2008A trader watches screens as he works on the floor of the New York Stock Exchange, September 29, 2008.REUTERS/Brendan McDermidJohn Hussman warns of negative S&P 500 returns over the next decade.Hussman's analysis shows the market capitalization of nonfinancial stocks at all-time highs.Hussman's called the 2000 and 2008 bubbles, but his fund's performance has been poor.John Hussman probably isn't someone you want to take investment strategy advice from.The president of the Hussman Investment Trust is seemingly always bearish, even in enduring market rallies, and his Hussman Strategic Market Cycle Fund (HSGFX) is down 55% since December 2010.But Hussman does produce some interesting valuation analysis that's frequently cited by investing legend and GMO cofounder Jeremy Grantham, and now may be a good time to take stock of where the market sits historically, and what that could mean for future returns.After all, the S&P 500 is coming off back-to-back years of at least 23% gains.