Studies compare the working memory of small children with a range of tests, involving for example the recollection of a set of numbers or the repetition of those numbers backwards. Children who spoke a second language revealed better results than their monolingual counterparts, as well as exhibiting an ability to manage a heavier cognitive load. Similar studies have been performed with adults, yielding similar results.
For those questioning the benefits of bilingualism, evidence such as this reveals the significant effect it has on the fitness of the brain. Working memory is an crucial function for processing information and responding to the world across all areas of life. Keeping this function fit and well-trained is essential, and what better and more engaging way to do it than with a second language?
Yang, E 2017, 'Bilinguals' working memory (WM) advantage and their dual language practice', BrainSci vol. 7, no. 7, p. 86.
2018, 'The advantages of a bilingual brain', BBVA Open Mind, 26 April.