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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.


  1. #1 Meredith Warshaw
    October 23, 2007

    It seems straightforward that being able to distinguish speech sounds is important for learning language. Anything that interferes with the ability to do that is going to make it harder to develop the building blocks for language such as learning new words. So it’s not surprising at all that differences in ability to distinguish sounds at 6 months predict language ability 18 months later. In my mind, the real question is what factors influence the ability to discriminate between speech sounds, and how does one tell early on which are just innocuous differences in rate of developmental vs. those due to problems with hearing, delays in auditory processing ability, etc.

  2. #2 Sally LeRoy
    October 23, 2007

    Looks like the paper is gated so I will admit I didn’t read it. However, just looking at the graphs you included in the post, the results don’t seem to be statistically significant. The variance/error is way too large and the sample is way too small for those trend lines to be meaningful. I’d be very cautious about drawing conclusions from such a small amount of date.

  3. #3 Dave Munger
    October 23, 2007


    I agree, the sample size is quite small. The results I’ve reported are significant. The effect size is quite large, and that makes up for the small sample.

    What’s more, they also gathered data at 13 months and 16 months, with a higher retention rate, and found the same results.

    In addition, they report on their ongoing research with a larger sample of 40 children, and again they say these results are replicated.

    I’ll be interested to see those results when they are finally published in peer-reviewed form.

  4. #4 Gibran
    October 24, 2007

    One possible aspect which is related and may be more difficult to analyze quantitatively is how much is the baby ‘talked to’;
    Exposure to regular well-formed speech from a parent may be a factor here. It may seem obvious, but some parents continue to talk with their babies as if in a conversation. I feel that one of those social factors is exposure to the nuances of the language Early on.

  5. #5 FhnuZoag
    October 24, 2007

    A preprint of the paper is available at , my fellow academiphiles.

    Included are some test statistics, suggesting that the findings are at least somewhat significant.

  6. #6 m dreyer
    October 24, 2007

    It’s interesting that for kindergarteners, phonemic awareness ability, the ability to discriminate, sequence, and manipulate sounds in words, is one of te strongest predictors for success in learning to read and write. This effect is very well eatablished by research.

  7. #7 Ann
    October 24, 2007

    I don’t think there is strong evidence that early language aquisition is associated with higher intelligence. Often, later speakers catch up with early speakers, just as later readers often catch up with early readers. It is possible that some of us are gifted with a “ear” for speech.

  8. #8 Damien
    October 25, 2007

    Hah. I started talking about 36 months, and grew up to be quite verbal. My hearing’s not the best though, and my father’s was worse, so maybe there’s something there.

  9. #9 Frans van Otten
    October 28, 2007

    Without even having read the paper, I would like to add one other point of view. There may be a difference in learning ability that underlies the observations. But there may also be a difference in (the level of) desire to interact with the outside world. Which might influence both the initial test and the child’s progress in speaking ability.

  10. #10 Bruce Poisson
    November 5, 2007

    Haven’t had a chance to read the paper, but it’s worth noting that the test at six months may have nothing to do with language ability. The test may be vowel discrimination, but variance in 6mos babies may be entirely due to some other measure of intelligence (processing speed, rule-learning, etc)- it shouldn’t be surprising that smarter babies learn language earlier. It would be interesting to see if a similar preferential looking procedure on a non-linguistic task (say, discriminating pure tones) also correlated with later ability, or if their 6mos test also was associated with better performance on non-linguistic measures of the older babies. Without these sorts of controls, it’s hard to know exactly what’s being tested. Older work has shown that trials to habituation (another design used in infant studies) for all kinds of stimuli correlates with later intelligence..

  11. #11 Frank Rollins
    November 7, 2007

    So the conclusion is there really isn’t a definitive answer as to what made them pick up a language or talk at an earlier stage. I didn’t see nurturing involved whereby the parents are just so active in talking to the infant. I do understand the significance and factors that affect language learning at an early age or as an adult while enrolling a study abroad program.

  12. #12 ipek
    February 5, 2009

    I do not believe that the ones who can talk earliar are more intelligent than the others. This is non sense. We have brains. And some use some parts of it more than others. Or some use some parts which are not used by some others. Brain is used differently by each of us.

    My son started to write at 2 years old. I was shocked when i saw him writing both in English and Turkish. And he knew all the numbers. Besides he was painting better than i did. Now he is almost 4. But he is still not talking properly. No real sentence.

  13. #13 Grandma D
    August 11, 2009

    I was looking for information on a three year old that can barely talk. He is the youngest of 5 children. He babbles and can say a few words.(baby, train) but other than that he doesn’t speak any discernable words. I don’t think it’s a hearing problem, because he understands when you tell him to do something. He turned 3 in June.

    I have another grandson who turned 3 last Oct. He has been talking for a long time and has a very large vocabulary and can articulate very well and carry on a converstation. He has a very vivid imagination also. He knows his colors and can count to 10. How can we get the younger one to talk more? Should I worry? I can’t help but compare the two.

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