The steadily dropping marriage rate both contributes to income inequality and further entrenches it.
The well-off marry the well-off, form what she calls "collaboratives," earn lots of money and, having few children, "make out like bandits."
Those with less education make less money and are more likely to cohabit instead of marrying. They have more children, make less money, and their children don't do well either.
I suspect Marcus has identified a major cause of the hollowing-out of the middle class. This is a useful article.