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Some people are really good at logical reasoning. They quickly recognize that a particular argument is well-structured and that one point follows clearly from the next. Other people seem more strongly swayed by whether the information fits with what they already know. They can quickly determine that a conclusion fits with their beliefs, but they are not necessarily that good at determining whether the argument itself is well-structured.

Consider a simple example. I could tell you that an animal either breathes chlorine or oxygen but not both and that an animal I observe does not breathe oxygen. The conclusion is that the animal breathes chlorine gas. This argument is logically well-structured, but it probably doesn’t fit with any of your existing beliefs, because you probably haven’t encountered any animals that breathe chlorine gas.

A big difference between these ways of evaluating an argument is that logical reasoning is based on the structure of the argument, while beliefs are based on the similarity between what you observe and what you have seen before. An easy way to tell that logical reasoning is at play is if you can substitute any symbol into the argument to evaluate its structure. The example I just gave could be written as Either A or B, but not both. Not B. Therefore A.

An interesting paper by Valerie Thompson, Gordon Pennycook, Dries Trippas, and Jonathan Evans in the July, 2018 issue of the Journal of Experimental Psychology: General looked at the relationship between measures of and performance on tests that involve reasoning and belief. They were interested in whether people with higher measures on intelligence test are able to quickly evaluate the structure of arguments.

To test this possibility, they gave people several measures known to be related to IQ. Differences between people in performance on these tests often predict individual differences in success on logical reasoning problems. They also showed participants arguments with a logical structure where the conclusion either did or did not fit with their prior beliefs. So, the final conclusion might be something people often see (like This animal breathes oxygen) or something they do not believe happens (like This animal breathes chlorine gas). After seeing a problem, participants were instructed either to respond whether the argument was logically valid or whether it fit with their beliefs.

Some of the problems were set up so that the structure and belief responses were congruent. That is, there was either a logically valid argument that had a believable conclusion or a logically invalid argument that had an unbelievable conclusion. Some of the problems were incongruent. That is, they were either logically valid with an unbelievable conclusion or logically invalid with a believable conclusion. These incongruent problems are particularly interesting, because they put a response based on reasoning at odds with a response based on belief.

Of the participants in this study, those with the lowest intelligence scores were much better when responding to items based on whether the conclusion was believable than whether it was logically valid. Those with the highest intelligence scores showed the reverse pattern. They were better at responding that an argument was logically valid than when it was believable.

A second study obtained a similar pattern for the congruence of beliefs with statistical information rather than logical validity. In this second study, participants sometimes had to respond quickly, so that their answers reflected their rather than a slow deliberate reasoning process. The higher a person’s IQ score, the more likely that person was to respond best when evaluating an argument based on statistics rather than belief.

These results suggest that one consequence of IQ differences is that they lead people to focus on different kinds of information. People with higher IQ scores find it easier than those with lower IQ scores to keep in mind the abstract structure of an argument, and so they naturally focus on structure. As a result, their intuitions when faced with an argument are based on the structure of the argument.

Those people with lower IQ scores found it harder than those with higher IQ scores to keep the abstract structure of an argument in mind. As a result, their intuitions are generally focused on whether the conclusion of an argument fits with their existing beliefs.

This difference may help to explain why IQ differences affect learning in school. Many of the most difficult things we learn in school involve information that is counterintuitive. The theory of evolution requires that we see a continuity in the animal species on Earth. Complex geometry uses spaces that do not exist in our experienced 3-dimensional world. The harder it is to evaluate the statistics and structure around an argument, the harder it is to wrap your head around conclusions that fly in the face of your existing beliefs.