Commonalities, differences, and differences that matter between monolingual and bilingual development.

This commentary makes the argument that the child-internal and child-external sources of individual differences in bilingual development are much the same as the sources of individual differences in monolingual development. It makes the further argument that the operation of the child-external influences results in differences between monolingual and bilingual development in the rate and sometimes in the outcome of language acquisition. An argument is made for the scientific and practical value of understanding the differences between monolingual and bilingual development, and future directions for research are suggested.