Here's a fascinating visualization of how inequality in the United States has evolved over the years. Red means lower inequality within a state, green means higher inequality:
One notable trend here: The Deep South, as well as New York and California, saw the fastest growth in income inequality over time. Eventually, though, all states start shading green by the late 2000s as inequality grows. The financial crisis, in particular, saw a sharp rise in income inequality.
Why is this happening? Back in 2011, the Congressional Budget Office investigated the rise in U.S. income inequality between 1979 and 2007 and concluded that it was driven by two big trends. First, business income has become more heavily concentrated at the top, likely due to the growth of high incomes in privately owned professional firms such as law, medicine, and finance. Second, capital income — dividends, rental income — has become much more unequally distributed as well.
Related: Mark Thoma has a couple of other animated inequality maps that are worth checking out.