According to Naomi Schaefer Riley’s new book Till Faith Do Us Part, 36% of American marriages are now interfaith (when all brands of Protestantism are lumped together). This is up from 15 percent in 1988 and 25 percent in 2006.

But there’s a significant outlier to the national trend toward intermarriage. My own part-member family notwithstanding, Mormons are the least likely of any religious group to marry outside the fold, at just 12%.

Here are seven reasons Riley gives for the low rates of interfaith marriages among Mormons. The first is obvious; a few others make good sense when you stop to think about them; and the last one is surprising but likely all too true.

1) The theology of eternal families confirms same-faith marriage as a goal for all Mormons.

This statement is going to seem obvious to Latter-day Saints, who are schooled from diaperhood that their families can be together forever—if their parents are married in the temple. But while Mormonism is hardly unique in its theological belief that families can be eternal, it makes that belief concretely contingent upon a particular wedding ceremony in an LDS temple, to which only orthodox Mormons are admitted.

2) Mormon young people serve missions at exactly the time when many might drift away from organized religion.

Mormons, Riley says, don’t countenance the notion of a prolonged adolescence for twentysomethings. Even as the general culture makes more allowances than ever before for “emerging adults” to find themselves, possibly experiment with other faiths, change geographical locations frequently, and date (and maybe even cohabit with) multiple partners, Mormonism sends its college-age people on missions to learn responsibility and take personal ownership of their faith. And when they return, they are encouraged to marry as soon as possible—to other active members of the Church. Moreover, the Church makes meeting other eligible Saints easier with singles wards, which aren’t perfect but certainly contribute to the formation of endogamous unions.

3) Mormons prohibit premarital sex.

Marriage ages for Mormons, while creeping up slightly, are still well below the national average. Since people who marry later in life are significantly more likely to marry someone of another religion or no religion, the Mormon prohibition of premarital sex—and the lower marriage ages that tend to result from it—have protected Mormonism against interfaith marriage.

4) The LDS Church is run by laypeople and thus requires a significant time commitment on the part of an entire family.

Looking past the important twenty-something years of dating, Riley explores how interfaith families respond to the later challenges and complexities of raising children when the partners don’t agree on religion. This is difficult in the LDS faith, where so much is expected of ordinary members. It’s not just a matter of which church to attend; what about tithing? Will we pay it, and to whom? Will the kids go to early-morning seminary? If so, who’s going to get up at 5:00 to drive them? Etc. Mormons, Riley says, are expected to have high levels of religious commitment, which may be offputting to prospective non-Mormon spouses (though this theory undermines the book’s overall argument that most young interfaith couples blithely assume early on that love will conquer all and don’t plan in advance for possible areas of conflict).

Mormons are loving and welcoming toward part-member families. 5)

This seems on the surface to be a counterintuitive argument—if Mormons are kind and accepting of interfaith marriages and the people in them, as Riley claims from her interviews and research (and as our family has experienced firsthand, with only a few exceptions in two decades), wouldn’t the opposite be true? Wouldn’t there be more interfaith, part-Mormon marriages? Riley says that in Mormonism, there is no stigma attached to being in a part-member marriage. For example, there is no shaming of interfaith children (like one story in the book of an evangelical Sunday School teacher who told one of her students that Mommy was going to hell because she didn’t come to church--!). But instead of creating more interfaith marriages, this persistent, long-term welcome mat actually cuts down on such marriages because . . .

6) As many as one-third of marriages that start out as part-Mormon become same-faith marriages over time.

Aha. Because of Mormonism’s strong emphasis on missionary work, approximately a third of part-member marriages will become same-faith marriages when the other spouse converts, sometimes many years down the road. (Incidentally, non-Mormon wives are almost twice as likely to convert to Mormonism as non-Mormon husbands.) These numbers are far higher than postmarital conversions in other religions, particularly in Judaism. There are several stories in the book of non-Jewish spouses who decided to convert but had to repeatedly bang on the door of the synagogue to be accepted, since conversion is not the norm. Mormons, by contrast, exude a “calm and quiet confidence that there are important truths to be found in the LDS faith” and that “their community is one that people should want to join.”

7) People don’t like Mormons.

This to me is the most surprising reason for the low rates of part-Mormon marriages. Frankly, a whole lot of Americans flat-out don’t like us, or at least don't know much about us. Interfaith marriage tends to increase when a religious group becomes assimilated, which is slowly happening with Mormons. But 2007 research indicating that "only 53% of Americans had a favorable opinion of Mormons."* (And oddly, post-election surveys after the much-ballyhooed 2012 “Mormon Moment” show that those numbers have barely budged since 2007.)

Speaking for myself, I was sorry to read that the overall rates of marital dissatisfaction and divorce are noticeably higher for interfaith than for same-faith couples. Such problems have not been my experience in being married to a 100% awesome Protestant husband. Tonight is the 25th anniversary of the evening we met, so forgive me if I’m a little gushy.

People ask me sometimes whether it’s hard for me that my husband is not Mormon. Or they want to know, on a practical level, how we make our interfaith family work. If you’re interested, you can read this article I wrote about that subject ten years ago. Not much has changed since then except that my husband is now Episcopalian instead of Methodist, and our daughter—who was given the right to choose for herself when she turned eight, the Mormon age of accountability—has generally followed in his Episcopalian footsteps, with time off the Canterbury Trail now and again to attend YW activities and LDS ward potlucks.

Sure, it’s complicated. And sure, there are compromises, but a healthy marriage is built on mutual compromise. I’ve no desire to change my husband, and he is equally respectful of my choices. I am proud to be in the 12%.

The images of the wedding rings and the Mormon temple wedding are from Shutterstock.com, and are used with permission.

* I had misunderstood this stat in the original post and corrected it on 5/10/13. Thanks to a reader for pointing this out! And in case you're interested, the Pew study is referenced on p. 189 of Riley's book.