Critical period for first language: the crucial role of language input during the first year of life

•A critical period for first language is different from that of a second language.•Different critical periods with different time frames for different language domains.•The critical period for syntax ends at around age one year.•Congenitally deaf children who do not receive input in the first year develop syntactic impairment.•Children with thiamine deficiency during 1st year also develop syntactic impairment.

The critical period for language acquisition is often explored in the context of second language acquisition. We focus on a crucially different notion of critical period for language, with a crucially different time scale: that of a critical period for first language acquisition. We approach this question by examining the language outcomes of children who missed their critical period for acquiring a first language: children who did not receive the required language input because they grew in isolation or due to hearing impairment and children whose brain has not developed normally because of thiamine deficiency. We find that the acquisition of syntax in a first language has a critical period that ends during the first year of life, and children who missed this window of opportunity later show severe syntactic impairments.