A paper published last fall by William Hunter in the Canadian Journal of Learning and Technology called 'Critical Thinking in Asynchronous Online Discussions: A Systematic Review' caught my eye. It was is behind a paywall so I'm not even linking to it. But I wanted to follow up and conducted a search hoping to find a free copy of the paper. No luck, but I did find a 2017 dissertation by Bill Hunter's student called 'Transformative Learning and Critical Thinking in Asynchronous Online Discussions: A Systematic Review', which I reference here (74 page PDF). After a longish discussion of literature reviews in general, the core of the findings are presented after page 51, looking for (following Anderson, et.al.) indicators of instructional design and organization, facilitating discourse, and direct instruction. DiPasquale concludes "The presence of the instructor as a designer and facilitator is an imperative for ensuring that the type of critical thinking and reflection that are necessary for transformative learning are promoted during AODs." I think this goes beyond the evidence, but it's certainly discussed in the literature.