According to a recent small study, learning a 3rd or a 4th foreign language is easier for multilingual people as they have trained their brains to learn languages.
The study compared the brain activity of bilingual and multilingual people in attempt to understand a new foreign language. It turned out that the pattern of brain activation for multilinguals was similar to that for bilinguals, but the brain activation was much more sensitive and much faster.
Researchers explain that bilinguals have only 2 points of reference and approach a new foreign language as a separate idea to learn from scratch. Multilinguals, on the other hand, use their knowledge of 3 or more languages to make generalizations and to build on prior knowledgein in their brains an accumulated way. That is why multilinguals who are proficient in their 3rd language, are able to learn another foreign language faster and more efficient compared to bilinguals.
Curious? HERE is the source