Researchers have shown that babies will remember the language they hear around the time of their birth, even if they're later raised to speak something different.
The new study suggests language learning can be subconsciously locked away in the first six months after birth, making it easier to relearn a birth language later on in life, especially in terms of its pronunciation.
The team behind the research says it's good news for those adopted internationally who want to reconnect with their mother tongue.
"Even in the very early months of life, useful language knowledge is laid down," said one of the researchers, Mirjam Broersma, from Radboud University in the Netherlands.
"What has been retained about the birth language is abstract knowledge about what patterns are possible, not, for instance, words."
While previous studies have shown that babies can pick up language patterns in the womb and at the very start of their lives, the aim of this new research was to figure out how much of that learning could be retained if they didn't continue speaking the language.
To figure this out, researchers enlisted 29 Korean-born Dutch speakers and 29 native Dutch speakers and asked them to identify and reproduce three Korean consonants over a two-week period of training. The Korean-born Dutch speakers no longer had any conscious knowledge of how to speak Korean.
The sounds were chosen as being especially unlike anything in the Dutch language, and the volunteers' efforts were then rated by native Korean speakers.
The participants who were born in Korea showed significantly more improvement in their pronunciation over the two weeks compared to those who were born in the Netherlands. They were also better able to pronounce the Korean sounds to begin with.
In other words, something appears to have stuck from their very earliest months as babies, even though these participants were then brought up in the Netherlands, only speaking Dutch.
"One of the most interesting findings was that no difference showed in the learning results of those Korean-born participants adopted under six months of age and those adopted after the age of seventeen months," says Broersma, suggesting that we get plenty of learning done during that first half a year.
The way young infants pick up and learn languages is fascinating, but it's also difficult to study – getting a panel of babies together to talk about their experiences isn't really an option.
However, we've seen plenty of recent research showing that even in the earliest months of our lives, we're developing important language skills, and the new study backs up research from 2015 looking at how our very first language stays with us.
In that research, the brain activity of volunteers showed traces of recognising a first language, even if it wasn't practiced any more – though in this case the focus was on switching languages after the first three years of life rather than the first six months.
So if you've long since lost the use of your first language, why not give it another try? You might be able to pick it up faster than you think.
The findings have been published in Royal Society Open Science.