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New research provides additional evidence that political ideology can interfere with logical reasoning. The findings, published in the scientific journal Thinking & Reasoning, shed light on how politically motivated reasoning impacts the ability to correctly evaluate syllogisms.

A syllogism is a kind of logical argument that applies deductive reasoning to arrive at a conclusion based on two propositions that are asserted or assumed to be true. ("All men are mortal. Socrates is a man. Therefore, Socrates is mortal.") Syllogisms can be valid or invalid, depending on whether the conclusion follows logically from the premises. Importantly, the validity of a syllogism depends on the form of the argument, not on the truth of the premises.

"I have always been interested in the psychology behind political opinions and how people judge whether a politically laden statement is true or false. Studying the ability to identify logically valid conclusions on policy issues felt particularly important in the supposedly post-truth world we live in," explained study author Julie Aspernäs, a PhD Student at Linköping University in Sweden.

The new study included a nationally representative sample of 1,005 Swedish adults. The participants first completed a brief training session to familiarize themselves with syllogisms. They were then shown a series of syllogisms and were asked to indicate whether the conclusion logically followed from the premises. The participants were explicitly instructed to disregard any beliefs about the content of the syllogisms and focus only on whether the argument was logically valid.

Syllogisms contained both non-political and political arguments. Non-political syllogisms included statements such as "If knthzor has two legs, then knthzor can not participate in Umpt; Knthzor can not participate in Umpt; Therefore, knthzor has two legs." Political syllogisms included statements such as "If the labor market is not fair, then the state should intervene to equalize income. The labor market is not fair. Therefore, the state should intervene to equalize income."

The syllogisms varied in logical validity (valid or invalid), difficulty, and ideology (left-leaning or left-leaning conclusion.) The political syllogisms also addressed a diverse set of issues, including labor markets, private health care, marketization of the school system, gender-neutral education, multiculturalism, military defense, asylum to refugees, and climate change.

The researchers found that participants tended to exhibit better accuracy in evaluating syllogisms when there was a match between the validity of the syllogism and the ideological position of the conclusion. Left-leaning participants performed worse on syllogisms where the correct answer was not aligned with leftist ideology, while right-leaning participants performed worse when the correct answer was not aligned with rightist ideology.

The findings indicate "that your judgment is likely tainted by a desire to believe what you want to believe," Aspernäs told PsyPost. "Many of us would benefit from a greater ability to detect conclusions that rest on flawed argumentation."

The results are in line with a previous study, published in 2020, which found that people more willing to accept logical conclusions that were consistent with their political beliefs compared to conclusions that were inconsistent.

In addition, another study published in 2019 has provided evidence that the ability to evaluate logical arguments was influenced by people's political views. "Liberals were better at identifying flawed arguments supporting conservative beliefs and conservatives were better at identifying flawed arguments supporting liberal beliefs," explained Anup Gampa of University of Virginia, a lead co-author of the study.

Aspernäs noted that ideology appears to interfere with logical reasoning regardless of whether a person holds right-wing or left-wing beliefs. "I would like to emphasize that we found flawed reasoning on both sides of the political spectrum, and that most of us engage in motivated reasoning from time to time albeit to varying extent," she said.

The study, "Motivated formal reasoning: Ideological belief bias in syllogistic reasoning across diverse political issues", was authored by Julia Aspernäs, Arvid Erlandsson, and Artur Nilsson.