How should I confront an under-performing coauthor? I'm working on a study with a coauthor; the ratio of the work distribution is about 90/10, me. On the one hand, this person was the one to approach me about collaborating on a project, and suggested a specific venue for presenting it. Yet since that initial invitation, I've been the one to come up with the topic of study, the methodology, compiling the resources we'd need for data collection, and to date have done more of the data collection so far. I've basically called all the shots in this study, and have had all the ideas (I always ask for their thoughts, but they basically agree with everything I say). I've indirectly brought this up once, indicating we were pressed for time and pointing to everything I had to do without pointing out how little they were doing. They offered to help pick up the slack, but then never followed through. 

At this point, I think I need to be more direct about the issue; I don't want to be mean, but I also want to make clear that a) I'm carrying too much of the burden; it doesn't need to be 50/50, but what we have going on right now is really unfair to me; b) if they're unwilling/unable to shoulder more of the work, then I don't think we should continue on this project together.

I always struggle with directness, as I feel like I'm being rude. Any suggestions on how to nip this particular situation in the bud?

EDIT/UPDATE: I've since emailed my co-author. They responded in what feels like passive aggressive gas-lighting. Basically, the gist of their response was (this is not a direct quote), "it seems like you're the one who's overwhelmed and I'm questioning your commitment to this project. If it's too much for you or you have other priorities, we can take a step back...but I for one am ready to work."

I'm not even sure how to respond to this. They did offer one explanation for why I haven't seen any work on their part (they said they were working on it in a separate document instead of our shared document), which is fine and I can accept that, but turning the tables and going "it's not me, it's you" is really bothering me. I don't want to escalate this or seem like I'm making a big deal if it isn't one (yes, despite naming this gas-lighting, I can't seem to escape its effects), but I also don't want to stand for that kind of behavior. Any further suggestions on how to respond to this coauthor would be appreciated.
You certainly need to sort this out. Would you like to stay on good terms with this colleague going forward? I'm going to assume the answer is yes, and I would certainly encourage that. 

Being direct is indeed the best approach, and it can be rudely done or considerately. Perhaps the person is having personal problems at the moment? You do not know what is causing the problem, but it is not necessary to assume that the person is just an unreliable jerk. They might be, but you can give them a chance to explain the situation.

Since you are doing all the work, and the two of your are peers, you are really in the driver's seat. But you do have to talk--not about promises for the future at this point but explanations for the past non-performance. 

You might ask the person what they want, since their actions to date suggest a lack of interest in the project or some problem interfering with their ability to work on it.  That's a considerate way of giving them an opportunity to explain the situation. You are not being mean or accusing them of anything. And you will soon enough find out whether or not the person really does have a short-term problem or is actually unreliable. Then you will know what to do.