Philip Cohen, a sociologist at the University of Maryland, asks in a blog post: Which majors are most and least likely to marry each other? He used the American Community Survey.
I combined eight years of the ACS (2009-2016), which gave me a sample of 27,806 college graduates who got married in the year before they were surveyed (to someone of the other sex). Then I cross-tabbed the major of wife and major of husband, and produced a table of frequencies. To see how majors marry each other, I calculated a ratio of observed to expected frequencies in each cell on the table.
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The most homophilous major is theology, where the ratio is a whopping 31.
Actual is 31 times more than expected. (See the blog post for details.) No other cross-tab comes close.
These results are for bachelor to bachelor marriages, as it were. Cohen also has results for bachelor to less than bachelor marriages.
He makes the data available here.