A variation on the "serious" mindset is the "security" mindset. One example of that is "coyote fear". In northeast Ohio most predators were extirpated by around 1900. Over the past 30 years or so, though, as the area transitioned from a rural/agricultural zone to suburbs and exurbs, coyotes moved in. Some cadre of people thinks they are very dangerous animals. A subset of those hysterical people think the presence of coyotes warrants wearing a sidearm when walking around in suburban or exurban parks. In reality, a coyote is a medium sized dog that avoids people. Other predators like eagles, possums, foxes are similarly dangerous to pets and farm animals, but the coyote gets weird fantastical status among local suburban retards.
"Folk concept" is another way of saying people rely on a prepackaged story based model for interpreting reality. A hysterical retard lady who sees a coyote when she's taking out her trash, for example, won't be able to ascertain it's a medium sized dog that poses no threat and runs away as soon as it sees a human, she'll only see the folk story version of a deadly predator. In some places and among some people that folk version of reality becomes the consensus view and when there is fear or some minor risk involved, it can foster a sort of mass hysteria.
In other cases, an individual's anxiety and phantasmagorical horror show understanding of reality doesn't extend beyond their inner world. I saw an interesting bodycam video a few days ago where a 74 year old man pulled a knife on an 11 year old. The old man was afraid of a kid riding a bike on the sidewalk so he threatened the boy with a knife. The old man was also carrying a loaded gun. In the old man's mind, he was maneuvering in a world of threats to his person and his fear justified his actions. This is the basis of the security mindset: fear and anxiety. However, in his case, his delusions were proprietary, rather than shared so he was arrested.
Anyway, I think the "serious" mindset is a variation on this same theme. It's a folk world view, or maybe a religious belief about the nature of reality. Essentially, if you don't "work hard", that is, be serious, god or reality will "punish" you. In some circumstances people believe this literally, that is, being frivolous is a sin which results in future punishment.
However, these same people also realize the converse scenario isn't a sure thing, that is "hard work" does not necessarily lead to success. In fact, it is often frustrated, so the folk concept of "man plans, god laughs" or the line "The best-laid plans of mice and men oft’ go awry” sums that folk view up.
I think those two concepts are fused together in the minds of many people, maybe especially european descended people. They believe in hard work, but know "luck" or good fortune or "god's favor" is actually the dominant force in the universe.
This topic takes me back to the "play" approach versus the serious mindset. Kids and young animals play to "develop". Their lack of experience makes them necessarily open minded, and often willing to experiment and push boundaries of what they think they can do. In the context of the adult world this can lead to innovation and new ways of understanding the world, or even just provides an individual with an opportunity to have their own experiences rather than living inside a folk model or religious delusion of reality.
An important element of these models of reality is it imposes a sorting process on the population. Let's take that 74 year old crazy man as an example. In his community, his delusion is personal, so he's jailed and forced to accept the nominal world view. However, in some scenario where everyone is equally a shithead about danger and every other man is packing a side arm because they think they might be attacked at any moment, he'd be right at home and the kid in that story and his family would move. In the "coyote fear" community, if you were a nature boy you'd leave so you could have a pet coyote, or hang out in peace with the critters.
Ironically, the "play" mindset seems like the big time winner for humans at least in terms of generating wealth, and it also seems to be associated with intelligence, while the serious people can be pretty stupid. A "folklore heavy" or religious community is often stifling and/or materially poor.