Prof. Renato Corsetti , University of Rome ‘La Sapienza’

Bilingual children possess greater ‘cognitive flexibility’ than their monolingual peers. 
This idea was first proposed by Peal & Lambert (1962), and the literature of recent years has increasingly confirmed it. 
Cognitive flexibility reveals itself in a variety of forms of thinking, from spatial perception through logical-mathematical reasoning to various aspects of language processing, particularly those involving metalinguistic behaviour.  (For a review of the literature, see Reynolds 1991.  See also Baker & Prys-Jones 1998, and Pinto 2002.)
Monolingual and bilingual children therefore differ in their ‘language awareness’: their awareness of certain patterns – parts of speech, word order, endings, inflexion, agreement, and number – and of how these vary between one’s mother tongue and other languages. 
Good language awareness makes language learning significantly easier.  It is precisely for this reason that monolingual children gain so much from learning a second language. One can draw upon the heightened language awareness associated with bilingualism to increase competence in one's first language, and in any other language subsequently learned. 
As Byalistok (1988) has noted, important effects of this metalinguistic awareness are that it can facilitate the earlier acquisition of reading, and that it may relate to higher levels of attainment in a variety of curriculum areas.  Several studies demonstrate just such an effect on mother-tongue competence in children who learn a second language, even if only for one hour a week over a period of one year (Pinto & Corsetti 2001).
Language awareness is an important predictor of third-language acquisition (Jessner 1999), where the connections between two languages already known act as a stepping stone to the third.  To quote Safant Jordà (2005) directly: ‘Considering current research, we are able to assume that bilingual learners will acquire an additional language faster and more efficiently.’
Studies on third-language acquisition are now on the increase, mainly in Europe, where to the learner’s regional language is often added the national language, and later a language for wider communication, such as English (Broeder & Extra 1999).  But three-language situations can be found in other continents too (Rubagumya 1994, Tickoo 1996, Dutcher 1998).
These studies demonstrate that knowledge of a second language always assists the learning of a third.