Really what people need is an accurate sense of their performance (where they are at), an alignment with a goal (where they want to be), and a concrete action plan on how to make up that delta (how to get better).

I think instead of asking “does criticism or praise produce better outcomes on average over the duration of training” (my interpretation of your comment) I would ask “how can we ensure our assessments of student performance are accurate and how can we make sure it is translated to the most effective feedback per individual student.”

If you HAD to generalize to only positive/negative feedback, my guess would be that people need both, so an ideal value would be a ratio rather than binary. Say: 2 positives to every 1 negative (for example). But as a sometimes educator it’s really hard to keep track of that ratio for each student (unless it’s written or formalized in some kind of way).