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Usually a supervisor has to focus on many project at once but the PhD student on the other hand has only his project to focus on, PhD student is the person who usually does all the literature work as well as the experiments, so technically he is the master of that particular project then why the PhD student is never offered a chance to become the communication/corresponding author of those manuscript that are published by him on his work. One could argue that in the first 2 years of the PhD work he might have less command over the subject to which I might agree but what about in the last stage of the PhD program or after the PhD program for that matter. Why is he never offered a chance to become a communicating/ corresponding author?
Communicating/ corresponding author here represents the person who would respond to queries and reply to emails, where as the last author is usually the supervisor who wrote the project or took the lead.