Often our most firmly held beliefs come from our social groups, not our own experience. If something is self-evident, or easily proved, we don’t need to believe it. If we believe something, it’s because we can’t prove it’s true. If something can’t be proven true, we’re in danger of getting it wrong. So there’s usually no real personal benefit to a strong belief. Your default position should be to wait and see. So when we do hold a firm belief, there’s usually other humans involved. We hold these beliefs on behalf of a group to which we want to belong. Feigning interest in gossip about a friend of a friend. Pretending there’s a Santa Claus to keep the magic alive for your kids. Liking a band’s music because your friend likes it. Forgoing eating pork for your religion. We all adopt beliefs on behalf of our groups, because the social benefits of being in these groups outweigh the cost of holding these beliefs.
For most of human history, being kicked out of a tribe would mean death alone in the wilderness. Without modern technology we couldn’t live on our own. We’re social animals and we win against the other animals not with bigger teeth, sharper claws, stronger muscles, but by working together in groups. So we are predominately preoccupied with identifying how our beliefs will be interpreted by others, rather than examining if they’re true. In fact the more the belief differs from our model of reality, the more important it becomes to group cohesion, because it becomes a costly signifier that you’re “one of us”. If holding a belief is too costly, there will be a tipping point where you’d leave the group. So the costlier the belief you hold on behalf of the group, the greater your devotion to the group. We likely wouldn’t believe these things in isolation, but maintaining good relations with your spouse, family, tribe, church, military unit, or workplace can easily outweigh other considerations.
Adopting group beliefs help us blend in as one of the crowd, so as to not attract too much unwanted attention from those in charge. In groups whose power is threatened, it can be a matter of life or death. As Voltaire says, "It is dangerous to be right in matters on which the established authorities are wrong”. The rational strategy is often to believe the irrational. Those with the highest similarity of beliefs demonstrate loyalty with those in power. Flattery works even at the highest level, and it’s hard to climb the ranks (or avoid the chopping block) without showing you’re on side. However if group beliefs stray too far from reality – if they force adoption of obviously incorrect and costly beliefs – they’re in a position of instability. From there they can be violently and unpredictably corrected.
The exact opposite strategy – professing beliefs that run counter to authority – is a high-risk-high-reward approach. It makes you a target in the short term, in favor of potentially huge long term success if your band of rebels manages to topple the current regime. The founding fathers of the U.S.A. may have been executed if King George had his way, and regained control of the colonies. He didn’t, so they carved out a place in history, gaining positions of high status at an early age, with a rare opportunity to invent a new country. In less turbulent times, adopting really fringe or contrarian beliefs can be a form of Aposematism – a signal that you’re not worth attacking, for example brightly coloured poisonous frogs – which is likely the mechanism through which conspiracy theories persist. Holding fringe beliefs makes it clear you’re not willing to be recruited for the cause, and can get you out of social obligations entirely.