Few scenarios have seemed more promising for the unraveling of human mystery than the research pot of gold that many believe identical twins to be. Unlike the rest of the actual, messy human world, monozygotic twins represent an almost perfect study-in-the-making: two identical copies of the same genetic makeup placed into the world at nearly the same moment. One could hardly design better conditions for an experiment in the quest to lay bare the human nature recipe.

More and more, twin studies are becoming the vogue of researchers concerned with these questions. Consider, for instance, language acquisition. How is it—miracle of miracles—that we learn to speak? It’s not a new question, of course, and the verdict is decidedly still out. But, in the past few years, researchers have been making enormous strides toward pinpointing important questions about language acquisition. And they’ve been using twins to do it.

Prominent among those questions is one large one: How much of the differences between the various aspects of language in young children—things like vocabulary, expressive language (the language we speak) and receptive language (the language we understand)—can we chalk up, on one side, to genetics, and, on the other, to environmental factors?

“For language development at the low end, where early deficits in language may have long-term consequences, knowledge about specific causes may help practitioners to effectively help the child.”

The Difference between Knowing and Saying
Steven Reznick, a psychologist at the University of North Carolina, and his colleagues JoAnn Robinson at the University of Colorado Health Sciences Center and Robin Corley at the University of Colorado, have delved into the mysteries of how young children come about language; their research has produced some interesting clues. In a 1997 study to examine cognitive development during the second year, they looked at 408 pairs of identical and same-sex fraternal toddlers, aged 14, 20, and 24 months. Using the Bayley Mental Development Index, the researchers measured nonverbal ability, expressive language, and receptive language, and included tests of word comprehension, visual attentiveness, and memory for locations.

One of the things they found was that the number of words a child understands during the second year of life is a better predictor of future intelligence than the number of words the child is able to speak during the same period. This is no doubt a comfort for the parents of late-talkers. They also discovered that during the second year of life, environmental factors seem to more heavily affect the number of words a child understands, while both environmental and genetic factors play a part in determining how much a child talks.

Why is it important to be able to distinguish between environmental and genetic factors in something like language acquisition? Robin Corley says that once we can pinpoint specific problems and their causes—in this case, whether or not that problem is inherited or the result of something environmental—then intervention becomes a possibility.

“For the normal range of language variation, both expressive and receptive, at these young ages, the primary interest is how development occurs; how the patterns of genetic and environmental influences change over time. But for language development at the low end, where early deficits in language may have long-term consequences, knowledge about specific causes may help practitioners to effectively help the child.”

“Given,” Corley adds, “that we find a deficit that ought to be addressed—an unrecognized hearing problem, for instance—we can then possibly tailor an intervention. And there are surely cases where knowledge of the precise causes can be invaluable in formulating the treatment.”

Using the example of some hearing loss, a possible intervention might simply be to place a child at the front of the classroom or, when speaking to him, face him so he can gather visual cues.

“Genetics is involved in the extent to which language delay persists.”

The Source of the Problem
Another recent study has investigated the etiology of several measures of cognitive delay—including verbal and non-verbal—in over 3,000 pairs of two-year-old twins. The researchers asked parents to keep daily records of their children’s language development. This research is part of the on-going Twins Early Development Study, being conducted jointly by a number of institutions, including the Institute of Psychiatry in London and the University of Washington in Seattle.

This team of researchers found that environmental factors seemed to be much more important than genetics in language acquisition for normal toddlers. For children undergoing delayed language acquisition, however, the converse proved to be true: genetic factors appear to be much more important in determining the ability of these children to acquire language.

“We find that genetics is involved in the extent to which language delay persists,” says Robert Plomin, deputy director of the Social, Genetic, and Developmental Psychiatry Research Centre at King’s College, London, and one of many investigators in this study. As Corley suggests, this is good information for knowing when and how to intervene.

Plomin and his colleagues have now embarked on a molecular genetic study in order to ascertain if the persistence of language delay problems can be predicted early on. If they are able to predict such a thing, Plomin says, intervention measures might be taken to aid the child in learning.

Cautious Projections
There are those, however, who remain circumspect about twin research. Among others, count Harvard University’s intelligence expert and Professor of Cognition and Education Howard Gardner, among this group. Studies involving twins reared apart, says Gardner, “need to be approached with caution because identical twins reared apart are not randomly placed in the universe; they shared the same intra-uterine environment and are often placed with family, friends, or others in the same geographical environment.”

Even cautious observers like Gardner concede, however, that twin research is among the best tools we have to date in answering the kinds of sticky questions at the heart of human existence—like who we are and how we got to be that way. Twin research “tends to oversimplify the discussions of heritability,” he says, but “there is no question that [it] is the best way to get at familial, extra-familial, and genetic contributions to temperament, personality traits, and intelligence, however defined.”

Jerry Gabriel is a freelance writer currently residing in Columbus, Ohio. He holds degrees from The Ohio State University, Northern Arizona University, and the Iowa Writers’ Workshop. His fiction and non-fiction have appeared in a number of magazines and newspapers.

Dale, PS, et al. “Genetic influence on language delay in two-year-old children.” Nature Neuroscience, August, 1998; 1(4): 324-8.

Eley, TC, et al. “Genetic and environmental origins of verbal and performance components of cognitive delay in 2-year olds,” Developmental Psychology. July 1999; 35 (4): 1122-31.

Reznick, JS, et al. “A longitudinal twin study of intelligence in the second year,” Monographs of the Society for Research in Child Development. 1997; 62(1):I-vi, 1-154; discussion 155-60.