Language development in young children

By Dr Tan Seok Hui, Language Development Researcher

Oral language skills play an important role in children’s cognitive development. This article examines the contribution that early vocabulary development makes to children’s subsequent cognitive development, and reports the findings from a recent study on expressive vocabulary for local infants in Singapore.

Vocabulary development
Learning new words is an integral part of cognitive development in early childhood. Research has shown that a wide range of vocabulary size exists among young children as early as the age of 13 months1. A study on monolingual infants acquiring US English as their native language has shown that vocabulary size varies from 20 words to more than 200 words at 18 months of age1. Similar findings have been documented for bilingual children: The number of words Spanish-English bilingual infants can say and understand ranges from as few as 16 words to as many as 549 words2 at 18 months of age.

Studies documenting the normal variation in infant vocabularies often measure the number of words bilingual infants can say and understand in terms of their conceptual vocabulary size2. This is computed by adding all the words the child can say and understand in both languages, with the exception that translation equivalents or words (e.g., apple, epal) which represent the same concept (i.e., apple) in both languages are counted once only. For both monolingual and bilingual infants, data are obtained by asking parents to tick the words their toddler can say and understand, using a standardized checklist of words which young children at that age are likely to know. For an accurate measure of bilingual infant vocabularies, it is important to ask parents to report their child’s vocabulary for both languages, using a vocabulary checklist for each language3.

The importance of early vocabulary
Studies show that the number of words children can say and understand when they are in infancy can play an influential role determining children’s subsequent cognitive development. For example, infant vocabulary size predicts not only preschool language skills4, 5, but also learning and cognition, literacy skills, and school achievement in middle childhood6. It has been observed that children who are quick to learn new words at 2 years of age tend to have larger vocabularies at the same age; these children also tend to have better verbal and nonverbal cognitive ability at 8 years of age7.

Conversely, children who are identified to have delayed language at 2 years of age because they say and understand relatively few words (e.g., fewer than 50 words) at that age, tend to have language deficits in both vocabulary and grammar when assessed a year later, at 3 years of age8, 9. It may thus be crucial for speech-language professionals and health practitioners to be able to identify children who are at risk for language delays, and to have access to local vocabulary norms.

Early language milestones
Previous studies have established that bilingual children acquire their language milestones in the same time frame as monolingual children10, 11, 12, 13. With respect to vocabulary size, studies have demonstrated that bilingual children’s vocabularies are equivalent to those of monolingual children when words from both languages are counted together, with the understanding that words which express the same concept in different languages are counted only once10. It is however noteworthy that the concepts which bilingual children can label in both languages often make up only a small proportion of their total vocabulary during infancy10. Consequently, bilingual infants are found to have vocabularies which similar in size to those of monolinguals, regardless of whether the number of words bilingual infants can say are measured in terms of total vocabulary (adding words from both languages together) or total conceptual vocabulary (counting translation equivalents only once), with the latter being the more conservative measure10.

Normal variation among Singaporean infants
To date, there has been only one large sample study14 which documents the variation in vocabulary size among infants acquiring local languages in Singapore.* This study sampled 855 infants aged 18 to 20 months and 648 infants aged 25 to 27 months, among whom more than 96% of infants were exposed to two languages—English and a local language (e.g., Mandarin, Malay, Tamil). In the study, parents were asked to indicate the words their child could say and understand using a standardised vocabulary checklist, which they had secure online access to for the purposes of the study.

In the study, it was found that all infants had some expressive vocabulary at 18 months. Anecdotal evidence previously suggested that local infants exposed to two languages might have no expressive vocabulary even at 18 months. The data from this study however indicate that local infants can be expected to say their first words by 18 months. Moreover, at least half the infants at this age were reported to say and understand more than 50 different words. As expected, vocabulary size increased with age. Parents also reported more words in expressive vocabularies for girls than boys.

In sum, there is much empirical evidence to suggest that the number of words children can say and understand during infancy is an important indicator of their cognitive, as well as language, ability. Importantly, a recent local study documents vocabulary norms for local infants aged 18 to 20 and 25 to 27 months predominantly acquiring two languages, with results indicating that local infants do say their first words by 18 months.

