A team of American and British researchers studying 2-year-old twins has found that genetics, not the environment, plays the major role in the delayed acquisition of language among children who are having the most difficulty learning to speak.
The study, which looked at more than 3,000 pairs of twins born in England and Wales, was headed by Robert Plomin, research professor of psychology at the Institute of Psychiatry in London and Philip Dale, University of Washington psychology professor. It focused on both the entire range of normal variation among children and on children who ranked in the bottom 5 percent in acquiring language.
The researchers found that twins, whether identical or fraternal, generally scored very similarly in language at age 2. But the results from the children in the bottom 5 percent told a different story. If one identical twin ranked in the lowest 5 percent there was an 81 percent chance that his or her twin also would fall into that group. But if the twin was a fraternal there was only a 42 percent chance of the other being in the bottom 5 percent.
"This points to a genetic influence since identical twins have the same genetic makeup while fraternal twins are only 50 percent the same genetically," said Dale.
"For the broad range of children, environment or nurture appears to be more important. With fast, average and even moderately slow language development genetics or nature doesn't seem to matter as much as environment, though it does play some role. But what happens to children at the low or very slow end of language development is strikingly different. With these children, genetics is important and that's why it makes a whopping difference what kind of twin they are."
There's further suggestion for the effects of genetics among this group of children, according to Dale, if you assume fraternal and identical twins are treated similarly in the way parents talk and read to them.
"If fraternal twins are more different from each other than identical twins it is because they are genetically different," he said. "If the treatment is the same for identical twins and they both have low scores again the reason primarily must be genetic." In addition, because same-sex and opposite-sex fraternal twins have the same 42 percent chance of being at the low end of language development scale, it appears that boys and girls aren't being treated differently. This also suggests a genetic rather than an environmental influence, according to Dale.
Despite this apparent genetic link to a delay in learning language for some children, Dale emphasized that environment and the way parents relate to their youngsters is vital.
"For kids in general, environment really matters. The amount and the way parents talk to children is very important and influences how well they will learn language."
To examine language acquisition, the Dale-Plomin team was able to enlist the help of parents of 3,039 pairs of twins who are part of the Twins Early Development Study which is looking at all 7,756 pairs born in England and Wales in 1994. Their study consisted of 1,044 pairs of identical twins, 1,006-same sex fraternal twins and 989 opposite-sex twins. The parents were given a list of 100 words, representative of a larger inventory of common words that 2-year-olds use. Parents were asked to check off specific words that their twins use.
Children at age 2 have a huge variety in their vocabulary, ranging from those who speak no recognizable words to those who already use all 100 on the list. The average number of words produced from the list was 48 for the entire sample of twins, but just 4.2 words for the lowest 5 percent. Sixty-one children produced no recognizable words.
Dale said that an early delay in language acquisition doesn't necessarily mean a child will have language or reading problems later in life.
"We know half of them won't have problems and will catch up with their peers. The other half will have problems and we would like to know how to identify those who won't catch up. From this study, we know that part of the reason is genetic and we would like to look for specific genetic markers of both temporary and more enduring delay in future research."
Dale said the researchers are following the twins in their study to find which children do and don't catch up. So far, they have found no relationship between delayed language acquisition and other abilities such as spatial and non-verbal skills.
The study, which was published in the journal "Nature Neuroscience," reaffirmed known gender differences in language delay: more boys than girls have this problem. Analysis of data indicated that parents are not treating boys and girls differently.
"Boys are more at risk for this, just as they are for just about everything else," he said.
Other members of the research team include Emily Simonoff, Thalia Eley, Bonny Oliver, Thomas Price and Shaun Purcell of the Institute of Psychiatry in London; Dorothy Bishop of MRC Cognition and Brain Science Unit in Cambridge, England; and Jim Stevenson of the Centre for Research into Psychological Development at the University of Southampton.