Bitcoin is a social movement whose popularity splits along geographic lines in the U.S., Nobel economics prize winner Robert Shiller said.

“The East Coast is less into it than the West Coast,” Shiller said Tuesday in a Bloomberg Television interview with Tom Keene and Guy Johnson. “Silicon Valley is really into it. This to me shows that this is not a rational response to new information.”

Bitcoin stayed above US$6,000 on Tuesday after having dropped below that threshold for the first time since February over the weekend. The largest cryptocurrency has tumbled this year as regulators spanning the globe step up scrutiny of what critics say is a vehicle for fraud.

“It’s a social movement. It’s an epidemic of enthusiasm,” the Yale University economics professor said. “It is a speculative bubble. That doesn’t mean that it will go to zero.”

Asked about a possible comparison with the 17th-century tulip bubble in the Netherlands, Shiller responded: “Tulips are still valued, there are some expensive tulips.”