I’m sitting in my kitchen in London, trying to figure out a text message from my brother. He lives in our home country of Germany. We speak German to each other, a language that’s rich in quirky words, but I’ve never heard this one before: fremdschämen. ‘Stranger-ashamed’?

I’m too proud to ask him what it means. I know that eventually, I’ll get it. Still, it’s slightly painful to realise that after years of living abroad, my mother tongue can sometimes feel foreign.

Most long-term migrants know what it’s like to be a slightly rusty native speaker. The process seems obvious: the longer you are away, the more your language suffers. But it’s not quite so straightforward.

In fact, the science of why, when and how we lose our own language is complex and often counter-intuitive. It turns out that how long you’ve been away doesn’t always matter. Socialising with other native speakers abroad can worsen your own native skills. And emotional factors like trauma can be the biggest factor of all.

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It’s also not just long-term migrants who are affected, but to some extent anyone who picks up a second language.

“The minute you start learning another language, the two systems start to compete with each other,” says Monika Schmid, a linguist at the University of Essex.

Schmid is a leading researcher of language attrition, a growing field of research that looks at what makes us lose our mother tongue. In children, the phenomenon is somewhat easier to explain since their brains are generally more flexible and adaptable. Until the age of about 12, a person’s language skills are relatively vulnerable to change. Studies on international adoptees have found that even nine-year-olds can almost completely forget their first language when they are removed from their country of birth.