Theories of language development fall into one of two camps: empiricist or nativist. Empiricists believe language is a learnt behaviour. Nativists, on the other hand, believe we are born with some innate language ability. Empirical researchers focus on learning theories to understand how children acquire language skills, while nativists look for biological components responsible for the universal rules underlying all of the languages spoken by people.
For Lev Vygotsky---a Russian psychology researcher who began developing his empirical theories of cognitive development after the Russian Revolution in the early 20th century---children learn by solving problems with the help of other people, such as parents and siblings. Language develops as a tool for helping them solve problems more effectively. They learn the skill by practicing or modelling the language behaviours they hear being used around them. In his theory, language development is closely tied to social behaviour, putting him in the empiricist camp.
B. F. Skinner, an American psychologist best known for his work in behaviourism, proposed behaviourism as the basis for language development in a book published in 1957. The core of behaviourism is learning through reinforcement. The reinforcement takes different forms. For example, if a parent says to the child, "Can you say mummy?" and the child responds accordingly, the parent provides positive reinforcement. If the child uses language to make demands, such as asking for a biscuit, and the demand is granted, the child receives positive reinforcement for using language. This approach places Skinner in the empiricist camp of language development.
According to empiricist Jean Piaget, a Swiss psychologist known for studying how knowledge develops in children and in adults during the first half of the 20th century, language development is connected to a child's cognitive development. As the child moves through the different stages of cognitive development---sensorimotor, pre-operational, concrete operational and formal operational---his language skills change, too. For example, during the pre-operational stage, children can grasp the existence of things even when they cannot see those things. Likewise, they can use language to think about those non-present things.
Noam Chomsky, an American linguist and cognitive scientist, believes children are born with innate knowledge of the rules governing language. This makes him a nativist. His research during the late 20th century also suggests that the rules are universal among the known human languages. For example, Japanese and English seem very different, but both languages include verbs and in both languages verbs take an object. The difference is where the object of the verb is placed in the sentence. According to Chomsky, the reason children learn language so quickly is because they already know its rules.
Jerome Bruner, a nativist and American cognitive psychologist, believed language development comes easier to most children because of a combination of innate biological "endowments" and social encouragement. Bruner's research on the subject began in the 1960s. Bruner notes that even children who cannot distinguish between their thoughts and things attempt to use language, suggesting they are born with an inclination towards communication. The role of encouragement is to provide necessary support as the child develops linguistically.