After one year of scholarship in Spain, you surely master the language of the conquistadors like none around you, but why is everybody picking on you, saying you are boasting (believe me, I personally experienced something like this). In fact, people will be in the situation to find it hard to return to their native language.

This phenomenon is named "first-language attrition" and puzzled researchers for a long time: how was it possible to forget, even momentarily, words you have used fluently all your life.

Psychologist Benjamin Levy and Dr. Michael Anderson at University of Oregon found that this forgetting is not a passive fact, due to the simple disuse of the mother tongue language, but an active process inflicted by the brain itself that impedes us using words of the native language, which would make learning and speaking the new language harder. This forgetfulness is - in fact - an active adaptive strategy to better "catch" the second language.

The researchers used native English speakers who had made at least one year of college level Spanish to answer repeatedly the name of various objects in Spanish.

The more the students were using the Spanish words, the harder they found it to encounter the corresponding English labels for the objects.

In fact, using the foreign language inhibits the corresponding labels in the native language, and appears as "first language attrition". Nevertheless, the more fluent bilingual students were, far less prone to experience the attrition they were. Thus, "first language attrition" is a key factor during the initial stages of second language learning.

When we begin to learn a new language, our brain starts to actively inhibit our easily accessible native language words while trying to imprint in our mind a new idiom.

When bilingualism advances, the attrition turns less necessary, so the subjects in the study were better in shifting between the two languages.

It may look paradoxical, but "first-language attrition provides a striking example of how it can be adaptive to (at least temporarily) forget things one has learned."