Something to think about in average graphs of growth in America over time. Especially when looking at 1945-1973 time frames. I'll stick with cars and airplanes because I think they are representative.
At the start of that period America had a massive advantage of industrial infrastructure that had been bombed, blasted or otherwise destroyed. America didn't have to rebuild factories from rubble.
With cars, during that time frame, Americans liked the cars built domestically which were also quite competitive in costs. If you couldn't afford a shiny big car, perhaps you bought a smaller, less shiny one, but it was still made in America.
With passenger airplanes, worldwide (in the West), most were built, yep, in America. Certainly within America airlines had a few forays into foreign aircraft, but in general, most planes were built in Seattle, Long Beach, or San Diego. And those locations built a large majority of all the airplanes flown by western nations.
But by the 1970s things had started to change. "them thar furrin" cars had started to appear on the scene en masse, and by the 1980s it was a torrent. And for passenger airplanes, it was a little behind cars, but pretty soon, more planes came from outside the US, and some of the bigger airlines actually had more of "them thar furrin" planes than they had American aircraft, and were (when you looked at purchases) spending the majority of their spending on "furrin" aircraft.
There are numerous other industries that have followed that pattern; perhaps steel most famously.
So when you look at income disparity shifts, keep in mind that the real top end of income -- people like Jobs, Gates, and now "new entrants" like Zuckerberg, Brin, and Page -- the really bright guys -- they are, in a very real sense, creating new stuff that doesn't have global competition yet. And that's why they continue to see high income growth.
Median income comes from your "base" for lack of a better, technical term. And for things like cars, planes, steel... the rest of the world caught up to the US in the 1970s and the heavy global competition is what has slowed it down. We see even today the signs of it in technology today for the average IT worker -- first it was just a little bit of "outsourcing" simple tasks to places like India, but as time passes increasingly complex work is being shifted at an ever increasing pace. I think 15 years if you took median IT salaries and mapped out 1985-2025, you'd see good growth for the first 20 years, and then some serious flattening out during the last 10 or so.
In short, it's not taxation or tax policies that have made changes, it's global competition. If you want to make a difference, you have to foster policies that encourage growth into new areas where there is limited competition. Government is historically terrible at doing that (with a handful of small exceptions such as the early parts of the space program).
Steve
I agree with all of that but I'd like to add two more points.
1. In addition to the "head start" the USA had after WWII (as the only unharmed country of the Western or Eastern civilizations), they not only could take advantage of the lack of competition. They also used their newly-gained worldwide nimbus and influence to fill the power vacuum, which was left by the European countries and the British Empire, and to install their own system of economical exploitation, namely in South America, Africa, the Middle East and East Asia. It is the reason, why until today countries, that are actually rich with resources, have a population suffering from poverty.
Partly it happened in competition with the Soviet Union during the Cold War (which actually wasn't that cold but cost millions of lives) and partly because there simply was no one who could stop them.
During the 1950's and 1960's the USA were busy with installing regimes or oust governments to their liking all over the world. In most cases they simply bought the loyalties of third world countries with money. Millions of millions went into the pockets of he heads of governments, securing the USA's access to resources. If they met resistance with political leaders who weren't corrupt, the CIA hatched rebellions and revolutions, until an America-friendly government was installed.
But times changed and slowly the USA started to lose control over their vassal states. The process started with the revolution in Iran, which for the first time made the USA look like a paper tiger. But also in other countries the people realized, that they had been puppets in the hands of American politics and weren't willing to play the game anymore.
The lousy image of the USA especially in South America today is a result of those reckless post WWII politics. Guys like Hugo Chavez in Venezuela might be crazy fools but they don't come from nowhere. They are a direct result of the USA's imperialism (yes, I'm using that word, because that's what it was!).
With the ongoing loss of control over former marionette states, the USA also lost a lot of their influence and economical power. They no longer automatically have a better position in the world, especially when it comes to competing with the reinvigorated European economies and the new rising Asian economies. And this exposes the weaknesses and neglects of past decades.
2. Much of the economical success of the USA is based on debt. The USA's wealth is built in borrowed money. The world knows it and is losing more and more trust in the "leading nation's" ability to recover from economical crises like recent ones. Each new blow will be harder to parry. And there will be more blows.
With the vanishing power and the loss of direct influence on other countries in the world, the actual weakness of the U.S. economy started to manifest itself. It already started in the 1970's, when Europe was rebuilt and started to catch up again. In my opinion the current economomical crisis in the USA isn't a new problem but an already existing weakness, that was present all along through the decades and was only covered up by convenient circumstances. Otherwise they wouldn't have such problems recovering, while in Europe things are already returning to pre-crisis levels.
As said above, the times of convenient cirumstances are over. The USA aren't able anymore to politically or economically dominate the world, like they did after WWII. The future of the USA doesn't look too bright. Debts are enormous, unemployment is high, exports are low and the Dollar is losing more and more of its appeal as the world's leading currency.
The problem, however, is, that most Americans are still thinking that their country is kind of invincible and that things will eventuelly be sorted out. Well, it's not that easy anymore. This realiziation still has to find its way into the heads of most U.S. citizens.