Citations to previous literature are extensively used to measure the quality and diffusion of knowledge. However, we know little about the different ways in which a study can be cited; in particular, are papers cited to point out their merits or their flaws? We elaborated a methodology to characterize "negative" citations using bibliometric data and natural language processing. We found that negative citations concerned higher-quality papers, were focused on a study's findings rather than theories or methods, and originated from scholars who were closer to the authors of the focal paper in terms of discipline and social distance, but not geographically. Receiving a negative citation was also associated with a slightly faster decline in citations to the paper in the long run. social studies of science | citation analysis | bibliometric techniques | natural-language processing | negative citations S cientific knowledge is a key input for economic prosperity (1-3) and evolves thanks to the complementary contributions of different scientists. The norms that regulate the scientific community coordinate this endeavor (4-6). Citation of previous work is one such norm and a major means of documenting the collective and cumulative nature of knowledge production. Citations allow for the establishment of credit and the identification of scientific paradigms and their shifts (7, 8); they measure the impact and quality of discoveries and, by extension, of a researcher, an institution, or a journal (9, 10). Studies rely on citation data also to analyze the diffusion of scientific ideas, the creation and evolution of scientific networks, and the role of top scientists and inventions (11) (12) (13) (14) (15) (16) (17) . Less attention has been devoted to the different intentions behind a citation. In particular, although papers may often be cited because a current study is consistent with past work or builds upon it, a reference can sometimes be made to point out limitations, inconsistencies, or flaws that are even more serious. These "negative" citations may question or limit the scope and impact of a contribution, a scholar, or an entire line of research. Criticisms expressed through citations could also be part of the "falsification" process that, according to Karl Popper, characterizes science and could be a signal of the solidity of a field (18). For example, the recent criticisms and eventual dismissal of the evidence of gravitational waves and ultrafast expansion of the universe in the "big bang" were interpreted as developments in the study of the origins of the universe (19). Even for findings that are eventually confirmed, critiques may be beneficial in the process. For instance, the Copernican revolution benefited from and was refined by Tycho Brahe's observations about inconsistencies in the heliocentric view, despite the eventual falsification of Brahe's theory (20) . A thorough classification and understanding of different types of citations, and in particular of negative citations-their incidence, distribution within research fields and across time, their location within a paper, and the connections that they establish between studies and scholars-is therefore a valuable exercise to understand the evolution of science. This enhanced classification may also offer current repositories of scholarly work (such as Google Scholar, PubMed, ISI Web of Science, and Scopus) an opportunity to improve their search and ranking algorithms; by extracting more information from citations, we can uncover more information, reject false knowledge more rapidly, and ultimately enhance the scientific discourse. Such a classification, however, is difficult to perform and would have been impossible just a few years ago. Recent advancements in natural-language processing (NLP) (21) and in the ability to parse and analyze large bodies of text, however, now allow us to reconstruct the context in which a citation was made, and therefore to understand why a given study was cited in the first place. We developed a method to identify citations that question the validity of previous results and to analyze their incidence and patterns to determine their role, relevance, and impact using bibliometric data, NLP techniques, and domain experts. In this study we provide evidence of (i) how negative citations are expressed, (ii) their incidence or frequency, (iii) the types of papers that receive these critiques and the types of papers that make the critiques, (iv) the parts of a study that are negatively cited (e.g., the theory, the results, the implications, etc.), (v) the relationships between the citing and cited authors, and (vi) the consequences of a negative citation in terms of future citations. To guarantee homogeneity of the analysis and define a feasible testing ground, the analysis in this paper was based on 15,731 full-text articles in the Journal of Immunology (1998Immunology ( -2007 and the 762,355 citations contained in those papers. Details of our procedures are in Materials and Methods and Supporting Information. Out of 762,355 citations from 15,731 articles in the Journal of Immunology (1998Immunology ( -2007, we identified 18,304 as negative (about 2.4% of the total). The 762,355 citations referred to 146,891 unique papers, and of these papers 10,405 (about 7.1%) received at least one negative citation. Thus, although the incidence on a per-citation basis is relatively low, a nontrivial number of papers received at least one negative citation. On the one hand, the low frequency may be evidence of a limited, uninfluential role of negative citations, or of Significance Providing a detailed classification of the types of citations that an article receives is important to establish the quality of a study and to characterize how current research builds upon prior work. The methodology that we propose also informs how to improve the citation process, for example by having scientists attach additional metadata to their citations. The approach is scalable to other fields and periods and can also be used to identify other types of citations (e.g., reuse of methods, materials, empirical tests of theory, and so on). Finally, our methods provide online repositories such as Google Scholar, PubMed, ISI Web of Science, and Scopus with a way to improve their search and ranking algorithms.
Citations as a metric to measure the impact of paper have been controversial, especially due to the existence of questionable citations REF .
The incidence and role of negative citations in science
Proceedings of the National Academy of Sciences of the United States of America
Proceedings of the National Academy of Sciences of the United States of America
Medicine