Ninety percent of people on this planet are stupid.
Think about it. Think broadly. Don't use a mis-representative sample of your friends or acquaintances, since they're much more likely to be similar to you, i.e. smart. Exclude yourself, since merely pondering about conceptual problems distinguishes smart people. No, think about everyone else, about the people you generally try to ignore or to avoid, be they angrily yelling on the street, or making mindless mistakes at their dull jobs, or mindlessly quoting passages of a customer service manual. They're the people who don't enjoy thinking; and since they prefer to avoid the resulting insights about the future, they make the bad decisions and expect the other 10% to take care of the resulting mistakes.
If you're up to it, step outside and try to apply this mindset to the first few immediate problems that you notice. If you notice a scam--be it a letter from a Nigerian Prince or an offer to go on a so-called "free" cruise--and you wonder if anyone falls for it, use the 90% rule. Scamming is profitable for a reason, right? Safety warning: although now they're a necessity to avoid lawsuits, how did they become so widespread? Would parents really did let their kids put plastic bags over their heads? Would anyone really accidentally ingest some household cleaning liquids? You've always known the answer, but you had no explanation for it beforehand. Well, now you do.