If you're human, then chances are you believe something that "everybody knows" to be true. And if you're a thinking human, then chances are you know that such beliefs can turn out to be not so true after all. Everybody knew saturated fat is bad for you, after all. But now they don't and the opposite is true. Fancy that! Whether it's beliefs, emotions, or behaviors, social contagion is a real thing, and it's the shared nature of these phenomena that hold communities together in one relatively...