Critical period for first language acquisition may be shorter in autistic children than in typically developing children

The goal of this study was to differentiate between two hypotheses regarding syntactic-language comprehension deficits in autistic adults. One hypothesis suggests a persistent, age-independent barrier, such as sound hypersensitivity or social avoidance, which may hinder acquisition of syntax throughout life. Another hypothesis proposes an age-dependent factor, such as a shortened critical period for language acquisition. These hypotheses predict distinct trajectories for language learning-rates. The first hypothesis expects autistic individuals to consistently exhibit a slower learning-rate compared to neurotypical individuals across all ages. In contrast, the second hypothesis predicts that autistic individuals will initially acquire language at a rate comparable to their neurotypical peers but will experience an earlier decline in learning-rate. To test these predictions, we analyzed language learning-rates in 15,183 autistic and 138 neurotypical individuals, 2 to 22 years-of-age. At age 2, both groups showed comparable learning-rates. In neurotypical individuals, this rate remained stable from ages 2 to 7. However, in autistic individuals, the learning-rate began to decline exponentially starting as early as 2.3 years, with an earlier onset of decline observed in those with more severe autism. These findings strongly support the second hypothesis, indicating that language deficits in autism may be caused by a shortened critical period.