how do you know?
This sounds like scientism.
Because I have experience with good rationalists, and the kind of people they have relationships with, and I am a bayesian so I can assign degrees of belief to propositions that I haven't tested directly. In this case, it seems reasonable that similar people have similar relationship-behaviors, and so my existing knowledge is relevant.
Rather like "how do you know that the fastest dog in the world can't outrun a formula one car?" - I know this with high certainty because I believe that similar animals behave in similar ways.
Poignant short story about truth-seeking that I just found. Quote:
The full thing reads like a flash tour of OB/LW, except it was written in 2001.