Vocabulary size in 5- to 46-year-olds with Down syndrome
- Natalie Mason (Boys Town National Research Hospital)
- Susan Loveall (University of Nebraska - Lincoln)
- Emily Buss (University of North Carolina - Chapel Hill)
- Heather Porter (Boys Town National Research Hospital)
- Lori Leibold (Boys Town National Research Hospital)
Correspondence: natalie.mason@boystown.org
Background: Vocabulary knowledge is a key component of language and is associated with speech perception, academic outcomes, and social participation. For individuals who are neurotypical, vocabulary knowledge increases rapidly during childhood and adolescence and continues to grow into older adulthood, reflecting cumulative learning and life experiences (e.g., up to 65 years in Kavé, 2022). For individuals with Down syndrome, vocabulary is often cited as a “relative strength” within their language profile (Abbeduto et al., 2007). Yet, their vocabulary knowledge follows a delayed developmental trajectory when compared to typical development, moderated by cognitive, educational, hearing, and linguistic factors (e.g., Filipe et al., 2022). Few studies have evaluated vocabulary knowledge across the lifespan, however, and limited data are available for middle-aged and older adults with Down syndrome.
Research questions:
Do age-related changes in vocabulary size follow the same trajectory for children and adults with Down syndrome and children and adults who are neurotypical?
Are language scores for individuals with Down syndrome more variable across the lifespan relative to individuals who are neurotypical?
Method: As part of an ongoing study to understand hearing and communication outcomes for individuals with Down syndrome across the lifespan, more than 100 individuals with Down syndrome (ages 5-46 years) and a similar number of individuals who are neurotypical (ages 4-51 years) completed a standardized measure of receptive vocabulary (PPVT-V). An asymptotic regression model was fitted to the data to characterize the association between age and vocabulary size for each group and compare performance between groups.
Results: Vocabulary scores for both groups showed continual growth throughout childhood to ~20-25 years before leveling off. However, vocabulary scores were, on average, lower for the participants with Down syndrome than for participants who are neurotypical. Variability of the residuals in the regression model were about a factor of 2 greater for participants with Down syndrome than neurotypical data, indicating greater individual differences. Although scores for most participants with Down syndrome fell below the 95% prediction interval for participants who are neurotypical, a handful of adults with Down syndrome performed within the normative range. Differences in age effects between groups appear to be driven primarily by reduced asymptotic performance, with relatively modest evidence for a slower rate of word learning in children with Down syndrome compared to children who are neurotypical.
Conclusion: The performance trajectory of individuals with Down syndrome mirrored that of the neurotypical participants, just with lower overall scores across the lifespan. These findings highlight the importance of characterizing vocabulary development across the lifespan and the need for supports that extend past early childhood.
