I'm writing one of my first academic papers and I'm not sure whether etc. is too informal. Should I use et cetera instead?
I would recommend not using etc. in an academic paper. And if you do, please be sure you are using it correctly. See this good explanation about using etc. Here's an excerpt:
It isn’t that writing that contains et al. or etc. is bad writing, it’s just that it is completely possible to construct meaningful sentences without using them. In fact, in most cases, it is probably preferable not to use them since both are badly overused, and technically speaking, they have definite meanings and specific usages that often do not apply in the cases they are used. More specifically, etc. is NOT to be used to complete a clause that starts with such as or for example.
To use etcetera in a sentence is to imply that the the reader already knows the rest of the set it is referring to, not, as it is so often used, as a placeholder for an undefined set. (Note that etc. is fine to use when referring to an infinite set, which is, by definition, a known set.)
As an editor, I would almost always ask for a revision of a sentence that contains etc. It usually can be reworded more precisely and better without using this word. And quite honestly, many authors use it incorrectly.
I think the problem is not the phrase itself, but the fact that it assumes the reader will know what "the rest" is. As a rule, in academic papers, this is a difficult assumption to make, because the nature of an academic paper is to define and spell out information.
So if it is appropriate to include this at all, then the expression "etc." is fine - it is not the formality that is the issue, but the implications.
Garner in Garner's Modern American Usage has a good entry on this. He says:
Writers should generally try to be as specific as possible rather than make use of this term. Still, it would be foolish to prohibit etc. outright because often one simply cannot practicably list all that should be listed in a given context. Hence, rather than convey to the reader that a list is seemingly complete when it is not, the writer might justifiably use etc. (always the abbreviation). In text, a substitute such as and others is usually a better choice.
The term etc. should be reserved for things, not for people.