Research suggests that assimilative motivation is largely responsible for the development of native-like speech in both first and second languages. Analysis of anecdotal evidence of early bilingual development, studies of dialect acquisition, bilingual immersion programs, and studies of language pidginization illustrating the development of assimilative motivation in children shows that this motivation begins to decrease in adolescence in most speakers. This does not assume that learners must be assimilatively motivated in order to develop a high second language proficiency level, because many learners who are instrumentally motivated develop enough second language facility to satisfy communicative language function requirements. However, without assimilative motivation, these learners are not likely to acquire all the characteristics of native-like speech. If the effects of assimilative motivation are to be understood thoroughly, actual observational studies must be performed on subjects of varying ages placed in diverse second language environments. Only through direct observation of individuals immersed for extended periods of time can the quality and quantity of interaction between second language learners and second language community members be measured. (MSE)