Final answer:
Chomsky's theory of universal grammar is based on the natural ability of children to learn language effortlessly across cultures and the existence of a critical period for learning language, supporting the idea of an innate biological predisposition for language acquisition.
Step-by-step explanation:
Noam Chomsky's theory that all human brains are equipped with a universal grammar is based on evidence from the ease and consistency with which children learn language across various cultures and the existence of a critical period for language acquisition. His critiques against the behaviorist approach to language learning, such as B. F. Skinner's theory of reinforcement, led to his proposition that a Language Acquisition Device (LAD) is an innate biological mechanism that enables children to rapidly learn and master language. This innate capacity, coupled with the observation that all humans are potentially capable of learning any language from birth, suggests that language learning is not solely based on environmental factors but is also a biological predisposition.
Furthermore, the similar developmental patterns of language acquisition in children from vastly different cultural and linguistic backgrounds provide compelling evidence supporting the existence of a universal grammar. The biological features of the human brain that facilitate language comprehension and production, even without formal instruction, also underline the theory of an inherent language faculty. Chomsky's ideas have greatly influenced our understanding of language development and cognitive science.