The share paid by the top 20% has gone up because the percentage of the national income accumulated by the top 20% has gone up by far more. If you have 100 people, 20 of whom make 100,000 each and 80 of whom make 10,000 each, and everyone pays a flat 10% tax, then the top 20% will pay 200,000 total while the bottom 80% pays 160,000 total. Then adjust it over time so the top 20 make 1,000,000 each while the bottom 80% make 20,000 each, but give the top 20 a 5% rate instead, and the top 20 pay 1,000,000 total while the bottom pay 320,000 - and so those top 20% who are making 50 times more than everyone else complain about how they're 'unfairly' paying too much, despite paying half the tax rate. It's a gross simplification, but the effect you're noting is not the wealthy paying an unfair amount; it's an effect of growing concentration of wealth into fewer and fewer hands, which is not a stable situation.