Wealth is highly concentrated as is income taxation:
“The individual income tax share that the wealthiest Americans pay has steadily increased over time.
• In 1980, the individual income tax share paid by the top 1 percent of income earners was 17 percent. By 2004, the tax share for this same group had more than doubled, to 37 percent.
• The share of income tax paid by the top 5 percent and top 10 percent of income earners has also increased during the same time period, albeit not as drastically.
–In 1980, the share of income tax paid by the top 5 percent was 35 percent. In 2004, the share paid by this group had increased to 58 percent.
–For the same years, the top 10 percent of income earners paid 48 percent and 71 percent, respectively, of income tax.
• Over the same period, the share of pretax income that the top 5 percent and 10 percent earned increased from 21 percent and 31 percent to 29 and 39 percent, respectively.
Whereas the top 10 percent of income earners earned 39 percent of the pretax income, they paid 71 percent of individual income taxes.”
Source: http://www.hoover.org/research/factsonpolicy/facts/6771827.html.