The primary objective of this chapter will be to discuss the literature on third or additional languages that examines the question of whether the prior knowledge of two or more languages, as well as the learning experience gained in acquiring these languages, have a significant impact on cognitive development and on the language acquisition process.

It is a basic principle of human cognition that learners rely on whatever they perceive as relevant to the task at hand. In a discussion on relevance theory, Wilson and Sperber (2006: 612) explain that 'the human cognitive system has developed in such a way that our perceptual mechanisms tend automatically to pick out potentially relevant stimuli, our memory retrieval mechanisms tend automatically to activate potentially relevant assumptions, and our inferential mechanisms tend spontaneously to process them in the most productive way.' During the language acquisition process, the most relevant piece of information for the learning task is likely to be the knowledge of other languages already in the mind as well as the experience gained in acquiring such knowledge (see also Pennington, 1999; Ringbom, 1986).

Prior language knowledge and prior learning experience were not always regarded in a positive light. If we examine the literature of the past century, we find that scholars maintained very different positions about bilingualism over the course of time, and that trends of thoughts were particularly strong and influential in shaping research directions. A useful chronological dividing can be identified in 1962, when Peal and Lambert published their seminal paper on bilingualism entitled 'The Relation of Bilingualism to Intelligence'. The paper is now known for having set in motion theoretical and methodological changes much needed at that time, and for having marked the beginning of new essential developments in research, as explained in this chapter.