As of Friday, a $10,000 investment in the S.& P. stock market index* would have grown to $11,733 if invested under Republican presidents only, although that would be $51,211 if we exclude Herbert Hoover’s presidency during the Great Depression. Invested under Democratic presidents only, $10,000 would have grown to $300,671 at a compound rate of 8.9 percent over nearly 40 years.