Research studying the acquisition of both spoken and sign language also lends support to this theory. A study examining the acquisition of both spoken and sign language by children of deaf parents indicated that the children acquired both languages in the same manner as children learning two spoken languages. Moreover, the study indicated that bilingual children, including those learning both spoken and sign language, were able to acquire various linguistic concepts at the same rate as monolinguistic children. While bilingual children tend to include mixed language in their speech, children acquiring both spoken and sign language have shown indications of simultaneous language mixing by both speaking and signing certain words together. Though it is only possible to observe this type of language mixing when the child is learning both spoken and sign language, it provides support for the Dual Language System Hypothesis, indicating that the simultaneous operation of separate two language systems could be a cause of language mixing by bilingual children.