According to Kenji Hakuta, Professor of Education at Stanford University, comparing monolingual and bilingual groups of the same age in terms of intelligence level, social status, cognitive flexibility, or any other parameter, those of the bilingual ones are always better. 

We can confidently confirm that the world-renowned immersion method is 100% effective! Since 2004, Saulės Gojus has been successfully running bilingual English-Lithuanian (and German-Lithuanian up until 2020) language groups, where children of Lithuanian families learn English naturally and without stress, and children of non-native speakers learn Lithuanian. Each group has two teachers, one of whom communicates only in Lithuanian and the other only in English, on a one-person-one-language basis: through daily teacher interaction, games, songs, stories and hands-on activities, the children are smoothly introduced to the English language. Neuroscientific research shows that this early second language learning mirrors the acquisition of the mother tongue, so that by the time the child leaves kindergarten, he or she has already acquired a basic toolbox of English skills that will allow him or her to continue to build up these skills in primary school.