as a result, the child will find this outcome rewarding, thus enhancing the child's language development. UG reemerged to prominence and influence in modern linguistics with the theories of Chomsky and Montague in the 1950s–1970s, as part of the "linguistics wars". In 2016 Chomsky and Berwick co-wrote their book titled Why Only Us, where they defined both the minimalist program and the strong minimalist thesis and its implications to update their approach to UG theory. According to Berwick and Chomsky, the strong minimalist thesis states that "The optimal situation would be that UG reduces to the simplest computational principles which operate in accord with conditions of computational efficiency. This conjecture is ... called the Strong Minimalist Thesis (SMT)." The significance of SMT is to significantly shift the previous emphasis on universal grammars to the concept which Chomsky and Berwick now call "merge". "Merge" is defined in their 2016 book when they state "Every computational system has embedded within it somewhere an operation that applies to two objects X and Y already formed, and constructs from them a new object Z. Call this operation Merge." SMT dictates that "Merge will be as simple as possible: it will not modify X or Y or impose any arrangement on them; in particular, it will leave them unordered, an important fact... Merge is therefore just set formation: Merge of X and Y yields the set {X, Y}." Chomsky's theory Chomsky argued that the human brain contains a limited set of constraints for organizing language. This implies in turn that all languages have a common structural basis: the set of rules known as "universal grammar". Speakers proficient in a language know which expressions are acceptable in their language and which are unacceptable. The key puzzle is how speakers come to know these restrictions of their language, since expressions that violate those restrictions are not present in the input, indicated as such. Chomsky argued that this poverty of stimulus means that Skinner's behaviourist perspective cannot explain language acquisition. The absence of negative evidence—evidence that an expression is part of a class of ungrammatical sentences in a given language—is the core of his argument. For example, in English, an interrogative pronoun like what cannot be related to a predicate within a relative clause: *"What did John meet a man who sold?" Such expressions are not available to language learners: they are, by hypothesis, ungrammatical. Speakers of the