Conciliationists say that when you meet an epistemic peer who disagrees with you, you should alter your credence towards theirs. While there are counterexamples to conciliationism here is a simple argument that normally something like conciliationism is correct without the assumption of epistemic peerhood:
That someone’s credence in a proposition p is significantly below 1/2 is normally evidence against p.
Learning evidence against a proposition typically should lower one’s credence.
So, normally, learning that someone’s credence is significantly below 1/2 should lower one’s credence.
In particular, if your credence is above 1/2, then learning that someone else’s is significantly below 1/2 should normally lower one’s credence. And there are no assumptions of peerhood here.
The crucial premise is (1). Here is a simple thought: Normally, people’s credences are responsive to evidence. So when their credence is low, that’s likely because they had evidence against a proposition. Now the evidence they had either is or is not evidence you also have. If you know it is not evidence you also have, then learning that they have additional evidence against the proposition should normally provide you with evidence against it, too. If it is evidence you also have, that evidence should normally make no difference. You don’t know which of these is the case, but still the overall force of evidence is against the proposition.
One might, however, have a worry. Perhaps while normally learning that someone’s credence is significantly below 1/2 should lower one’s credence, when that someone is an epistemic peer and hence shares the same evidence, it shouldn’t. But actually the argument of the preceding paragraph shows that as long as you assign a non-zero probability to the person having more evidence, their disagreement should lead you to lower your credence. So the worry only comes up when you are sure that the person is a peer. It would, I think, be counterintuitive to think you should normally conciliate but not when you are sure the other person is a peer.
And I think even in the case where you know for sure that the other person has the same evidence you should lower your credence. There are two possibilities about the other person. Either they are a good evaluator of evidence or not. If not, then their evaluation of the evidence is normally no evidence either for or against the proposition. But if they are good evaluators, then their evaluating the evidence as being against the proposition normally is evidence that the evidence is against the proposition, and hence is evidence that you evaluated badly. So unless you are sure that they are a bad evaluator of evidence, you normally should conciliate.
And if you are sure they are a bad evaluator of evidence, well then, since you’re a peer, you are a bad evaluator, too. And the epistemology of what to do when you know you’re bad at evaluating evidence is hairy.
Here's another super-quick argument: Agreement normally confirms one's beliefs; hence, normally, disagreement disconfirms them.
Why do I need the "normally" in all these claims? Well, we can imagine situations where you have evidence that if the other person disbelieves p, then p is true. Moreover, there may be cases where your credence for p is 1.
I always have difficulty thinking through credence-talk, since I think 'credences' are post-hoc fictions, so I might be off here. But on the assumption that they are real, how much does their existence really imply that normally people's credences are responsive to evidence? I suppose your argument wouldn't require more than a very weak interpretation of this -- if I'm reading it correctly, it just requires that if someone's credences are low, that indicates that they have some kind of evidence, of whatever quality, against it (so it doesn't require the responsiveness to be very sensitive). But even taking it in this weak sense, I wonder if there's a case for taking it this broadly rather than in a much more restricted form (e.g., normally, people who have seriously inquired have credences responsive to the evidence).
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