"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 la
Contact: Joel Schwarz
University of Washington