When Jim was about 13 months old, I happened to be enrolled in a graduate level developmental psychology class. Our big term paper assignment involved observing two children at different developmental stages. I decided it would be cool to do a “longitudinal study” of Jim’s language development over the course of the semester — my “two children” would be one person — Jimmy (as we called him then), at the beginning and end of the semester. The period from 13 to 16 months old is often a very important period in language development, when many babies begin to understand not only words but also phrases, and when they start to produce many words as well.
Unfortunately for me, Jim’s language development didn’t cooperate with my class schedule. He didn’t produce any new words between 13 and 16 months — he had found that pretty much all his requirements could be taken care of with three: “na-na” (no, bad), “da-da” (yes, good), and “bah-pul” (give me food — now — preferably in liquid form!).
Nora, on the other hand, was producing many words by 13 months, and many more at 16 months. But she was born about 19 months too late to be of use in my developmental psychology class. So why is it that some kids talk early and others talk late? Part of the answer comes from a study by a team led by Feng-Ming Tsao. Tsao and his colleagues tested the language ability of 26 six-month-old infants, then re-evaluated them at 13, 16, and 24 months of age.
The test they gave at 6 months was very similar to the study we discussed a few weeks ago (in fact, it was conducted in the same lab). The babies sat on a parent’s lap and listened to one of two prototypical sounds (/u/ or /y/), which was repeated every two seconds. Every so often a different sound was played, and if the babies looked in the direction of the different sound, they got to see a toy bear bang on a drum. Once they reliably responded, they moved on to a second phase where they were tested until they got 7 out of 8 correct responses. During this phase researchers wanted to know how many trials it would take until the task was learned. Finally they were randomly tested on 30 trials to get an overall accuracy score.
As it turned out, the second phase — number of trials until the task was learned — was most reliably correlated with future language ability. Take a look at this graph charting the results for the 13 toddlers the researchers were able to locate at age 24 months:
The fewer trials babies took to perform the task at age 6 months, the more words they were able to produce at age 24 months. Just this simple task of distinguishing between two different vowel sounds was significantly correlated with speaking ability 18 months later! A number of other measures of language ability also correlated with performance at 6 months. Consider this graph:
Once again, the toddlers who produced more grammatically complex language at 24 months were the babies who could more readily distinguish between vowel sounds at age 6 months. The researchers examined other factors as well, such as the socioeconomic status of the parents, and the only other factor in language development of the 24-month-olds was whether the mother had a job, which was weakly (and non-signficantly) correlated with production of irregular words at 24 months. None of these external factors had made a difference in the performance of the task at 6 months.
Unfortunately, this study can’t show us why ability at 6 months correlates with ability at 24 months. Perhaps different kids have different language abilities that are manifested as early as 6 months old. Perhaps some unknown environmental factor plays a part. Perhaps the better-performing children are simply more intelligent than the others. The team is working on a follow-up study of a larger group, which controls for some of these factors while following the kids for a longer period.
Tsao, F., Liu, H., & Kuhl, P.K. (2004). Speech perception in infancy predicts language development in the second year of life: A longitudinal study. Child Development, 75(4), 1067-1084.