My own thoughts were that in those with lower IQ, there is a greater need for concreteness because they have less capacity for critical thinking and dealing with matters of uncertainty.
Oh, for some reason I figured this was a self-evident thing, which is why intuition was all I needed for my assurance. That is, I would think no explanation was necessary for such an obvious conclusion to be reached. So there must have been some underlying motivation/agenda for the study to have been done. Was it merely for statistical analysis, discrediting of religion, or to make some social scientists with low self esteem feel somehow intrinsically better or more valuable than other people? Such studies really do not prove much, since they often state an obvious, but almost always have an intention behind them.
What people are able to understand or believe or not is determined by many many variables, such as memory retention, cognition, comprehension, age, socio-cultural bias, the culture and era they were born in, access to education(the quality and breadth of which is highly variable), etc. No amount of providing an education, empirical evidence, reasoning with someone, over any amount of time, can keep them from willfully refusing to believe, or even wanting to know of, something outside of their desire or capacity to comprehend it. I would say it's not really their fault, because for the most part they are concreted in their worldview. And trying to convince them of something they can't or won't come to realize by their own volition and ability is a lot like trying to p iss up a rope. Also, you can lead a horse to water, but good luck trying to make it drink.
Conversely, I've met many "open-minded" college graduates that cannot change a car tire, explain how their toaster works, or say who painted A Sunday Afternoon on the Island of La Grande Jatte(nor do they care to learn); and yet, because they have invested years and gone deeply into debt to become knowledgeable, they feel some need to express their superiority. But, unfortunately, when someone pays for their own indoctrination, they end up learning a great deal of information about a very narrow slice of life(and all that only theoretical), with very little quantity or diversity of life experience to base any real knowledge on. I would say colleges, by and large, end up telling most students
what to think rather than
how to think. Consider
Srinivasa Ramanujan, who made more complex contributions to deep math than professors at Cambridge, and that simply by studying a single library book on applied mathematics he had been given when he was 16. You cannot put in what God left out, and you cannot take out what God put in. The capacity and volition for developing an ability is either there or it isn't, but it doesn't determine whether a person is good or bad.