A study by Ned Davis Research shows that, over the last century, the market has performed better when a Democrat is in the White House and Republicans control Congress. Since 1901, this constellation of political power has produced an annualized gain of 9.0%. When Democrats dominate both the presidency and Congress, annualized returns have been 7.3%. When Republicans control both branches, annualized returns have been 7.0%. The lowest gains coincided with a Republican president and Democratic Congress, a mere 2.2% annualized return. The reasons for these numbers are not clear and remain open to argument says MarketWatch’s Mark Hulbert.