The top 20% of American households have gone from making 43% of all US income in 1968 to 52% in 2018, according to the Pew Research Center, and the trend doesn’t look likely to stop any time soon.

There are a variety of reasons for the increase. Tax policy, the hollowing out of middle-class jobs, and the rising financial returns on college education compared to a high-school education have all contributed. Another meaningful reason: How Americans choose their partners.

Far more than they did 60 years ago, high-income Americans tend to marry others with high incomes. In 1960, only 0.4% of married couples both made the top 20% in income for someone their age, according to Quartz’s analysis of US Census data. In 2018, that number reached 7.4%. If couples married at random from the income distribution, that number would be about 4%. (To consistently look at historical trends, we only examine couples of the opposite sex; the US Census did not historically collect data on same-sex couples.)