1. Almost all humans acquire at least one language before the age of five. Although people gain vocabulary throughout their lifetimes, even young children are able to understand and produce complex sentences with complicated meanings. How do children accomplish this remarkable feat in such a short amount of time? Do adults learn language differently from children? Linguistic researchers have long debated the answers to these questions, but most agree that both nature and nurture are involved in language acquisition. They disagree, however, about how much linguistic knowledge children have from birth—and thus whether genetics or experience is more important in language acquisition. 
2.  For many linguists, biological factors are the most important in language learning. Some argue that some linguistic knowledge must exist in our brains from birth because children cannot possibly encounter every feature of their language before the age of five. This argument, called the poverty of the stimulus, contends that children must have an innate language ‘device’ to make up for their limited exposure. Indeed, these linguists point out that nearly all children can produce the same kinds of complex sentence structures by the age of five, even without having heard them before.
3.  Many researchers have theorised what this innate linguistic knowledge must look like. One popular theory is universal grammar. This theory posits that all languages have the same basic structural foundation. That foundation is the innate knowledge universal to all humans. While children are not genetically predisposed to speak a particular language, a universal grammar gives them certain linguistic information as a starting point, which allows them to readily acquire the rules and patterns of whichever language they are exposed to.
4. Not all linguists, however, believe that an inborn capacity for language is the most important factor in language acquisition. These researchers place greater emphasis on the influence of usage and experience. They agree that there is an innate component that helps children acquire language; however, they argue that this component is not specific to language. Instead, it is part of humans’ general ability to perceive and organise patterns in the world. These linguists argue that the poverty of the stimulus is a myth: children are exposed to a wealth of linguistic structures over the course of five years. From this input, children act in many ways like mini-statisticians. They gather data and determine language patterns and structures from what they have observed. They also hone in on a particular way of saying things because they have been exposed to that structure more than any other. 
5. Linguists on both sides of the debate are still working to explain the different language learning capabilities of adults and children. Early childhood seems to be a critical period for mastering certain aspects of language, such as proper pronunciation—just ask a native English speaker struggling to roll Rs in secondary school Spanish. Children also tend to have a heightened ability, compared to adults, to learn second languages—especially in natural settings. While adults frequently learn to speak new languages proficiently and may even have some advantages when studying in a formal classroom, they usually do not learn as quickly and easily as children. Indeed, few adult language learners would be mistaken for native speakers of their non-native tongue. Are these varying capabilities a result of differences in how adults and children are exposed to a new language? Are they the result of biological changes that occur at the onset of puberty, or do both biology and experience come into play? 
6. While our understanding of language acquisition is incomplete, this pursuit is well worth the effort. ‘We still don’t understand how a child learns its first language, why some children have language disorders or how children and adults learn a second language,’ explains Professor Joan Maling, ‘and we still don’t understand what happens when a stroke or a disease such as Alzheimer’s seems to wipe out a person’s knowledge of language.’ Unravelling the process of language acquisition promises not only to help scientists answer these questions and countless others, but also to explain fundamental features of learning and the human brain.

Adapted from the US National Science Foundation, ‘Language and Linguistics: Language Learning’