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To organize the world and efficiently use cognitive resources, children learn to place objects, people, and locations into categories.

In a broad sense, a category—like fruits—is a mental representation of a collection of real things that share similar features, such as the presence of seeds.

In this instance, the characteristic of having seeds is a "hidden" or internal feature—one not easily observed—that can be used to link items to form a category.

However, sometimes the feature—or features—on which a category is based can be readily perceived. For example, if a child sees an animal that has four legs, a tail, and barks, they’ll likely categorize it as a dog.

Importantly, children can take what they know about a single item belonging to a category, like their pet dog, and use that information to identify and make inductive inferences—educated guesses—about unfamiliar category members.

For example, if a child knows that their dog wags its tail when it’s happy, then they can guess that a new tail-wagging dog is also happy.

Based on the pioneering work of Frank Keil, this video explores how to investigate the criteria 5- to 9-year-olds use to categorize objects by designing stimuli that cross categorical boundaries, and explains how to perform such a transformational study and interpret categorization data. We will also look at how psychologists use this technique in other applications, for example to explore how children make inferences.

In this experiment, researchers present 5-, 7-, and 9-year-old children with 16 stories in which items are physically altered so that they resemble something else.

Eight of these stories center around living animals or naturally occurring objects, like zebras or diamonds—collectively called natural kinds—and eight deal with man-made, non-living items referred to as artifacts, such as cups or pots.

For instance, in a natural kind-focused transformation story, children are first shown a "before" image of a naturally occurring item or animal, such as a raccoon.

They’re then told that doctors surgically transformed this thing, for example by attaching containers of foul-smelling liquid to the raccoon, and shaving and dying portions of its fur so that it becomes black and white with a puffy tail.

Importantly, the researcher emphasizes that such operations result in the item being transformed so that it represents a different object of the same type: a living natural kind is made to look like another living natural kind.

Children are finally presented with an "after" image of the subject, and asked how they’d categorize it post-surgery—if it’s a raccoon or a skunk.

Their answers are rated on a scale of 1 to 3. In this system, if children shift their categorization—if they assert that the shaved raccoon is now a skunk, as it resembles this animal in appearance and smell—they’re given a score of 1.

In contrast, if children are unsure of how the transformed thing should be categorized—if it’s some “ra-kunk” hybrid of the two—they’re given a score of 2.

Finally, if children don’t change the initial categorization of the item—saying that the animal remains a raccoon, despite its altered physical state—they’re assigned a score of 3.

Here, the dependent variables are the average scores children receive across natural kind and artifact stories, which can be used to gauge children’s tendency to resist changing their identification of transformed objects.

Based on research performed by Frank Keil and colleagues, it is expected that children will readily change their categorization of inanimate artifacts, but not of living natural kinds, presumably due to some internal, unchangeable feature that animals possess.

Before the experiment begins, create cards with images of pre- and post-transformation items. Design 16 such transformation vignettes—eight dealing with artifacts, and eight dealing with natural kinds.

When the child arrives, greet them and introduce them to the concepts of doctors and surgeries.

Present the 16 transformation vignettes to them in a random order: “The doctors took a coffeepot that looked like this. They sawed off the handle, sealed the top, took off the top knob, sealed the spout, and sawed it off. They also sawed off the base and attached a flat piece of metal. They attached a little stick, cut a window in it, and filled the metal container with bird food. When they were done, it looked like this.”

“After the operation, was this a coffeepot or a birdfeeder?”

If in any instance the child gives an ambiguous or hybrid response—like saying that a coffeepot transformed into a birdfeeder is now a “coffee-feeder”—also ask them to call it by either its pre- or post-transformation name. This encourages children to think about their answers, as well as warrants different responses across all of the vignettes.

Question the child in a free-form manner about the reasons behind their response, in order to determine the features or principles of the post-transformation item that led them to their conclusion.

For each vignette, record the child’s response for future analysis.

When the child has completed all 16 vignettes, have two independent raters read their responses and code them on a scale of 1–3.

To analyze the data for each of the two types of transformations—either natural kinds or artifacts—plot the average children’s score as a function of age.

Perform an analysis of variance to determine if there are differences between the three age groups or two types of transformations.

Notice that as age increases, the average scores of children for the artifact transformations remained relatively constant, hovering around 1.25. This indicates that children at all ages studied readily change their categorization of altered artifacts.

In addition, these results suggest that the perceptual features of post-transformation items in these instances—what a child sees—informs their categorical placement. In contrast, for natural kind transformations—especially those involving animals—children’s average scores increased as a function of age. This indicates that as children get older, they gradually represent category membership as an internal, unchangeable aspect of animals.

Now that you know how researchers are using transformation scenarios to better understand how children categorize general artifacts and natural kinds, let’s look at other applications of this technique.

As you’ve learned, natural kinds can be either living items—like plants or animals—or inanimate objects, like minerals.

Thus, some researchers are further looking into how children come to categorize these different types of natural kinds, and if any differences exist between these processes.

Such work has demonstrated that whereas children quickly learn to categorize animate natural kinds—and resist the tendency to change their categorization of animals post-transformation—a similar ability for inanimate natural kinds develops more slowly.

Some psychologists have suggested that parental influence may play a role in this phenomenon. For example, a parent may emphasize that an animal should be categorized as a fish if its parents were fish, and if it lays eggs that also yield fish.

However, given the advanced subject matter of what makes an inanimate natural kind—like the organization of electrons in its atoms, or the elements that constitute it—parents may not have similar discussions with their children about such items.

Other researchers are looking at how children apply facts about a single member of a category to make inferences about other items in the same category.

For example, psychologists could tell children that a mouse is more active at night and likes to eat cheese, and then show them a collection of realistic pictures composed of mice with different coats, other rodents, and unrelated animals—like a cow.

By either including or removing labels from these images, researchers can evaluate the extent to which textual category labels, in addition to morphological features, factor in children’s ability to infer that other mice have the same characteristics.

You’ve just watched JoVE’s video on how children develop the ability to categorize—and make inferences about—natural kinds and artifacts. By now, you should know how to design and use transformation stories to investigate this phenomenon, and collect and interpret results. You should also have an understanding of how psychologists are using this method to investigate other aspects of the categorization process.

Thanks for watching!