An attempt is made in the present paper to resolve inconsistencies between the results of recent studies which have reported that bilingualism is associated with positive cognitive consequences and earlier studies which suggested that bilingualism might adversely affect cognitive and scholastic progress. Because recent studies involved balanced bilinguals and were carried out in "additive" bilingual settings, the bilingual subjects in these studies are likely to have attained a high level of competence in the second language (L2) at no cost to their level of competence in the first language (L1). However, earlier studies tended to involve bilingual subjects from language minority groups whose L1 was gradually being replaced by their L2. Thus, it is not surprising that many of these earlier studies produced evidence of a "balance effect," i.e., that a bilingual paid for his L2 competence by a lowering of his L1 competence. On the basis of the differences in linguistic competence attained by the bilingual subjects in earlier and more recent studies it is hypothesized that the level of linguistic competence attained by a bilingual child may mediate the effects of his bilingual learning experiences on cognitive growth. Specifically, there may be a threshold level of linguistic competence which a bilingual child must attain both in order to avoid cognitive deficits and allow the potentially beneficial aspects of becoming bilingual to influence his cognitive functioning. (Author)