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Twins The same factors that give rise to uncanny physical similarities between twins--similarities are mirrored in genetic s

admin2010-05-09  47
问题                                           Twins
    The same factors that give rise to uncanny physical similarities between twins--similarities are mirrored in genetic similarities that can have more negative impact. Australian researcher David Hay has found that one type of identical twin seems to be plagued with developmental difficulties, especially in language acquisition, and that both parents and teachers add to these difficulties by responding more positively to the older of the two.
    Hay has found that just as many identical twins are "mirror images" of one another--with the facial structure, dominant hand and even fingerprints of one exactly reversed in the other---their brains also seem to reflect the same phenomenon.
Questions about Identical Twins
    "It is almost impossible to find identical pairs (of twins with the same brain structure)," says Hay, who presented some of his findings on twins at the International Society for Twin Study Conference in Amsterdam in September. "There is generally no relationship between how their brains function; the twins work and think differently." "The left half of the brain controls language skills in one twin while in the other it is the right half," he continues. "So one twin may have difficulty in the reception of language and the other in communication."
    "We still have piles of questions about identical twins," he adds.
    Hay heads one of the world’s largest studies on twins at LaTrobe University in Melbourne, and has studied 587 sets of twins and their siblings over the last 10 years. He has found that both genetic and social factors combine to put twins at a disadvantage, but that steps can be taken to set the youngsters on the path to normal development. The twins who experience the bulk of the problems are the second of two types of identical twins, says Hay. The first type, comprising one-third of all identical twins, results when the fertilized egg splits three to four days after conception. These embryos have separate placentas. The second group forms when the egg divides four to eight days after conception. They share the same placenta, and are "mirror image" twins in about 70 percent of the cases. This group is at the highest risk for a range of social and learning problems, as well as for congenital abnormalities such as spinal bifida and cleft palate. Hay points out that twins traditionally have been known to have difficulties acquiring language sills." They’re older when they say their first word, their sentences are shorter and baby talk persists longer that it does with other children, ’ he says. "They are also known to develop a secret language they use to communicate with one another." Hay has found that this exclusive language is usually the result of one twin omitting letters and syllables as he or she hurries to get a message across before the other one interrupts." In a sense their language is very adaptive to their own environment, but unfortunately, maladaptive to other situations," he says. "Until recently it was believed language problems gradually diminished until around the time of starting school for all children. But now it appears that twins in particular develop a shaky foundation in this area that has to be corrected early."
Problems with Twin Boys
    Hay has also found that "mirror image" identical twin boys are especially at a loss in trying to pick up language skills in school. They are at a double disadvantage, he notes, because boys are normally slower at language acquisition than girls are. According to Hay, as many as 90 percent of the "mirror image" boys at primary level had some reading problems while 23 percent were seriously learning-disabled. But environmental factors contribute even more heavily to twins’ troubles, Hay believes. Identical twins, his research shows, have cause to challenge their parents’ fairness. Parents tend to lavish more time and attention on the first-born twin, who is often easier to manage and healthier as a baby. As well, the first-born twin is often brought home from the hospital before the second twin, causing parents to "see the second child more negatively," says Hay. This distinction continues into primary school, where the first-born twin is seen as more dominant, independent and better-coordinated than the second.
Problems with the Second-born Twins
    "Surprisingly, teachers confirm these observations without knowing the birth order of the twins," says Hay. "When asked about the twins’ behavior, they reported a lot of problems with the second-born twins, saying they seemed withdrawn or depressed. "Parents should be aware of the way they can stereotype their children." he adds.
    The parents also have a role, as does the twins’ own relationship, in exacerbating their language difficulties.
    First, says Hay, parents of twins tend to spend less time with their children individually, so they have less exposure to adult language than do other children. Because twins spend so much time with each other, they reinforce each other’s relatively undeveloped language. Finally, twins have to compete with each other for adult attention creating the situation in which they finish each other’s sentences, and interrupt each other, further adding to their already poor language habits.
    From his findings, Hay has established some flexible guidelines for parents of identical twins. One suggestion is for parents to send their twins to preschool centers on alternate days so that contact with other children will accelerate language development. The other twin will benefit from the individual attention of the parent or caretaker who has generally been too busy attending to the basic needs of two.
    Many American and European models favor separating twins regardless of the particular case. Hay firmly believes in an individualistic approach to this question, however, in which the nature of the teacher, parents and twins can all be taken into account. "I’d say our only set policy is that there is no set policy."
    Hay’s work is complemented in the United States by Ronald Wilson’s Louisville Twin Study at the University of Kentucky. Wilson and other Louisville researchers have been focusing on the significance of twin temperament in child-rearing and development and other factors in twin development for the past 27 years.
Twins’ Temperament
    The Louisville researchers have examined a large number of twins from infancy to early childhood, weighing genetic against environmental influences on temperament. They have had difficulty; so far, learning much about inborn contributors to temperament, at least as they show up in twins’ first nine months of life. Wilson has found that during those months, the babies’ moods and behavior were primarily affected by feeding and sleeping cycles, colic, fatigue and other normal aspects of infancy.
    However, after nine months, individual temperaments became more obvious as the babies’ schedules became established. Then Wilson began noticing distinct differences between babies who were easy-going and those who were testier and more irritable.
    Interestingly, while temperaments became relatively fixed in the twins, the researchers also noticed that some identical twins underwent synchronized patterns of change in temperament. At times they shared similar periods of fussiness or amiability, and at others went through opposite ones, with one twin having a good-natured period and the other a cantankerous one. Wilson notes that one cause for these curious patterns is a gene-action system seemingly preset to switch on and off at certain developmental phases in people’s lives. Another cause for such swings is the baby’s environment, which can make the crucial difference between a well-adjusted child and a difficult one. Twins, it seems, don’t serve to put the nature/nurture question to rest so much as they serve to illustrate the subtle interplay between heredity and environment.
The Other Twin Puzzles
    Hay, in the meantime, is continuing to explore as many sides of the twin puzzle as possible. One of the more exotic studies he’s undertaken involved a three-month stint to Antarctica in search of a rare freshwater worm.
    "The worm is able to regenerate itself from the left or right side of its body when cut in half," he says. "It is potentially a very good model for the development of □mirror image□ twins."
    Hay also sees a trend towards more identical twins in the future. More fraternal twins are born to older woman, but a higher incidence of identical twins has been linked to older women who also conceive shortly after they stop taking oral contraceptives. The pill is currently the most popular form of contraceptive for women who plan to still have children.
Hay has found that both ______ and social factors combine to put twins at a disadvantage.
选项
答案genetic
解析 根据第5段第2句：He has found that both genetic and social  factors combine to put twins at a disadvantage，可知双胞胎处于劣势是基因和社会因素共同作用的结果。
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