As with life expectancy, there is a linear relationship between access to a university education and income in the United States.
“The most striking fact about the increase of inequality in the United States is the collapse of the share of total national income going to the bottom 50 percent, which fell from about 20 percent in 1980 to a little more than 12 in 2018. Such a dramatic collapse from an already low level can only be explained by a multiplicity of factors. One such factor was the sharp decrease in the federal minimum wage (in real terms) since 1980. Another was significant inequality of access to education. It is striking to discover the degree to which access to a university education in the United States depends on parental income.”
– excerpt from Thomas Piketty’s Capital and Ideology.