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Noble: Motivated reasoning

It’s hard to believe hand-washing was once considered controversial. In the 1840s, Ignaz Semmelweis discovered that when medical students performed autopsies in the morning and delivered babies in the afternoon, maternal mortality was twice the rate of the wards where midwives delivered babies. The midwives did not perform autopsies, thus Semmelweis concluded that particles from the autopsies were contaminating the mothers via the medical students’ hands.

He instituted a hand-washing routine that drastically reduced maternal deaths. However, when he promoted the benefits of hand-washing, he was widely derided because his findings conflicted with accepted dogma. What is more, his theory tacitly blamed doctors for the death of their patients. Semmelweis died without his discovery achieving widespread acceptance.

In the years that followed, Florence Nightingale, Louis Pasteur and Joseph Lister contributed to the growing understanding of germ theory and demise in the belief in miasmas as the cause of illness. In the ensuing decades, the benefits and practice of hand-washing would gain traction, especially in the medical community. However, it was not until the 1980s that the United States developed national hand-washing hygiene standards — 140 years after Semmelweis’ initial discovery.



Similarly, Louis Pasteur discovered that treating liquids like milk with heat prevented bacterial contamination. Milk is nutritious because it contains all the essential amino acids and calcium. But precisely because it is a nutritiously rich beverage, microbes love it as much as kids. Not properly treated, milk can be a carrier of deadly diseases such as tuberculosis, diphtheria, and salmonella.

Pasteur made this discovery in the 1860s, but it was decades before pasteurized milk was available in large U.S. cities. In the early 20th century, President Theodore Roosevelt’s surgeon general released a report claiming that most childhood deaths were attributed to consuming contaminated milk. Chicago passed an ordinance requiring pasteurization of milk in 1908 — one of the first cities to do so.



The first state to adopt a pasteurization law was Michigan in 1947. The federal government would not require all milk sold or distributed via interstate commerce to be pasteurized until 1987, more than 120 years after Pasteur’s discovery.

When did public awareness of climate change emerge? Most people I have asked estimate first hearing about climate change around 20 years ago. As this article makes clear, the scientific community has deliberated climate change for more than a century. As a senator, President Joe Biden introduced legislation in 1986 to study climate change. Moreover, Eunice Foote demonstrated the greenhouse effect, the concept underpinning climate change, in 1856.

Why are scientific findings met with resistance and often take decades or longer to achieve widespread acceptance? There is no single answer to that question, but there are several illuminating insights, both to the method and process of rejecting new scientific information.

Psychologist Matthew Hornsey said: “Beliefs are difficult to budge, because people don’t act like scientists, weighing up evidence in an even-handed way. When someone wants to believe something, then they act more like lawyers trying to prosecute what they already want to be true.”

It’s a view supported by University of Virginia professor Jonathan Haidt, author of “The Righteous Mind,” who points out — as did Nobel Prize winner Daniel Kahneman — that humans are not rational. Most of us usually make decisions based on emotion or intuition rather than reason.

Using brain scans, Haidt found when research subjects were asked moral questions, their brain activity pattern showed that they reached conclusions quickly and supplied reasons after the fact that justified the conclusion they arrived at.

This process has been labeled confirmation bias. When people look primarily for information that confirms what they already believe, they are engaging in confirmation bias. Its close cousin is motivated reasoning, a type of confirmation bias whereby information conforming to existing beliefs is deemed more credible and conflicting information is discarded or disregarded.

One reason people engage in motivated reasoning is to maintain congruence with the belief structure of the group they are aligned with — typically religious or political. Therefore, evidence counter to the belief of one’s identity group is threatening and rejected.

It is a wonder, given the strength of emotions, resistance to change and power of group identity, that scientific discoveries are ever adopted. However, awareness of the stew of influences that impact belief in science has the potential to make everyone more thoughtful and deliberative, the first step is acknowledging that rationality is a goal, not a given.


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