WASHINGTON (MarketWatch) -- The U.S. economy entered a recession in December 2007, a committee of economists at the private National Bureau of Economic Research said Monday. The economy reached a peak in December and has been declining since, according to the business cycle dating committee of the NBER. The committee does not judge a recession as two consecutive quarterly declines in gross domestic product; rather, it looks at four key monthly economic indicators, including employment, industrial output and sales. Employment peaked in December.