I came across this interesting map showing income equality using the GINI index (the higher the number, the greater the inequality). The U.S. shows greater inequality than other advanced nations (no surprise there). On the other hand, I didn't expect South Korea to be rated as having greater equality than Japan or New Zealand.
In the 1960s, the U.S. index was below .40. The wealth gap has been growing since then with no sign of abating.