Researchers have suggested that the lag in using words does not mean that shy kids are not able to understand what's being said.
The new study tests four possible explanations for the association: that shy children practice speaking less and so their speech becomes delayed, that children with delayed speech become shy because they have difficulty talking, that shy children understand what's being said but are simply reticent to speak, and that shy children's speech is actually normal while outgoing children's speech is above average.
The research team, from CU-Boulder's Institute for Behavioral Genetics (IBG) and the Department of Psychology and Neuroscience, found consistent evidence to support only the hypothesis that shy toddlers were delayed in speaking but not in understanding.
For the study, the researchers looked at information collected on 408 sets of twins at 14, 20 and 24 months of age, when children's language skills are rapidly expanding. The data on inhibition and speech characteristics of the 816 toddlers came from parent reports and researcher observations. Notes were made both of the child's ability to repeat sounds and answer questions as well of the child's ability to follow directions.
The research team looked for patterns in how the children's behavior changed over time, noting whether an increase in shyness, for example, followed or preceded a delay in speech. The result-that shy toddlers understand more than they indicate through talking-is both good news and bad news.
The new study has been published in the journal Child Development.