To outline his fears about the U.S. economy, Raghuram Rajan picked a tough crowd.

It was August 2005, at an annual gathering of high-powered economists at Jackson Hole, Wyo. -- and that year they were honoring Alan Greenspan. Mr. Greenspan, a giant of 20th-century economic policy, was about to retire as Federal Reserve chairman after presiding over a historic period of economic growth.

Mr. Rajan, a professor at the University of Chicago's Booth Graduate School of Business, chose that moment to deliver a paper called "Has Financial Development Made the World Riskier?"

His answer: Yes.

Mr. Rajan quickly came under attack as an antimarket Luddite, wistful for old days of regulation. Today, however, few are dismissing his ideas. The financial crisis has savaged the reputation of Mr. Greenspan and others now seen as having turned a blind eye toward excessive risk-taking.