The core assumption of this theory is that the innate propensity, which is represented by LAD or Language Acquisition Device, is activated when the child hears a language. The LAD consists of Language Universals or the rules or principles common to all human languages. Whenever the child hears a language, he/she tries her best to match them with the internal rules. The rules are capable of forming hypotheses about the rules of the child’s mother tongue. By gradual checking and re-checking of the hypotheses, the child is able to rectify or enlarge his/her language data. The innatists believe that the child alone can learn a language. He/she does not need feedback from others. It considers language learning as a creative activity.
Innatism is by far the most accepted & modern theory of L1 acquisition after behaviourism. The theory however, lacks sufficient scientific evidence since the idea of innate propensity is an abstract quality. It is not possible to prove its core assumption as strongly as behaviourism. Above all the theory is not as effective in the practical learning/teaching situations as it was supposed to be. It is believed that the child can learn correct language even if he/she is exposed to wrong language structure. But in reality it is not possible; researches have proven that the child tends to learn language more similar like the one he/she is exposed to.