What she found was that the most religious students (based on self-reported religiosity, their frequency of religious service attendance, and their religious observance) also had the fewest friends from other races.
What’s more, Protestant or Jewish (but not Muslim, Hindu or Buddhist) students also had the fewest mixed-race friendships. That’s probably because these are the two major religious groups.
These two effects were independent – so the most mono-cultural people were the most religious Protestants and Jews. This held even after controlling for a bunch of other factors, including the racial diversity of the college, the diversity of their previous school, and the race of the student.
And on top of all this, belonging to a religious club reduced the chances of inter-racial friendship still further! That wasn’t the case with other clubs (except explicitly ethnic clubs – and even here the effect was smaller than for religious clubs).
Now, the interesting thing about these three factors – religiosity, religious denomination, and membership of a religious club – isn’t that they weren’t highly correlated. That means that they seem to have independent, additive effects.
I guess this makes sense in a way. It’s been well known for decades that there is a strong correlation between racism and religion. That is, the more religious someone is, the more likely it is that they’re racist. This is counterintuitive if you only look at the dogmas of religion and their pretense to inclusion, but we all know how well intuition works.
Of course my own anecdotal experiences confirm that religious people are more racist than the non-religious; a mother of an ex gf of mine apparently disapproved of interracial relationships (which was ours, but she didn’t know about it) and was one of the reasons why she didn’t approve of Obama, who was running for president at the time. Other religious parents of friends of mine also didn’t approve of interracial relationships going so far as to disown children for marrying outside of their race.
This probably isn’t a knock on religion per se; it probably means that racism and religion both appeal to the same sort of brain module that separates us vs them. This is evidenced by the fact that there is also a relationship between religiosity and nationalism. So it could be that people become religious just because they are also the type of people who are predisposed to racism and nationalism.