 The provisions of the United States Internal Revenue Code regarding income taxes and estate taxes have undergone significant changes under both Republican and Democratic administrations and Congresses since 1964.Since the Johnson Administration, the top marginal income tax rates have been reduced from 91% for the wealthiest Americans in 1963, to a low of 35% under George W Bush, rising recently to 39.6% (or in some cases 43.4%) in 2013 under the Obama Administration.Capital gains taxes have also decreased over the last several years, and have experienced a more punctuated evolution than income taxes as significant and frequent changes to these rates occurred from 1981 to 2011.