Source: NASA/Wikipedia Commons

When I was growing up, the list of planets in our solar system included a small planet far from the Sun called Pluto. Then, a number of objects about the same size as Pluto began to be discovered. Some were even bigger than Pluto. A body of scientists, the International Astronomical Union, redefined the term planet and changed the classification of Pluto from “planet” to “dwarf planet.”

A number of non-astronomers have found this demotion of Pluto hard to take. Indeed, the backlash has been so strong that some NASA scientists want to reclassify Pluto and several other large bodies in the solar system as planets.

Why do people have a hard time reclassifying Pluto?

This question was explored in a paper by Alexander Noyes and Frank Keil in a 2017 paper in the Journal of Memory and Language.

These researchers point out that many categories people form are based on the opinions of experts. Most people don’t know how to define a planet, what constitutes a particular microbe, or the factors that make something a quark. Instead, we assume the experts know what items belong in these categories, and we generally accept their definitions.

Once something is placed into a category, though, other factors take over. In particular, people generally act as though category members share some unseen essence that makes them a member of a category. We assume that there is something about microbes, planets, and quarks that make them what they are. This idea is called essentialism.

You can even use this belief in essences as joke. Suppose someone asked you, “Is John a painter?” You could say—dismissively—“Well, he paints.” This works as an insult, because categorizing someone as a painter seems to suggest that they have some essential characteristic that makes them a painter above and beyond the action of painting.

In several experiments, Noyes and Keil gave people examples of scientific concepts and asked them several questions about the degree to which they believed that category had an essence. The questions focused on whether the category had boundaries, was objective (versus subjective), could be discovered by experts, and whether knowing something was a member of the category was meaningful. These questions relate to how strongly people believe the particular category has an underlying essence.

When people were asked whether they would accept an expert’s definition for a new scientific object, they generally agreed to do so. However, when people were told that an object was having its classification changed by experts, people were less willing to accept the change. The more that they felt the object had an essence, the more strongly they resisted the change.

Of course, people don’t always resist changes in categories. For example, some categories are created strictly by rules. In baseball, the strike zone is defined by the rules of the game. Indeed, when I was young, the strike zone consisted of the area over home plate at a height from the batter’s knees to the batter’s shoulders. Now, the strike zone goes only from the knees to the middle of the batter’s torso. People accept this change, because it is not a natural kind. It is defined as part of the game of baseball.

In addition, people are willing to accept a change in categories when they find out that the original classification was based on fraud. In one final study in the paper, participants were told that an initial classification of an item was based on results of studies that were fraudulently reported. In this case, they accepted new definitions by experts.

These results demonstrate what happens when the rules for doing science conflict with the underlying psychology of people’s concepts. Even people who are prone to accept the authority of scientists for certain kinds of concepts can have difficulty changing their beliefs about how things are classified. This does not come from a mistrust of science, but rather from the difficulty changing beliefs about what lies at the essence of something that used to belong to a particular category.

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