The answer to your question is about as politically-charged as a freight train full of dynamite. Economics has always been a "dismal science" that engenders intense debate.

There are some areas of agreement:

1. That wealthy people are paying a slightly-lower effective tax rate now than they were 50+ years ago

2. Income taxes on wealthy people have made up make an increasingly larger share of federal revenues over time. In 1979, 56.5% of tax liability was carried by the top quintile, in 2005, even after tax cuts, it was 68.9%.

3. Wealthy people account for a growing majority of federal personal tax receipts (In 2013, filers earning over $250,000 paid $603 billion in income taxes. By 2014, these taxpayers paid $700 billion in income taxes, about 55% of total income taxes paid).

There's also further agreement regarding the effective taxes paid by the wealthiest Americans in the past. Yes, the top tax rates were extremely high, but they were so high that few people reached those tax brackets. The handful of people who did, often didn't pay those ultra-high rates, anyways.

Left-wing economist Thomas Piketty and some colleagues write:

"In the 1950s, top 1% income earners paid 40%-45% of their pre-tax income in taxes, while bottom 50% earners paid 15-20%. ... today: top earners pay about 30%-35% of their income in taxes, while bottom 50% earners pay around 25%."

Scott Greenberg over at the right-leaning Tax Foundation has a similar perspective on the effective rates in the past:

There is a common misconception that high-income Americans are not paying much in taxes compared to what they used to. Proponents of this view often point to the 1950s, when the top federal income tax rate was 91 percent for most of the decade. However, despite these high marginal rates, the top 1 percent of taxpayers in the 1950s only paid about 42 percent of their income in taxes. As a result, the tax burden on high-income households today is only slightly lower than what these households faced in the 1950s.

The data shows that, between 1950 and 1959, the top 1 percent of taxpayers paid an average of 42.0 percent of their income in federal, state, and local taxes. Since then, the average effective tax rate of the top 1 percent has declined slightly overall. In 2014, the top 1 percent of taxpayers paid an average tax rate of 36.4 percent.

All things considered, this is not a very large change. To put it another way, the average effective tax rate on the 1 percent highest-income households is about 5.6 percentage points lower today than it was in the 1950s. That’s a noticeable change, but not a radical shift.

How could it be that the tax code of the 1950s had a top marginal tax rate of 91 percent, but resulted in an effective tax rate of only 42 percent on the wealthiest taxpayers? In fact, the situation is even stranger. The 42.0 percent tax rate on the top 1 percent takes into account all taxes levied by federal, state, and local governments, including: income, payroll, corporate, excise, property, and estate taxes. When we look at income taxes specifically, the top 1 percent of taxpayers paid an average effective rate of only 16.9 percent in income taxes during the 1950s.

All in all, the idea that high-income Americans in the 1950s paid much more of their income in taxes should be abandoned. The top 1 percent of Americans today do not face an unusually low tax burden, by historical standards.

Why is this? Greenberg and other authors point out a few factors:

1. Less than 10,000 households actually made more than $200,000 (about $2 million on today's money). The 91% tax rate kicked in for very few people.

2. The people the higher tax rates applied to didn't pay it. As tax rates increased, wealthy people just reported less income. Were they making less? Perhaps they were doing less risk-taking and investment activity. Or were they just making the same and reporting less? Was it plain old tax avoidance? This is one of those economic points that gets politically charged and there's limited data, anyways. Either way, as tax rates went up, revenue didn't increase proportionally.

3. Wealthy people used deductions, tax shelters, and loopholes to reduce their effective tax rate.

If you're talking about high historical tax rates, lower-income people are the place to look. In 1958, everyone making up to $5,000 annually, was subjected to an effective 20% tax rate, most of it income taxes. Keep in mind there were also far fewer tax credits and tax deductions for working-class and middle-class people than their are now.

Although effective income tax rates for low income people gave fallen thanks to policies like the Earned Income Tax Credit, working- and lower-middle-class people have been slapped with higher payroll taxes to pay for larger entitlement spending. Piketty again:

In the 1960s, payroll taxes amounted to 5% of the pre-tax income of bottom 50% earners; today they exceed 10%. In fact, payroll taxes are now much more important than any other taxes—federal and state—borne by the bottom 50%.

Bottom line, it's complicated.