The limited research on bilingualism with which I am familiar, from the standpoint of a psychologist who teaches human development, suggests that there are no negative consequences for children who speak two languages. The research deals mainly with kids whose parents are of one language at home and their school teaches English. Some people fear that teaching children another language at home may interfere with their learning English in schools. I know people with such fears. No research that I am aware of supports those fears. You might have a look at the following articles:
Brislin, R. (1993). Culture's influence on behavior. Fort Worth, TX: Harcourt Brace Jovanovich.
Diaz, R. M. (1983). "Thought and two languages: The impact of bilingualism on cognitive development." Review of Research In Education 10: 23-54.
Fillmore, L. W. (1989). "Teachability and second language acquisition." In M.L. Rice & R. L. Schiefelbusch (Eds.), The teachability of language. Baltimore: Paul H. Brooks.
Hakuta, K., & Garcia, E. E. (1989). "Bilingualism and education." American Psychologist 44:374-379.
On January 14, 1998, Professor Rider added the following:
The most important thing to consider is that it is the quality of bilingual education that counts most in facilitating cognitive development: Quality of each of the two or more languages that the child learns. Children who learn a second language, or who grow up with two different languages, where one language does not replace the other but supplements the other show the greatest benefits. One of Diaz' conclusions is that higher degrees of bilingualism (i.e., the more competent in each language the child becomes) are associated with greater cognitive flexibility and improved concept formation. Why this is so remains speculative, but it probably is related to the fact that, with two distinct languages in a child's repertoire, the child has different frames of reference for concepts; different ways of looking at things in the world provided by the different languages.
A recent study by Kimbrough Oller (1995: Early speech and word learning in bilingual and monolingual children: Advantages of early bilingualism; a paper presented at the meeting of the American Association for the Advancement of Science, Atlanta) compared groups of children who grew up with both Spanish and English versus children who spoke only English. The bilingual group actually spoke better English than the English-only group. These results apply only to children who learn Spanish and English simultaneously at a young age.
In contrast, another a recent study by Grace Yeni-Konshian (1995: What happens to our first language when we learn a second language? Paper presented at the meeting of the American Association for the Advancement of Science, Atlanta) looked at children who moved to the United States from Korea between the ages of 2 and 24 years. For children who began speaking English between the ages of 6 and 8 years, they were proficient at neither Korean nor English. So the message is, early bilingual education is good, later education is not so good.
I am not bilingual. But friends of mine who are (mostly friend who speak English and a Native American language, although I know a couple of English/Spanish speakers, too) proficient at more than one language tell me that moving in and out of two or more languages is like moving in and out of two different worlds. Speaking one language amounts to thinking in one way; speaking another language means thinking in another way. Cognitive skills are measured almost exclusively in terms of verbal functioning. And that is probably why children proficient in multiple languages show cognitive benefits, while children NOT proficient in multiple languages (but only marginally competent) do not show cognitive benefits.
From a Native American bilingual speaker:
I am a Chippewa-Cree who learned to first speak Cree (thank God) and later learned to speak English just before I went into the first grade which was difficult because I didn't know what was being said. I took my Dick and Jane book home and with sheer determination and resolution I learned to read and speak English. Thus by the end of my first year in school I learned enough to pass into the second grade. I am now a middle-aged women aging gracefully and I am an avid reader of all subjects and authors--I mean I devour them. I have a keen interest in writing which I intend to accomplish in due time. Now, on to the real reason why I'm responding. I read the article [above] on the scientific studies on the subject. And being a dual speaking individual, I feel I can say without a doubt that speaking two languages has its advantages particularly in the areas of cognition and concept. I can mentally translate English into Cree using a lot of visual imagery which is like seeing what is being said literally inside your head. I sometimes find that I have to do this in order to fully grasp what is really being said. I do find that the Cree language is more precise and vivid; whereas, on the other hand, I find English can be very vague (sometimes I think it's deliberate). Another advantage is that I feel a real sense of communion when I speak Cree to another individual who understands it; in other words, we are talking!