This came by email. It's a paper investigating changes in educational homogamy over time::
The Changing Role of Education in the Marriage Market: Assortative Marriage in Canada and the United States Since the 1970s by Feng Hou and John Myles, Statistics Canada: Executive summary Educational homogamy — the tendency of men and women with the same level of education to be married to one another — has important implications for social stratification, family income inequality, and intergenerational mobility. While absolute levels of educational homogamy have unambiguously been rising, whether or not relative rates of educational homogamy, net of changes in the educational composition of husbands and wives, have also been rising has been a contested issue in the literature. Changes in relative rates are indicative of whether the function of education in mate selection is strengthening or weakening.
In this paper our aim is to answer three questions. First, what is the overall trend in the relative rate of educational marital homogamy in Canada and the United States over the three decades from the beginning of the 1970s to the turn of the century? Second, are the highly educated more likely now than in the past to marry within their own education level or to marry down, and are the less educated more likely than in the past to marry within their own educational class or to marry up? Third, do women and men experience different trends in educational homogamy given that the rapid increase in women's educational attainment relative to men's implies declining opportunities for women to marry better-educated men but rising opportunities for men to marry more-educated women?
Our analysis of marriages among young adults shows that the overall level of both absolute and relative rates of educational homogamy have unambiguously increased in both countries over the three decades. In Canada, 54% of couples had the same level of education in 2001, up from 42% in 1971. In the United States, some 55% of marriages consisted of couples with the same level of education in 2000, up from 49% in 1970. These overall trends have been driven mainly by changes in the association of husbands' and wives' education rather than by changes in the relative supply of more- and less-educated partners. In Canada, the change in association, net of changes in the distributions of wives' and husbands' education levels, accounts for almost 10 percentage points of the 12-percentage-point increase in educational homogamy from 1971 to 2001. In the United States, the change in association accounts for 4 percentage points of the 6-percentage-point increase.
In both countries, intermarriage across education levels occurs primarily between adjacent education levels, and intermarriage across more than one education level is relatively rare. Therefore, changes in intermarriage between adjacent education levels dominate the overall trends in educational homogamy.
Rising educational homogamy has been driven mainly by changes at the top and at the bottom of the educational hierarchy.
Declining intermarriage between those with university degrees and those with less education was a major factor in both countries. In Canada, the relative rate of intermarriage between the university educated and those with only some post-secondary education fell by 38% and in the United States, by 45%. Whereas Canadian trends were quite similar for men and women, the decline in the United States was almost entirely driven by declining intermarriage among university-educated men.
Similarly, the odds of intermarriage between high-school graduates and those with less than high-school completion fell by 30% in the United States and by 58% in Canada. In Canada, this was a common pattern among both men and women while in the United States the trend was mainly confined to men.
There were two important exceptions to the general trend towards rising homogamy. The first was an increase in intermarriage between women with some post-secondary education and men with high-school graduation in the United States. The second was a small increase in intermarriage in the 1990s among male and female university graduates in Canada and among female university graduates in the United States, suggesting that levels of educational homogamy among the better educated may be stabilizing.
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One question that comes out of this is how much of the changes in inequality over time might be attributed to these changes. As I noted, I'm a bit pressed for time today, so I wasn't able to look into this as much as I'd like, but here's a bit on the question:
A model constructed by the economists Raquel Fernández and Richard Rogerson, published in 2001 in The Quarterly Journal of Economics, led them to conclude that “increased marital sorting” — high earners marrying high earners and low earners marrying low earners — “will significantly increase income inequality.” A 2003 analysis by Gary Burtless, an economist at the Brookings Institution, found that a rising correlation of husband-and-wife earnings accounted for 13 percent of the considerable growth in economic inequality between 1979 and 1996.
Burtless himself does not think that assortative mating is necessarily becoming more prevalent. In fact, he says he believes that “the tendency of like to marry like has remained roughly unchanged over time. What have changed are the labor-market opportunities and behavior of women.” In this conception, men have always married women of their own social class, but such stratification was obscured by the fact that the female halves of these couples often did not work or pursue advanced degrees. Now that women who are in a position to do so are attending college and graduate school and joining the professions, the economic consequences of Americans’ assortative mating habits are becoming clearer.
More here (this is preliminary work, but it suggests that causation may run in the other direction, i.e. that inequality causes assortative mating). If anyone can add more, that would be appreciated.