(A lot of ink has been shed on the topic of how to rationally update your beliefs with respect to other sources. I don't mean for this little essay to contribute to the overall discussion. It's just something that I realized, on a more gut-level, recently.)
I have a lot of friends who I think are smart. They sometimes tell me things. And because they're smart, I tend to take what they say rather seriously. More seriously than, say, if I were to read something on my own which claimed the same thing. But, what if my smart friend just offhandedly said something? It seems that I still take them rather seriously, especially if I don't know that they're being offhanded about it.
I mean, obviously, it's good to report your credences and all that, but that sort of normative behavior happens less than you'd think. Anyway, the point here is that I seem to assign greater confidence to people I know compared to sources I read.
So what really happens is something like Alice tells me, "You know, Owen, I heard that X." And I'll take that as rather strong evidence for X. Except that, if I assume Alice generally reads the same stuff as I do, then her claim for X is really means something like "I read some stuff which convinced me of X". Which just ends up being a claim about the existence of evidence for X. But if Alice isn't thinking too hard about all this and sometimes just passes on stuff she's reading to me, then I might be unduly weighting these claims, i.e. more than if I had just seen the evidence myself.
The obvious recourse here is to ask people for their sources when they claim stuff. More generally, when people tell me X, it's not really some "other" more trustworthy source. I think I should just infer the existence of evidence for X which my friend saw. The glaring exception here is if my friends are incorporating several sources of information; in which case, they would have stronger evidence for X.
And, honestly, they probably do this for the most part. It's the edge cases I'm worried about.
Social evidence is very compelling. One might wonder, "Why do I by default assign higher credences to claims made by people I know?" The rational explanation is that if I know my friends have good epistemic hygiene, then I can place high confidence in what they tell me. But of course that's probably not it. People generally back up their friends and trust them, even in cases where their friends have questoinable epistemics.
Like just about everything in life, there's a plausible evo-psych argument that talks about group cohesion, alliances, and coalitions.