Self-upvotes are universally despised and heavily frowned upon, but not currently against standing Site Rules. I think this gives some authors the impression that there's some permissable cause for it, when the only acceptable reason for it (to counter malicious downvoting) basically never comes up as an issue, and should be dealt with by staff on a case by case basis. I would like to see it added to Site Rules formally, under User Obligations, phrased something like:
You will not vote on your own submitted materials. If you believe a particular user has downvoted you for reasons other than the article's merits (malicious downvoting), you may contact a member of Senior Staff and inform them of your belief. Only if directly and formally instructed by Senior Staff acting in an official capacity may you upvote your own materials.
I'm not proposing a dragnet to hunt down any and every self-upvote in past articles, though I would expect SS to make sure they themselves have no standing self-upvotes in their own works. If an author is still around and they have a self-upvote on one of their pieces, we would ask them to remove it, but that's basically it.
