When the Mormons left behind the Midwest and set off for the Rocky Mountains in the middle of the 19th century, they were not a random sample of Americans. Neither were other groups of frontier settlers, like the type who could make it in the Wild West. Today, the offspring of the Wild West settlers are among the most criminal whites, with Arizona leading the nation in the incarceration rate of the white population.
Mormons, though, were not selected for their rugged individualism and willingness to solve problems with violence. What traits did set them far apart from the average of their day? Well, whatever would incline one to put so much blind faith in a New Age cult ("new religious movement"), that they would trek out toward the inhospitable and untamed frontier because their leaders said that's where we need to go if we want insulation for our cult to practice plural marriage. Too bad if it means you have to leave your families and communities behind.
They would also have had to believe that their latter-day prophet had translated an ancient religious book written in Egyptian on golden plates (the Book of Mormon). And that the Garden of Eden of the Old Testament was really located in the American state of Missouri, and that the holy Israelite temple of Zion in the Old Testament was also to be restored in America.
A lazy atheist response would be that all followers of religion are gullible sheeple, how are Mormons any worse?
In fact, neither of the two major world religions today -- Christianity and Islam -- owed their initial growth to their founding prophet claiming to have discovered and properly interpreted a physical object, one that no one else saw, and using a translation method that no one else could verify. That should have alerted any normal person at the time to the strong possibility that Joseph Smith was a con man.
Jesus didn't claim to have discovered a long-lost tablet that would give the Israelites the true fullness of Mosaic laws. He preached his own take on existing laws, and listeners were either persuaded or not of his interpretation, moved or not moved by his exhortations. He wasn't trying to pull a fast one on his audience.
Mohammed's appeal was based on revelations that came to him from angels -- that's a supernatural claim that can't be verified by curious listeners, unlike the physical basis that Smith claimed for his prophecy. Either you believed Mohammed's claims about angelic revelation or you didn't, but it wasn't because he was employing flimflam techniques.
This suggests that present-day Mormons are the offspring of a population that was unusually suggestible and impressionable (to use more neutral terms than gullible), to be hooked in the first place. And to have stayed with the cult for so long in such inhospitable circumstances, they would also have had to be unusually eager to please their social superiors and to dread "making waves" against the collective.
Before turning to some hard data, let's take an impressionistic look at a Mormon woman explaining some of her religion's beliefs (video here). She's from southern California, but would blend right in among Mormon women of her generation in Utah. Overall impression: ditzy (not pejorative), free-spirited, anxious about fitting in, hopes that you like her by the end of her presentation.
That's way different from the stereotype that folks east of the Rocky Mountains have about stuffy, stern, sober Mormons. They were originally from Yankeedom, but they're more like a New Age splinter group of Puritans. New Englanders are infamous for being dour and crotchety, whereas Provo, Utah is "Happy Valley" -- even if used dismissively, the nickname still shows the peachy-keen Stepford Wives atmosphere that is more typical out West.
Getting more quantitative, we can look at the rates for con man schemes, which the government keeps close enough of an eye on for there to be national data. Google "Utah fraud per capita" and peruse the endless news reports of the Mormon capital being the most vulnerable to Ponzi schemes, "affinity fraud" (trust me, I'm one of you guys), and the like. These articles go back regularly to at least the 1990s.
Fraud per capita
Utah: Where the con is on
Utah County is hotbed of white-collar crimes
Affinity fraud: Fleecing the flock
Utah named a top Ponzi state -- again
And so on.
The scammers target LDS Church members preferentially, so it does seem to be related to Mormons specifically, and not their ethnically similar neighbors around the state. It doesn't seem to reduce to "Scandinavians are high-trust and vulnerable to con men". It's particular to followers of Mormonism.
And it doesn't generalize to all religious groups, as though the Catholics, Presbyterians, and Baptists were just as easily duped by con men.
That suggests that Mormons have been selected for higher levels of being suggestible and eager to please.
We'll see those traits come up again when we look at their temple initiation rites, which have been a constant selection pressure for those traits, right up to the present day.