Steven Landsburg on the "stagnation" of incomes

This is from his book "Can You Outsmart an Economist?"

The table shows that from 1980-2005, all 4 groups (white men, nonwhite men, white women and nonwhite women) all had increases in median income (adjusted for inflation) of at least 15% while the overall gain was just 3%.

The reason is that the lower paying groups grew more than the higher paying groups, keeping the average increase low.