New evidence suggests that the earliest traces of a language can stay with us into adulthood, even if we no longer speak or understand the language itself. And early exposure also seems to speed the process of relearning it later in life.

In the new study, recently published in Royal Society Open Science, Dutch adults were trained to listen for sound contrasts in Korean. Some participants reported no prior exposure to the language; others were born in Korea and adopted by Dutch families before the age of six. All participants said they could not speak Korean, but the adoptees from Korea were better at distinguishing between the contrasts and more accurate in pronouncing Korean sounds.

“Language learning can be retained subconsciously, even if conscious memories of the language do not exist,” says Jiyoun Choi, postdoctoral fellow at Hanyang University in Seoul and lead author of the study. And it appears that just a brief period of early exposure benefits learning efforts later; when Choi and her collaborators compared the results of people adopted before they were six months old with results of others adopted after 17 months, there were no differences in their hearing or speaking abilities.

“It's exciting that these effects are seen even among adults who were exposed to Korean only up to six months of age—an age before which babbling emerges,” says Janet Werker, a professor of psychology at the University of British Columbia, who was not involved with the research. Remarkably, what we learn before we can even speak stays with us for decades.