It depends on the source of the funds and the nature of the postdoc’s duties. If the postdoc is on a “fellowship”, either paid by the university or by something like an NIH trainee fellowship, then they are not employees. If the employee is hired to do work for a project, on the other hand, which is typical when funded on an NSF or DARPA grant, then they are employees. This is MIT’s info page, which I believe is reasonably representative: https://postdocs.mit.edu/postdoctoral-position/defining-post...
Science had an article on that in 2002 that went into more detail on how this system came about (basically, vague laws that have ended up interpreted through a series of IRS rulings). I think but am not 100% sure that even though this article is about 20 years old, the current situation is still similar. https://www.science.org/content/article/postdocs-and-law-par...
Science had an article on that in 2002 that went into more detail on how this system came about (basically, vague laws that have ended up interpreted through a series of IRS rulings). I think but am not 100% sure that even though this article is about 20 years old, the current situation is still similar. https://www.science.org/content/article/postdocs-and-law-par...