New Haven, CT -- The number of words toddlers understand is far more important than the number they speak during the second year of life, according to a recent study of twins by Yale University psychologist J. Steven Reznick. The study, which was designed to show how a child's genetic makeup and environment influence the development of language and other cognitive skills, should reassure parents about the general intelligence of toddlers who are less talkative than their playmates.
In tests of 408 pairs of same-sex twins -- half identical and half fraternal --Professor Reznick and colleagues at the Institute for Behavioral Genetics at the University of Colorado, Boulder, found that language comprehension rather than spoken language at ages 14 and 20 months better predicted how well the twins would do on intelligence tests at 24 months. Furthermore, the study found that environment had a greater impact on how many words a child understood, while genetic and environmental factors each played a role in how much a child talked.
"Some children are born to be chatty while others are born to be reticent about sharing their growing vocabulary knowledge," says Professor Reznick, who has developed ways to track eye motions in infants and toddlers to find out what they know but aren't saying. "Don't worry about the child who is not talking much if that child seems to understand what he or she hears and is asked to do."
The children were given a number of verbal and non-verbal performance tasks as part of a test battery called the Bayley Scale of Mental Development. To test language comprehension, toddlers also were asked to find the cat when shown slides of both a cat and a shoe, for example. Researchers then tracked eye motion to determine which slide the child looked toward.