Stephen Hawking’s A Brief History of Time1 begins with a famous anecdote about a “little old lady” who challenges a scientist’s public lecture on astronomy, insisting that in fact the earth is flat and rests on the back of a giant tortoise. When the scientist asks what holds the tortoise in its place, the lady is ready for him: “You’re very clever, young man, very clever … but it’s turtles all the way down!” Too often, it seems to us, both the guiding assumption and core insight of sociopsychological accounts of mass opinion is that “it’s groups all the way down.”