Babies may be more language-savvy than scientists thought: A study of people who were adopted as babies suggests that infants younger than 6 months may grasp crucial abstract information about their native tongue. They seem to store this information for years even if they don’t hear their native language in the interim.

In the study, published in the journal Proceedings of the National Academy of Sciences, researchers studied Dutch-speaking adults who had been adopted as babies. They found that these people, who had moved from South Korea to the Netherlands as infants, were better at learning sounds that are unique to the Korean language, compared with Dutch speakers with no Korean experience.

There was no difference in language understanding between the adoptees who had come to the Netherlands before 6 months of age and those who had left Korea as toddlers. The researchers found that both groups learned the Korean-specific sounds faster than the Dutch-speaking control group.

“We showed first that the knowledge retained by the early adoptees was as useful as the knowledge that the older adoptees had,” study researchers Jiyoun Choi, of Hanyang University in Seoul and Anne Cutler, of the Max Planck Institute for Psycholinguistics in the Netherlands, wrote in an email to Live Science. “And second, we showed that the knowledge was abstract in nature.”

Previously, researchers knew that, at 6 months of age, babies can distinguish between a broader range of sounds than adults. But by the time they’re about 9 months old, infants lose the ability to detect sounds that aren’t important in their native tongue. For example, native Korean speakers might have trouble distinguishing between the “r” and “l” sounds in English, while a native English speaker would be mystified by a series of “p,” “t” and “k” sounds that have more variations in Korean than in English.

These findings had been taken to mean that babies don’t know their native phonology—the abstract rules governing their language’s sounds—until after 6 months of age. (However, some studies show that babies can determine whether a sound is in their native tongue when they’re just days old.) There had never been any direct evidence showing that babies lacked this abstract knowledge, Cutler and Choi told Live Science; it was always just an inference.

The researchers found the answer in adoptees from Korea. When the international adoption program between Korea and the Netherlands opened up, it started slowly, and many of the adopted kids were older than 1 year, Cutler and Choi said. As the program became more established, younger babies began to come over in greater numbers, in part because of the preferences of adoptive parents.

As a result, the researchers were able to recruit a group of 29 Dutch-speaking adults who came to the Netherlands either before 6 months of age or after 17 months of age. They matched this group to a control group of Dutch speakers who had never been exposed to Korean, and then tested both groups on their ability to distinguish between voiceless alveolar stops—a kind of sound articulated by the cutting off of air and without the vibration of vocal cords. Dutch has just one of these kinds of sounds, a “t” sound, whereas Korean has three.

Abstract thinking

Before training, the adoptees and the non-adoptee control group were equally bad at distinguishing the Korean sounds. Over a week and a half of learning, though, the adoptees proved faster at picking up the differences. They were significantly better at distinguishing the sounds at the midpoint of the study, compared with those who’d had no exposure to Korean.

Once the researchers took into account other factors that could have affected the results, such as the people’s current ages and their sex, there was no difference in the language-learning abilities between the adoptees who moved to the Netherlands as toddlers and those who moved to the Netherlands in early infancy.

This finding indicates that even as very young babies, infants can detect abstract patterns in the language they hear around them.

“At 3 to 6 months, there is abstract phonological knowledge,” Cutler and Choi told Live Science.

In a study on the same data published earlier, the researchers found that this long-buried knowledge also gave the adoptees a boost at not just recognizing, but pronouncing, the Korean-language sounds.

“We expect that there is going to be a new wave of research addressing what three to six-month old infants can do!” Cutler and Choi wrote. The researchers are now working on a variety of projects to investigate how sound perception and sound production are linked, how adults learn second languages, and how infant brains process sounds in the first year of life.