Twin study suggests rationality and intelligence share the same genetic roots

A recent study involving twins has shed new light on the relationship between intelligence and rational thinking. The findings indicate that the ability to make rational decisions, often seen as a separate skill, is actually very closely tied to general intelligence. In fact, the study suggests that being irrational, or making illogical choices, might simply be another way of measuring lower intelligence.

Rationality, in this context, means making good decisions based on logic and available information, avoiding common pitfalls like jumping to conclusions or being swayed by gut feelings when they are wrong. Some experts believed that rationality was a unique skill, distinct from intelligence as measured by standard intelligence tests. They proposed that rationality involves a special ability to override our initial, intuitive thoughts and engage in more analytical thinking. This “rational mindset,” they argued, might be missed by traditional intelligence tests.

Others, however, suspected that rationality and intelligence were more closely related, with rational thinking simply being a natural outcome of higher intelligence. To explore this question in depth, Professor Timothy C. Bates of the University of Edinburgh decided to investigate the genetic basis of both rationality and intelligence.

“If ‘rationality is what IQ tests miss,’ then this would be very important,” Bates told PsyPost. “It would imply that a lot of decision making is unrelated to cognitive ability and that we need new tests to prevent and understand some of our most egregious mistakes. So it seemed important. And we could use twins to test this in ways not used before.”

To examine the roots of rationality and intelligence, Bates turned to the Brisbane Adolescent Twin Study. Twins provide a unique opportunity to tease apart the influence of genes and environment on various traits. Identical twins share nearly all of their genes, while fraternal twins, like regular siblings, share only about half. By comparing how similar identical twins are to each other compared to fraternal twins in a particular trait, researchers can estimate how much of that trait is influenced by genetics.

Free daily newsletter

Bates analyzed data from 1,570 individuals, including both identical and fraternal twins. All participants completed tests designed to measure both their general cognitive ability and their cognitive rationality. Cognitive ability was measured using a combination of tests that assessed vocabulary, numerical reasoning, and the ability to identify patterns in sequences of letters and numbers. These types of tests are commonly used to gauge different aspects of intelligence.

Cognitive rationality was assessed using a specific test known as the Cognitive Reflection Test. This test presents individuals with problems designed to trigger an intuitive but incorrect answer. For example, one question asks: “A bat and a ball together cost $1.10. The bat costs $1 dollar more than the ball. How much does the ball cost?” The intuitive, quick answer is 10 cents, but the correct answer, requiring a bit more thought, is actually 5 cents. The Cognitive Reflection Test uses several such questions to see how well people can resist misleading intuitions and arrive at the logically correct answer.

The results showed that both intelligence and rational decision making were strongly influenced by genetic factors. Bates was surprised by “the strength of the heritability of rationality: It is really a great little IQ test!”

In addition, when Bates tested whether there was a separate factor that could account for rational thinking in addition to intelligence, he found that this extra factor did not improve the explanation of how people performed. Instead, the same general mental capacity that drove vocabulary and puzzle-solving also accounted for performance on the rational decision making test. Both sets of scores loaded heavily on a single shared factor. This supports the view that rational thinking is not a separate ability but is actually an indicator of broader cognitive skills.

Bates outlined four key takeaways: “1) We found that irrationality, far from being what IQ tests miss, is one of the best IQ tests available. 2) We found that irrationality, far from being unrelated to genetics and more of a mindset, is among the most heritable of psychological traits. 3) Irrationality is making mistakes which are unnecessary: wrong decisions when we have all the information we need, and some simple logic means there is no reason for the error. We found that realizing what information is available, and applying some simple logic, is almost all of the cause of cognitive irrationality. 4) Cognitive ability explained nearly all of cognitive irrationality, and much of the overlap was genetic.”

Although the study featured a large sample of twins and made use of widely accepted methods to measure intelligence and rational decision making, there were some limitations and open questions for future research. The measure of rational thinking used only a few items and might not capture every aspect of how people make decisions in real-life contexts. Different tests, especially those involving larger sets of problems, could confirm or refine the results. It would also be useful to see how these findings play out in childhood and adolescence, as well as in different cultural settings. Future work could look for influences from personality traits, learning experiences, or motivational factors that might add further detail to how intelligence and rational decision making interact.

“It would be great if rationality research unified with intelligence research,” Bates said. “We have, it turns out, been working on the same problem, in the case of intelligence research for a century, so there’s a lot, I think, that might be of use for other fields.”

The study, “Cognitive rationality is heritable and lies under general cognitive ability,” was published in the journal Intelligence.