The taxonomic assumption groups objects together based on shared characteristics.

When children are learning the names for new objects, they use different strategies to learn faster. In addition to assuming that the same object name applies to objects that are the same shape (shape bias) and assuming one name per object (mutual exclusivity), young children also assume that new words apply to objects that belong to the same category (taxonomic assumption). Categories are groups of items that share characteristics such as their function (i.e. a sippy cup and a glass both belong to the category of “cup”), their status as living or non-living (i.e. sparrows and airplanes both fly in the sky, but only sparrows are birds), or even their smell or taste (i.e. fruits are sweet).

Sometimes the taxonomic assumption works with shape bias. Examples of these two methods working together include a toddler sorting dogs (one category of animal, one general shape) from birds (separate category of animal, different general shape).

Sometimes these techniques work against each other because the taxonomic assumption can used to group objects together even if they have a different shape.

For example, imagine a toddler is familiar with footballs, tennis balls, and eggs. If presented with these three objects, what do you think would happen if a researcher pointed at the football, called it a “dax,” and told the toddler to “find the other dax?”

If they were only using shape bias, we would expect young children to group the football with the egg because they’re both rounded in the middle and smaller on the ends.

In this situation, children are more likely to group the football and tennis ball together. Why?

Researchers think this behavior is because children naturally think that the new label, “dax,” applies to a category of objects, and since footballs and tennis balls both belong to the category of ball, it is likely that the “other dax” is the tennis ball, rather than the egg.This is an example of when the taxonomic assumption overrides shape bias.

The taxonomic assumption puts limits around what a word is most likely to refer to. It would not be helpful if, upon hearing the word “dog” in the presence of the family dog, a child assumed that everything that the fuzzy creature touched—his bone, his water bowl, his ball—were also called “dog.” However, very young children assume that words refer to categories, which gives them a head start on being able to learn the meaning of words.