Nearly half of Americans say that they definitely or probably couldn’t come up with $2,000 in 30 days, according to new research, raising concerns about the financial fragility of many households. In a paper published by the National Bureau of Economic Research, Annamaria Lusardi of the George Washington School of Business, Daniel J. Schneider of Princeton University and Peter Tufano of Harvard Business School used data from the 2009 TNS Global Economic Crisis survey to document widespread financial weakness in the U.S. and other countries. The survey asked a simple question, “If you were to face a $2,000 unexpected expense...