"Our situation is this: most of the people in this world believe that the Creator of the universe has written a book."

--Sam Harris, The End of Faith

Continuing from parts one, two and three...

In general, two things should drive our state of belief:  how plausible the theory seems and how much evidence there is in its favor (prior and posterior probabilities). 

Plausibility depends on our current state of knowledge.  It's fairly subjective. Yet, it's a good starting point.  Imagine that, when you are told some theory or hypothesis, prior to seeing any evidence for or against it, you state how likely you think it is that the theory is true.  You just make an educated guess.  For example, if someone told me that listening to music causes liver damage, I'd be very skeptical, because it's not easy to imagine the causal pathway from sound waves to liver damage (which is an organ that is seemingly unrelated).  Once you've decided on a prior degree of confidence, you then observe the evidence. Perhaps experiments on both humans and mice have investigated music and liver function.  Based on the evidence, you update your beliefs.  Weaker prior beliefs require stronger evidence to sway you.

I've noticed that humans have varying degrees of skepticism about equally plausible claims.  This variance depends on both the source of the claim, and on the implications of claim. 

Source of the claim

Who do you trust?  Scientists? The government? Your friends and neighbors?  The Bible?  The answer to this question seems to affect how skeptical you are.  If you believe in the Bible, for example, you are much less skeptical of scientists who claim to have evidence in favor of intelligent design than you are of scientists who present evidence in favor of evolution (confirmation bias).  Similarly, if you don't trust authority figures, you might be far less skeptical of your neighbor who claims vaccines are dangerous than you are of scientists who say they're not.  Alternatively, some people probably rely too much on experts.

Who you trust is probably driven to some extent by social networks, group cohesion and bonding.  It often feels good to go against the mainstream, and bond with your group.  Robin Hanson has written about this form of bias here:

emotional attachment... is a rationality no-no for factual beliefs, especially controversial ones. 

If you feel tempted to say "I believe X" and can feel your emotions swell with the evil pleasure of attachment via belief, watch out! 

And here:

You just can't fight "conformity" by indulging the evil pleasure of enjoying your conformity to a small tight-knit group of "non-conformists."  All this does is promote some groups at the expense of other groups, and poisons your mind in the process.  It is like fighting "loyalty" by dogged devotion to an anti-loyalty alliance.

Best to clear your mind and emotions of group loyalties and resentments and ask, if this belief gave me no pleasure of rebelling against some folks or identifying with others, if it was just me alone choosing, would my best evidence suggest that this belief is true?  All else is the road to rationality ruin. 

Implications of the claim

If pizza is my favorite food, I'm probably going to be very skeptical if someone tells me that consuming it will shorten my life.  I don't want to believe anything bad about pizza consumption, because I don't want to stop eating it.  This is a cognitive bias that I've observed many times.  On the other hand, if you like the implications then you are less skeptical of the evidence.  For example, if you are anti-abortion, you might be very quick to accept as true the claim that abortion increases the risk of breast cancer, even though the evidence in favor of it is very weak.

Religious beliefs and evidence

There seems to be a puzzling relationship between the plausibility of a claim and the evidence required to convince someone it's true.  As I said before, nearly all religions make claims that sound crazy, based strictly on what we have observed in our lifetime.  For example, we have never seen a talking snake, or a river full of blood, or really any miracles.  Claims of this nature should therefore be viewed with much skepticism.  It's not that you rule them out, it's that you require a lot of evidence in their favor. 

Surprisingly, though, people seem much less skeptical of claims made in religious texts, then they are of claims that at least appear to be physically plausible.  For example, if I said "scientists now think that hot dog consumption causes stomach cancer," most people would dismiss the statement.  Even though it does sound like something that could be true. The hot dog does spend some time in the stomach.  On the other hand, if I told people the story of Noah's ark, many would nod their head in approval.  This approval would come despite the fact that we've never seen a human that would be capable of carrying out such a task.  The more absurd claim receives less skepticism than the more plausible claim.  Why?  Why are people, who otherwise are skeptical and cynical, so willing to accept as true these laws-of-physics-violating events?  Why are we more skeptical of things for which we have evidence?