There was a theory among linguists that the brain’s capacity for language learning—language as a concept, a modality for thought—is finite. Scientists called the period from ages zero to five the “critical window,” within which a child had to gain fluency in at least one language, any language, or risk permanent cognitive damage. Once the window shut, learning anything became difficult, even impossible—without a language, how does one think, or even feel? The critical window remained “theoretical,” mostly because intentionally depriving children of language was deemed by ethicists too cruel an
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