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
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Contact: Joel Schwarz
joels@u.washington.edu
206-543-2580
University of Washington
22-Oct-1998