That's the most important part, isn't it? Beliefs of humans are path-dependent. Nobody can perfectly rationally re-evaluate all evidence they've encountered and compute a belief to hold. We always start with a belief, and adjust it over time. Some more eagerly than others, some more correctly than others, but it's always an additive shift. Integration of evidence, if you like. That's why when I see someone believing something that's clearly (to me) absurd, I'm mostly interested in how did they get there - something must have convinced them, and I found it useful to discover what it was, because often it's a piece of real evidence I haven't encountered before.