The parameter-setting theory proposes that universal grammar provides a set of binary parameters (switches) that can be set to produce different languages. The pro-drop parameter, head-directionality parameter, and null-object parameter exemplify this approach. Parameters explain how children acquire language rapidly—they need only determine the parameter settings for their language rather than learning grammar from scratch.
Identify parameters in the literature and explain cross-linguistic variation as different parameter settings. Show how a single parameter accounts for correlated properties across a language.
From the Universal Grammar hypothesis you know that Chomsky proposed that humans are born with an innate language faculty — a set of grammatical principles common to all languages. From the Minimalist Program you know that this faculty works through operations like Merge, that structure is built compositionally, and that the linguistic system aims for computational efficiency. But if UG provides the same principles to every child, how do children end up speaking languages as different as Japanese, Arabic, and English? Parameter theory is the answer: UG provides fixed principles and a finite set of variable parameters whose values are set by exposure to the child's native language.
Think of it like a light-switch panel. Every child is born with the same panel — the same set of switches — but the switches start in an unset position. As the child hears their native language, the input drives each switch to its correct position for that language. The pro-drop parameter is the clearest example. In Italian or Spanish, the subject of a sentence can be omitted: *Parla* means "He/she speaks" without an explicit subject pronoun. In English, omitting the subject is ungrammatical: *Speaks* is not a well-formed sentence. Parameter theory says Italian and English share the same underlying grammatical structure (UG's principles), but one switch — the pro-drop switch — is set to [+pro-drop] in Italian and [-pro-drop] in English. A child acquiring Italian hears sentences without overt subjects and sets the switch accordingly; a child acquiring English does not hear such sentences and leaves the switch off.
What makes parameter theory more than a convenient description is the claim that parameters cluster properties together — setting one parameter correctly should trigger correct knowledge of other grammatical properties even without direct exposure to them. This is sometimes called the clustering effect or parameter clustering. The pro-drop parameter, for instance, was originally argued to correlate with rich agreement morphology (languages with obvious subject-marking morphology allow pro-drop), inversion of subjects and verbs, and certain properties of embedded sentences. If a child hears enough subject-drop sentences to set the parameter, she should automatically produce the correct behavior on all correlated properties — explaining why children acquire grammar so rapidly and systematically despite the poverty of the stimulus.
The Minimalist Program refined parameter theory by asking where parameters are located in the grammar. Earlier versions placed parameters on specific grammatical rules; minimalism pushes them into the lexicon — specifically, into the features of functional heads (morphemes like tense markers, agreement markers, and determiners). The head-directionality parameter, for example, determines whether a language is head-initial (the head of a phrase comes first, as in English verb phrases: *eat apples*) or head-final (the head comes last, as in Japanese: *ringo-o tabe-ru*, literally "apple-eat"). In minimalist terms, this is a feature of functional heads that determines how they combine with their complements via Merge. This localization of parameters in functional morphology makes the theory more precise and more testable — and also more vulnerable to counterexamples, since the inventory of parameters and their precise formulations remain actively debated in the field.