Why do adopted children usually learn the language of their adoptive environment?

Adopted children learn the language of their adoptive environment because language is acquired through exposure and learning rather than physiological inheritance. While all typical humans are born with an innate faculty and drive to acquire language, no one inherits the ability to speak a particular language.1

Instead, children are biologically predisposed to master the language—or languages—to which they are routinely exposed from infancy. This process is driven by the child's interaction with their linguistic environment, where they internalize the vocabulary, phonological patterns, and grammatical structures of the speech they hear around them. Because this acquisition is based on environmental input rather than biological ancestry, adopted infants will naturally acquire the language of their adoptive parents, regardless of their own physical characteristics or the language spoken by their biological parents.12