Assortative mating is a mating pattern in which individuals with similar [characteristics] mate with one another more frequently than would be expected under a random mating pattern. Wikipedia, “Assortative Mating”  

..if matching in 2005 between husbands and wives had been random [rather than assortative]... the Gini coefficient would have fallen from the observed 0.43 to 0.34, so that income inequality would be smaller. Thus, assortative mating is important for income inequality." Greenwood et al, "Marry Your Like: Assortative Mating and Income Inequality," 2014

The argument in brief:  an increase in assortative mating, in which highly educated women are more likely to exercise the near-universal preference of women to marry men of higher education, income, or status (“hypergamy”) has led to increasing inequality. In earlier times, marriage across education levels and classes was much more common, as women often married up, dispersing wealth and reducing inequality among households. These days assortative mating has led to a concentration of income and wealth, as the best educated women hoard the most desirable men and saturate those men with dating and marriage choices, while less fortunate women increasingly choose to remain single rather than marry against their preferences. What we get is a shrinking middle class and growing gap between rich and poor.

Just getting started. Questions to ponder: just because in prior times marriage partners didn't make sure their education levels matched, does that mean that the choice of spouses was random? Or were assortative considerations still in play? Next.

Reference:

Jeremy Greenwood, Nezih Guner, Georgi Kocharkov, and Cezar Santos, 2014. "Marry Your Like: Assortative Mating and Income Inequality," American Economic Review, American Economic Association, vol. 104(5), pages 348-53. http://www.nber.org/papers/w19829