NEW YORK: As companies in the benchmark Standard & Poor's 500 begin to release lackluster second quarter results, rich valuations threaten to keep the U.S. stock market spinning in place. At 16.5 times forward estimates, the S&P 500, up less than 2 percent for the year to date through midday Friday, is about 10 percent more expensive than its historic average of 15, according to Thomson Reuters estimates.