When allowing users to provide feedback on other content, most websites allow users to provide both positive and negative feedback. StackExchange, for example, allows users to both upvote and downvote questions and answers, and YouTube allows users to both like and dislike videos.
Some websites, on the other hand, only allow users to provide positive feedback. Facebook is perhaps the most obvious example: users can "like" content, but not "dislike it". StackExchange kind of does this as well for comments: I can upvote your comment, but I can't really downvote it (though I can flag it, which is a bit different).
Are there any studies (either from actual user testing, or maybe from research in social psychology) on the effects of choosing one or the other? For example, why might Facebook not ever want to provide a dislike button? (I'm assuming there's a reason besides added software maintenance complexity.) Is it because adding a source of negative feedback might discourage some websites from adding like buttons (because while it's awesome to see that people like your content, it's pretty discouraging to see that even a single person doesn't), or is it perhaps because of reasons on the user side?
I'm looking for actual research on these questions, not armchair thought experiments.