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.)
The primary reason for this jump has been the rise of women in the workplace. In the past, men who made high incomes were often married to women who didn’t do paid work and were instead relied upon to do housework and take care of children. But things have changed. A generation of women advocated for equal job opportunities; at the same time, improved technologies for completing housework (pdf), like dishwashers and laundry machines, also emerged.
Fewer than 45% of women ages 25 to 54 were in the job market in 1960. Today, more than 75% of them work.
If high-income women married low-income men, the rise of two-income households wouldn’t have had much of an effect on inequality. In reality, though, women with high salaries are increasingly marrying high-earning men. In 1960, a woman in the top 10% of incomes for her age and gender married a man in the top 10% for his age and gender 11% of the time. By 2018, the share of marriages following this pattern reached almost 25%.
Economists call the phenomenon of people finding wives and husbands with similar characteristics to them “assortative matching.”
One characteristic that has become increasingly important to both men and women is level of education. A 2014 study on assortative matching published by the National Bureau of Economics Research (pdf) found that, in the 2000s, a man or woman who graduated from college was much more likely to be married to a person who also graduated from college than they were in the 1960s, even accounting for the country’s rising college attendance rates overall.
Graduating from college is strongly correlated with high incomes, so this means there a lot more highly educated, wealthy couples. The 2014 study found that the impact on inequality is substantial, causing a 25% increase in the US’s gini coefficient, a popular measure of inequality. Another set of researchers found increases in assortative matching on education in Denmark, Germany, the UK, and Norway.
Richard Reeves and Joanna Venator of the Brookings Institution point out this may perpetuate inequality across generations. Children of parents with the highest incomes are much more likely to go on to make lots of money.
The US is already a stratified society. The fact that Americans increasingly pair up with others at similar pay levels both underscores and exacerbates that fact.