How do you decide who is the first author of a paper?

Generally, the first author of a paper is either the principal investigator of the study from which the article’s findings derive or the first author is the person who wrote the majority of the article. In many cases, the first author may have filled both roles. Sometimes, trainees are given the privilege of taking the lead role in writing an article under the supervision of a mentor, in which case the two parties will agree beforehand who will be the first and corresponding author. Additionally, subsequent authors are usually determined based on their level of contribution to both the study and the writing of the article.