Education, productivity, and marriage

Arnold Kling reflects:

... we went from a pattern in which inequality was falling as the average standard of living was rising to a pattern of increasing inequality.

Two questions arise. First, why has education attainment stagnated? Second, why has average productivity growth remained high in the face of this stagnation?

My answer would be assortative marriage. We are dividing into a two-tier society, where the top tier produces children capable of obtaining advanced skills and the bottom tier does not.