http://hanson.gmu.edu/extraord.pdf

"Extraordinary claims require extraordinary evidence. But on uninteresting topics, surprising claims usually are surprising evidence; we rarely make claims without sufficient evidence. On interesting topics, however, we can have interests in exaggerating or downplaying our evidence, and our actions often deviate from our interests. In a simple model of noisy humans reporting on extraordinary evidence, we find that extraordinary claims from low noise people are extraordinary evidence, but such claims from high noise people are not; their claims are more likely unusual noise than unusual truth. When people are organized into a reporting chain, noise levels grow exponentially with chain length; long chains seem incapable of communicating extraordinary evidence."