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. “The question isn’t whether valuations are above average but by how much.