: the payoffs in their model depend on social impact (i.e., number of viewers) and participation is endogenous. A crucial difference in our model is the inclusion of externalities. The model could be generalized and solved using tools introduced in Hinnosaar (2018).

  29. 29.

    Note that there are other, more subtle differences in the research environments that may affect the outcomes. For example, in our setting, the added content comes from an outside source. Instead, in the settings of Kane and Ransbotham (2016), Aaltonen and Seiler (2016), and Zhu et al. (2020), the added content is created by the community.


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We are grateful to John Duffy, two anonymous referees, Yan Chen, Chris Forman, Willa Friedman, Shane Greenstein, David Hugh-Jones, Tobias Kretschmer, Giovanni Mastrobuoni, Ignacio Monzón, Juan Morales, Abhishek Nagaraj, Stefan Penczynski, Imke Reimers, Stephan Seiler, Ananya Sen, Matthias Sutter, and Michael Zhang for valuable comments. We would also like to thank the seminar participants at the University of Strasbourg, the Collegio Carlo Alberto, George Mason University, ParisTech 2019, Digital Economy Workshop 2019 (Católica Lisbon), ZEW Conference on the Economics of ICT, the Advances with Field Experiments 2019 Conference (University of Chicago), SED 2021 for valuable input.

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Correspondence to Toomas Hinnosaar.

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Hinnosaar, M., Hinnosaar, T., Kummer, M.E. et al. Externalities in knowledge production: evidence from a randomized field experiment. Exp Econ (2021).

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  • Knowledge accumulation
  • User-generated content
  • Wikipedia
  • Public goods provision
  • Field experiment

JEL Classifications

  • L17
  • L86
  • C93