Standard & Poor’s has been mired in controversy this month after sending out incorrect updates on both France and Brazil's sovereign debt and now academics have added to the agency's woes by finding it issued higher ratings after switching from an investor-pay to an issuer-pay model.

John (Xuefeng) Jiang of Michigan State University, Mary Harris Stanford at Texas Christian University and Yuan Xie at Fordham University tested whether S&P assigned higher bond ratings after it switched from being paid for its research by bond buyers to being paid by companies that issued bonds in 1974. Rival Moody's Investors Service had made the same move four years earlier in 1970.