Even outside sources can be cherry-picked.
Sources are OK evidence, people bringing sources are better evidence. When those advocating banning chimp testing focus on cases decades old, that tells me no similar case has happened recently. I don't assume that that case is typical of all chimp experiments, past and present, instead I learn the contrary.
If I find some study indicating that colleges discriminate against high school ROTC members, I don't really have much of an idea if what I stumbled upon is the strongest evidence for that. If sam tells me about it, I am confident no reliable study showing more discrimination has been conducted.
This requires a model of the poster that not everyone reading comments will have. That can be solved by downvoting such posters to indicate their untrustworthiness.
But as I said humans are not perfect Bayesian reasoners and are not good at adjusting for unreliable speakers. We almost always either adjust too far and reverse stupidity or don't discount enough and let cherry picked sources or skewed interpretations sneak into our brains. We'll update without even noticing.
I wanted to bring attention to two posts from Razib Khan's Discover magazine gene expression blog (some of you may have been readers of the still active original gnxp) on the polemic surrounding Pinker's The Better Angels of Our Nature.
Relative Angels and absolute Demons (and the related But peace does reign! )
I generally agree with some of his arguments, but found this quote especially as summing up some of my own sentiments: