I can usually tell if someone is Mormon or not. People ask me how I do it and I can't tell them anything definitive. Apparently, this issue is now being scientifically studied:

The study entitled, "On the perception of religious group membership from faces" can be found on PubMed. From their abstract:

. . . We tested whether Mormons could be distinguished from non-Mormons and investigated the basis for this effect to gain insight to how subtle perceptual cues can support complex social categorizations.
Tom Rees comments on this and other studies in Epiphenom in a piece entitled, "Hey, good lookin'... you must be a Mormon!" Science and Religion Today publishes his piece as, "Can Mormons Be Distinguished by Their Faces?" Tufts Journal published, "Spotting the Faithful."

From Tufts Journal:
The Tufts study by Rule, Nalini Ambady, professor of psychology and Neubauer Faculty Fellow in the School of Arts and Sciences, and James V. Garrett found that both Mormon and non-Mormon subjects were able to identify who was a Mormon more often than would occur by chance. It was published Dec. 7 in PLoS One. [See this link]
Only headshots were used in the study and researchers eliminated each facial feature until they got down to skin texture. They concluded skin texture was the definitive clue and they tied it to health and the fact that Mormons are healthier than most.

Again from Tufts Journal:
Even when the researchers removed important features, such as the eyes or mouth, the subjects were able to identify Mormons more often than would occur by chance.
Rees contests this:
 . . .a study last year that showed that, in the United States, the religious were not rated as being healthier. In fact, it showed that although people thought they were picking out the religious based on their healthiness, in fact they were not. So, the complete opposite of this new study then. Oh well, back to the drawing board.
Rees is ignoring that Mormons ARE healthier than most people. I'm not including any links to this fact because I've read about studies all my life that confirm this. They are too numerous to list. This fact is well established.

I think the researchers established that people can tell but the reason why is still open:
Mormons themselves, Rule says, believe “God is present in different people and you can observe this just by looking at them. Mormons say it’s about the holy spirit emanating from someone,” which is how they are able to identify each other.
I'm not sure if this has received an official cultural label but I'll call it the "glow theory." A commenter, "Jon" on "Can Mormons Be Distinguished by Their Faces?" explains this:
I am a Mormon and I believe that “faithful” Mormons are easy to spot.
In fact, while overseas in South America I was able to pick out a couple and identify not only that they were American but also Mormon from across a crowded hotel restaurant. I didn’t hear them speak until I walked up to them and point blank asked 1 if they were American and 2 if they were Mormon. They were shocked that I could so easily identify them, but Mormons do “glow” spiritually.
I've usually heard Mormons, and even others, use the term "glow" to describe this phenomenon. It even works for non-Mormons. My Branch President and his wife told me of a tourist visit they made to Jerusalem. They were walking down the street when they heard a man yell, "Hey, you Mormons!" They looked around and a Jewish shop keeper was gesturing at them. They pointed at themselves in wonder and he said, "Yes, you Mormons, you come into my shop." They obeyed and discovered his shop contained a variety of Mormon kitsch and other items that would appeal to Mormons. They asked how he identified them. He said, "By the glow."

I think the most famous story is the following entitled, "The Light in Their Eyes" by Elder James E. Faust:
I recently recalled a historic meeting in Jerusalem about 17 years ago. It was regarding the lease for the land on which the Brigham Young University’s Jerusalem Center for Near Eastern Studies was later built. Before this lease could be signed, President Ezra Taft Benson and Elder Jeffrey R. Holland, then president of Brigham Young University, agreed with the Israeli government on behalf of the Church and the university not to proselyte in Israel. You might wonder why we agreed not to proselyte. We were required to do so in order to get the building permit to build that magnificent building which stands in the historic city of Jerusalem. To our knowledge the Church and BYU have scrupulously and honorably kept that nonproselyting commitment. After the lease had been signed, one of our friends insightfully remarked, “Oh, we know that you are not going to proselyte, but what are you going to do about the light that is in their eyes?” He was referring to our students who were studying in Israel.
The researchers won't accept this explanation but it is probably what most Mormons believe. A commentator already posited the idea on a political science blog entitled Utah Data Points when reviewing the recent study.


I'll go even further. I can determine whether a Mormon has attended BYU. I was sitting in a institute class once and a guy walked in. I thought to myself, "He's so BYU he reeks." I discovered later he had just graduated from BYU.  Maybe I'll call this the "BYU Reeks Theory." 


Something tells me that label won't catch on.



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