Thomas Piketty, Emmanuel Saez, and their team of coauthors have constructed databases that document changes in the income distribution for lots of countries over (give or take) a century. Most of it comes from tax records, I believe, which has its problems, but it seems clear that the share of income going to the top [20/10/1]% has gone up over the last 2-3 decades in the US, and also some other countries. I like the graphs best.
Once you’ve taken a look, ask yourself these questions: (i) Why? (ii) What, if anything, should we do about it? Report back when you have answers.