Immigrants study

Study: Adult children of immigrants often lose touch with their native language
Even when children of immigrants grow up speaking their parents' language, by the time they reach adulthood, their preference is to speak English, according to recent research findings. Thirty-four percent of those individuals surveyed who grew up speaking their parents' native language at home reported still being proficient in the language as adults, according to a University of California at Irvine study of 5,700 young adults.
Rumbaut, R. (2007). The Children of Immigrants Longitudinal Study (CILS-III). http://cmd.princeton.edu/cils%20iii.shtml