The top 10% of earners -- households making about $250,000 a year or more -- are splurging on everything from vacations to designer handbags, buoyed by big gains in stocks, real estate and other assets. All this means that economic growth is unusually reliant on rich Americans continuing to shell out. Mark Zandi, chief economist at Moody's Analytics, estimated that spending by the top 10% alone accounted for almost one-third of gross domestic product.