This is my distillation of the 2016 paper Updating on the Credences of Others: Disagreement, Agreement, and Synergy by Kenny Easwaran, Luke Fenton-Glynn, Christopher Hitchcock, and Joel D. Velasco. Following their wish, I will refer to them as the collective ‘EaGlHiVe’ pronounced ‘eagle hive’ (concatenating the first two letters of their names). Unless indicated, I tried not to add substantive claims myself.
Despite my efforts to make this as clear and simple as possible, this post is rather technical. I expect readers who are unfamiliar with the fundamentals of probability theory and bayesian reasoning to struggle with parts of it. However, you don't need to be fluent in probability theory to get the gist of this post.

The main points

Sometimes, the probability estimates of others provide evidence for some proposition.
Ideally, we would want to do Bayesian Updating on the credences of others, but that often seems cognitively over-demanding.
Hence, we want to find rules or heuristics that come reasonably close to Bayesian Updating, while remaining tractable.
'Upco'(short for ‘updating on the credences of others’) promises to be such a rule. Upco yields the same result as Bayesian Updating under some technical assumptions about the shape of our likelihood functions for someone's credence regarding some claim.
Upco roughly requires you to set your odds for some proposition to the product of your prior odds and the odds of everyone whose credences you are updating on.
It remains mostly unclear to me under which conditions we should have likelihood functions that entail Upco, and how much these assumptions can be relaxed so that Upco still closely approximates Bayesian Updating.

The problem

Your friend is going through an intense dinosaur phase. They have spent the last few weeks researching the Quetzalcoatlus northropi, an extinct species of giant flying reptiles that are believed to be among the largest flying animals of all time. When you see your friend, they successfully maneuver your conversation towards the megafauna. In passing, they claim that the largest Q. northopi had a wing span of over 12 meters. They show you this picture:

By Matt Martyniuk (Dinoguy2), Mark Witton and Darren Naish - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=41

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