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.
Comparing the performance of identical twins, who share the same genes, with that of fraternal twins, who share on average about half of their genes, enabled researchers to determine which skills were linked primarily to genetics and which could be attributed primarily to a shared environment.
Nonverbal tasks, like stacking blocks or finding hidden toys, showed a strong genetic link and almost no environmental influence during the second year. Language comprehension, on the other hand, showed some genetic influence but the strongest effect was from the environment. Speech was influenced almost equally by environment and genetics.
The study, in press in "Monographs of the Society for Research in Child Development," underscored the importance of reading to and talking with infants and toddlers. "A child's environment influences word comprehension, but there is no evidence that talking more than normal will increase word comprehension," Professor Reznick cautioned. "A constant barrage of words would likely only confuse a toddler."
Hearing Impairment Can Hinder Word Comprehension
A child who cannot respond to simple commands or recognize words for some familiar objects, like eyes, ears and nose, by about 14 months of age should be checked by a pediatrician for a possible hearing impairment, he said. On average, children that age can say about 10 words but understand about 100 words. Most children start comprehending words between the ages of 8 and 10 months.
"Somewhere between the age of 14 and 20 months, language comprehension really takes off," he said. "It's as if the child finally grasps the concept that everything has a name. We call it the 'Adam insight' because children are suddenly motivated and excited to learn the label for every animal, every toy."
The research was part of the MacArthur Longitudinal Twin Study funded by the John D. and Catherine T. MacArthur Foundation, a long-term study now in its 12th year that provides one of the largest and most comprehensive data bases ever gathered to explore genetic and environmental influences on cognitive development in the second year of life.
Professor Reznick and his colleagues -- Robin Corley of the Institute for Behavioral Genetics and JoAnn Robinson of the Prevention Research Center for Family and Child Health at the University of Colorado Health Sciences Center -- found that girls did slightly better than boys on all the tests, perhaps because they develop language a little sooner or because parents interact differently with sons and daughters. Girls also appear to be more compliant and easier to test, Professor Reznick said.
While language development has been shown to be slightly delayed in some twin studies, that was not the case in the MacArthur study, Professor Reznick said, probably because the study included only twins of normal birth weight born no more than six weeks early. He added that 88.5 percent of the twins were from white families, 9 percent from Hispanic families and 2.2 percent from African-American families.
The finding that talkativeness is genetically influenced corroborates other research by Professor Reznick showing that 10 to 15 percent of children are born to be shy and that the tendency persists as they grow older. Signs of shyness in that study included clinging to their mothers and hesitancy in speaking and joining other children at play. Shy children responded to new situations with higher than average heart rates, blood pressure and muscle tension as well as greater pupil dilation and higher levels of norepinephrine, a brain chemical produced during stress.
Twin studies are uniquely useful for sorting out the complex interaction of environment and genetics in language development. "Talkative parents provide a rich linguistic environment that enhances language development, but they also provide a genetic impetus toward talkativeness. The MacArthur twin study helped us identify the link between genetics and talkativeness as well as the link between environment and word comprehension," he said.
Next, Professor Reznick will look for a link between word comprehension at 14 and 20 months and intelligence test scores later in life as the child starts school. He also is testing to see at what age children begin to remember and anticipate actions, as they do when they play peek-a-boo.
Note to Editors: J. Steven Reznick, Ph.D. 1982, University of Colorado, Boulder, is associate professor of psychology at Yale University and the Yale Child Study Center. He also is a faculty member of Yale's Bush Center in Child Development and the Yale Interdepartmental Neuroscience Program. His major research interests are in cognitive development in human infants, particularly the development of representation and intentionality as reflected in memory, future-oriented behaviors and language. For interviews, please contact him at 203-432-4630.