Assortative mating is when individuals with similar characteristics mate with each other more often than what would be expected through random mating. While humans have always assortively mated, it's plausible that this propensity has increased over the past century because a lot more men and women are able to act on their mating preferences. Thanks to technological, economic, and cultural progress, our mating choices are less constrained by poverty, fertility, patriarchy and small town living. More of us can hold out until we find a better fit.

Assortative mating is thought to contribute to income inequality when the better educated marry mostly each other and create a privileged class that hoards the best genes, parenting, education, and neighborhoods while the rest of society gets stuck in a socioeconomic rut. That's the theory anyway. What's actually happening on the ground? This:

2018 Marriage and Education.jpg

And this:

2018 Marriage and Education 2.jpg

The first chart confirms assortative mating among college graduates has increased steadily in the US since 1960. But so has the number of college graduates, from around 11% of adults age 25-34 in 1960 to roughly 35% today. Back in the day, most college graduates had to look outside the hallowed halls to find that special someone. There just weren't enough fish in that particular sea. Nowadays, colleges are well-stocked with potential life-partners.

However, the second chart reveals a substantial number of American men and women are not marrying their educational equals. College graduates are no exception. Per Pew Research Center analysis of US Census Bureau data :

"Among college educated newlyweds (including those with postgraduate and advanced degrees), nearly four-in-ten women (39%) married a spouse without a college degree, but only 26% of men did so."

Those female college grads marrying men without degrees aren't really marrying "down"; most are actually marrying spouses who out-earn them.* They are still exercising their preference for high status mates. Education is just one status marker; income is another.  Bottom line: associative mating is increasing in the US, as least among high achievers.

But what is the actual effect of this increase of assortative mating in the US? Does it really make inequality worse? Next

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* I'm guessing a lot of these men are business owners, managers, supervisors,  skilled technicians, and coding geeks.

References:

Greenwood, J. , Guner, N., Kocharkov, G., and Santos, C. 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

Qian, Y. (2017). "Gender Asymmetry in Educational and Income Assortative Marriage." Journal of Marriage and Family 79(2): 318-336.