"I don't know."
Gracy Olmstead writes a nice post noting how reluctant people are to admit ignorance of anything in our culture.
If someone asks me if human-caused climate change is a real problem, I answer, "How would I know? I've never studied climate science." But almost everyone I talk to seems to be a climate scientist, since they all have very, very firm positions on the issue.
Or take recessions, a topic I have studied for over a decade. When asked what causes them, I answer "When I first began studying recessions, I knew exactly what caused them. But the longer I study the topic, the less clue I have."
Nevertheless, my dermatologist assures me that he knows exactly what causes them.