Pearls of Practice

  • The number of words young children say and understand around 18 months of age is predictive of later verbal and nonverbal cognitive development.
  • Young children exposed to two languages simultaneously during infancy acquire their language milestones (e.g., first 50 different words in vocabulary) in the same time frame as their monolingual peers.
  • Local norms from a large sample study suggest that Singaporean infants have expressive vocabulary by 18 to 20 months of age.


* An adaptation of the questionnaire used in the study is available here. Parents can utilise this Singapore-specific questionnaire to assess their toddler's level of language development against local norms for expressive vocabulary.



  1. Fenson, L., Dale, P. S., Reznick, J. S., Bates, E., Thal, D. J., & Pethick, S. J. (1994). Variability in early communicative development. Monographs of the Society for Research in Child Development, vol. 59.
  2. Jackson-Maldonado, D., Thal, D., Marchman, V., Bates, E., & Gutiérrez- Clellen, V. F. (1993). Early Lexical Development in Spanish-Speaking Infants and Toddlers. Journal of Child Language, 20, 523–549.
  3. Patterson, J. (2004). Comparing bilingual and monolingual toddlers' expressive vocabulary size: Revisiting Rescorla and Achenbach (2002). Journal of Speech, Language and Hearing Research, 47, 1213 - 1215.
  4. Bee, H. L., Banard, K. E., Eyres, S. J., Gray, C. A., Hammond, M. A., Spietz, A. L., Snyder, C., & Clark, B. (1982). Prediction of IQ and language skill from perinatal status, child performance, family characteristics, and mother-infant interaction. Child Developmentz, 53, 1134-1156.
  5. Oliver, B., Dale, P. S., & Plomin, R. (2004). Verbal and nonverbal predictors of early language problems: an analysis of twins in early childhood back to infancy. Journal of Child Language, 31, 609-631.
  6. Hohm, E., Jennen-Steinmetz, C., Schmidt, M. H., & Laucht, M. (2007). Language development at ten months. European Child & Adolescent Psychiatry, 16, 149-156.
  7. Fernald, A., Perfors, & Marchman, V. A. (2006). Picking up speed in understanding: Speech processing efficiency and vocabulary growth across the 2nd year. Developmental Psychology, 42, 98-116.
  8. Buschmann, A., Jooss, B., Rupp, A., Dockter, S., Blaschtikowitz, H., Heggen, I., & Pietz, J. (2008). Children with Developmental language delay at 24 months of age: results of a diagnostic work-up. Developmental Medicine & Child Neurology, 50, 223-229.
  9. Thal, D. J., Reilly, J., Seibert, L., Jeffries, R., & Fenson, J. (2004). Language development in children at risk for language impairment: Cross-population comparisons. Brain and Language, 88, 167-179.
  10. Pearson, B. Z., Fernández, S. C., & Oller, D. K. (1993). Lexical development in bilingual infants and toddlers: Comparison to monolingual norms. Language Learning, 43, 93-120.
  11. Pearson, B. Z., Fernández, S. C., Lewedeg, V., & Oller, D. K. (1997). The relation of input factors to lexical learning by bilingual infants. Applied Psycholinguistics, 18, 41-58.
  12. Genesee, F., Nicoladis, E., & Paradis, J. (1995). Language differentiation in early bilingual development. Journal of Child Language, 22, 611–631.
  13. Junker, D. A., & Stockman, I. J. (2002). Expressive vocabulary of German-English bilingual toddlers. American Journal of Speech-Language Pathology, 11, 381-394.
  14. Low, Y.-L., Baggs, G., Tan, S. H., & Rosales, F. J. (2012). Nutrition and stimulation are positively associated with expressive vocabulary in young children. Oral presentation for the World Congress of Pediatric Gastroenterology, Hepatology & Nutrition, Taipei, Taiwan, 14-18 Nov, 2012.

This article is authored by Dr Tan Seok Hui, and the opinions, views and positions expressed in this article are those of the author.


Last updated on 20 June 2013

Please log in to access content.

Not an ANIC member? Sign up for a free account now!