Gross domestic product rose at a 2.6% annualized rate in the third quarter as the U.S. economy rebounded following two quarterly contractions. But, Kenneth N. Kuttner, the professor of economics at Williams College, said that the decline in personal consumption spending and a slump in housing investment are making it difficult for the U.S. economy to sustain its momentum, putting it at risk of a recession and a rise in unemployment next year.