"It doesn't seem quite clear to me. Would you care to provide some more information on the topic?"

There are a few areas of study that tease this out. The study of social contagion primarily. Studies of memetics. Studies of influence and persuasion that show the impact of social proof and repetition. Cultural histories that look at how fact-free mind viruses (such as nationalism or antisemitism) shaped societies. Studies about the neurological underpinnings of what kind of content people seek out (it's a part of the brain that's involved in pain). Studies into human tribalism and how our beliefs just mimic that of our in group.

  "And who exactly should have the authority to decide which ideas are allowed and which are not?"

There's no easy risk free way to draw a line, but at the same time drawing no line poses incredibly large downsides, so I'd say something like Canada's approach where democratically elected people decide is the best.