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I am new to the field of academia, and when performing a literature review for really any topic, I am puzzled as to why some papers have so many co-authors.
Most papers have only about 1-5 authors which I can understand but I can't seem to understand how 10-20 people could all meaningfully contribute to a single paper. How possible is it that co-authorship is being gifted?
Are there any reasons why so many co-authors could be justified?
Update 11/11/19: Some comments have suggested this question may be a duplicate. To clarify: I was not referring to large-scale taskforces that produce world-changing results such as the Higgs Boson project (What is the point of listing 1000 authors for a single scientific paper?). Those projects clearly play by different rules. I was meaning routine contributions to journals (such as a new formula or algorithm) where I struggle to understand how so many people could all meaningfully contribute to a small (albeit important) idea.