So there was great clashes when, you know, if you believe you shouldn't remove your shoes and someone's taking their shoes off, how can they do this? That actually was such a big clash in this case that they had to put a curtain down the middle of where they would worship.

But in the US, when you have two separate cultures, each with its right, each of which has come to exist in this political entity in the last couple hundred years, each feeling like, "I have the right to hold onto my culture," and that's what makes it difficult.

What sort of difficulties would happen when people of different cultures try to come together to worship? Tiny little things such as let's tell jokes with each other.

It happens a little bit more in the West, where there's more fluid - where everybody's originally from somewhere else. So they have a little bit more permission to do it. It happens the least, at the individual level at least, in the South, because the South has very strong, you know, set up black churches and white churches and a long history of that, and so it's a bigger social cost.

And different traditions stress different - so then there's that. I talked to an African American who says before she goes into an interracial church, she sits in her car and she listens to gospel music to get her fill, and she goes into an interracial church where they don't do gospel music, and she's ready to accept the other sorts of ways of worshipping. So there's that.

We've had people say, "Now when I go to work, I don't feel uncomfortable talking to people of different races, and I go up and introduce myself, and I start making a new friend I wouldn't have done otherwise."

We studied a mosque, and this is when we were at Notre Dame, and in this mosque they had people from a variety of countries, most of them immigrants. In some of the countries, when you go into a mosque you remove your shoes. To not do so could be punishable even by death in that nation. In other countries, it would be a great offense to remove their shoes when they come into the mosque, a sign of disrespect.