Now, I don't know, but my impression is that the appearance of confidence (by which I mean the things associated with confidence, like being tall, speaking in a loud voice, being a guy, having a higher level of social competence, etc...) have a lot to do with how much credit you get for doing a thing when it is obvious that credit is being shared.

I mean, that's just my impression, not a study or anything... but it is something I observe, and it's weird because my own bias is to assume that the less confident or less socially competent person did most of the work.