Learning a new language puts extra cognitive strain on children. This strain affects children differently than would an extra math course in school. According to Psychology Professor Erika Hoff, author of the book “Language Development,” learning multiple languages simultaneously limits the number of words that a child can learn in a set amount of time. For example, toddlers have the cognitive capacity to learn approximately 20 new words a month, but this number is for total words. When a toddler’s language input comes in the form of two languages, she might only learn 10 words in her native language a month, learning the other 10 words in her foreign language, which puts her behind in her native language.