Measles, climate change, low-fat diets, etc., etc.

We gain some knowledge via personal experience, but not much.  Most of what we accept as "true" we learned from others, an accumulation of authorities like mommy, the third-grade teacher, that really cool kid from home room, the neighbor, the guest speaker, the clergyman, the self-described "scientist," and that amusing guy at the bar.  Toss in cable TV and the internet, and we can be seriously wrong about a lot of serious things a lot of the time.

If  things are repeated often enough, and if the "authorities" have the appropriate hair and clothes, we're inclined to believe them.  Still, since we're frequently lied to or otherwise misled, and since different "authorities" often offer contradictory "truths," we are forced to pick and choose.  We prefer "truths" that are repeated a lot, expressed with great emotional conviction, and which don't contradict our preexisting beliefs.  People who disagree, by definition, must be idiots.

My beliefs, I assure you, are both scientific and morally superior.  You are entitled to disagree but, of course, contradicting me makes you an idiot.  So there.