In linguistics, the theory of universal grammar holds that there are certain fundamental grammatical ideas which all humans possess, without having to learn them. Universal grammar acts as a way to explain how language acquisition works in humans, by showing the most basic rules that all languages have to follow.

The basic idea of universal grammar, that there are foundational rules in common among all humans, has been around since the 13th century. In the following centuries this idea led many philosophers to try to design a perfect language from the ground up, taking into account what they felt were the core principles of all languages.

The most famous theory of the idea of a universal grammar was put forth by the linguist Noam Chomsky in the 1950s. Chomsky held that there was a universal grammar hardwired into the brain of all humans, and that all human languages had evolved on top of that universal grammar, and that children learned their native languages using the universal grammar as a support structure.

One of the main impetuses for the development of a modern theory of universal grammar is the question of how early language learners know that certain phrases are ungrammatical. Children acquire language by listening to native speakers around them. But, by virtue of being proficient speakers, native speakers don’t go around saying everything that is ungrammatical and saying it’s wrong. This is often called the Poverty of Stimulus argument, and universal grammar attempts to explain it by saying that a number of these restrictions are part of a universal grammar.

Universal grammar does not attempt to lay out many blanket statements that hold true for every single language on Earth. If it did that, after all, we would expect most languages to be roughly the same. Instead, we find an incredible range of languages. Instead, what a universal grammar seeks to do is to lay out propositions of the form, “If X is true, then Y will be true.” These structures lay out how all languages develop when faced with certain basic principles. Using these structures, students of universal grammar can attempt to state what word order a language might choose, what phonemes will be present, and other foundational traits of the language.

Another argument commonly leveled against universal grammar is that the theory itself is not actually falsifiable. Although it claims to be able to predict what new languages will be like, the sample size is small enough that when new languages are discovered the rules laid out must sometimes adapt to fit the new data. This would seem to undermine its validity as a strong predictive theory, making it more a cohesive set of observations about what we already know to be true.