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"To these scientists, and to biomedical researchers all over the world, twins offer a precious opportunity to untangle the influence of genes and the environment-of nature and nurture. Because identical twins come from a single fertilized egg that splits in two, they share virtually the same genetic code. Any differences between them-one twin having younger looking skin, for example-must be due to environmental factors such as less time spent in the sun. Alternatively, by comparing the experiences of identical twins with those of fraternal twins, who come from separate eggs and share on average half their DNA, researchers can quantify the extent to which our genes affect our lives. If identical twins are more similar to each other with respect to an ailment than fraternal twins are, then vulnerability to the disease must be rooted at least in part in heredity. These two lines of research-studying the differences between identical twins to pinpoint the influence of environment, and comparing identical twins with fraternal ones to measure the role of inheritance-have been crucial to understanding the interplay of nature and nurture in determining our personalities, behavior, and vulnerability to disease. Lately, however, twin studies have helped lead scientists to a radical, almost heretical new conclusion: that nature and nurture are not the only elemental forces at work. According to a recent field called epigenetics, there is a third factor also in play, one that in some cases serves as a bridge between the environment and our genes, and in others operates on its own to shape who we are."

As behavioral genetics enters a second century, the field's oldest research method remains both relevant and controversial. "Twins have a special claim upon our attention; it is, that their history affords means of distinguishing between the effects of tendencies received at birth, and those that were imposed by the special circumstances of their after lives." -- Sir Francis Galton, 19th century behavioral genetics pioneer, Inquiries into Human Faculty and its Development,1875 More than a century after Galton's observation, twin studies remain a favorite tool of behavioral geneticists. Researchers have used twin studies to try to disentangle the environmental and genetic backgrounds of a cornucopia of traits, from aggression to intelligence to schizophrenia to alcohol dependence. But despite the popularity of twin studies, some psychologists have long questioned assumptions that underlie them--like the supposition that fraternal and identical twins share equal environments or that people choose mates with traits unlike their own. The equal environments assumption, for example, has been debated for at least 40 years. Many researchers have found evidence that the assumption is valid, but others remain skeptical (see Further Reading below). Overall, twin studies assumptions remain controversial, says psychologist James Jaccard, PhD, a psychologist who studies statistical methods at the University at Albany of the State University of New York. In response, though, researchers are working to expand and develop twin study designs and statistical methods. And while the assumptions question remains a stumbling block for some researchers, many agree twin studies will continue to be an important tool--along with emerging genome and molecular research methods (see article page 42)--in shedding light on human behavioral genetics."

"BARBARA HERBERT, a former council worker living in southern England, discovered after the death of the woman she had thought was her mother that in fact she had been adopted. Among her assumed mother's papers, she found a name and address in Finland. When that produced no answer, she contacted the local newspaper in Finland. A reporter dug up the story. Her real mother had been sent to England, two months pregnant, in 1939. She had given birth, been sent back to Finland, and committed suicide at the age of 24. Mrs Herbert had a feeling the story was not over. She seemed to recall somebody saying, "There was another one." So she contacted Hammersmith Hospital, where she was born; and, sure enough, there had been twins. The Registrar-General refused to help her contact her twin. She took the Registrar-General to court, and won. That is how she found her sister. They met at King's Cross station in London. "We just said 'Hi' and walked off together, leaving our husbands standing there," says Mrs Herbert. "It seemed so natural." Mrs Herbert is a bit fatter than her sister, but she can think of no other important difference between them. Their intelligence quotients (IQs) were one point apart. They were tested again a year later; they scored ten points higher, but still only one point apart. Mrs Herbert and her sister Daphne are gold dust to geneticists. Unlike fraternal twins, who are the product of separate eggs fertilised by different sperm, identical twins are natural clones, produced when a fertilised egg splits in two shortly after conception. Such twins, when separated after birth, are thus a scientific experiment designed jointly by nature and by society. They have the same genes but have been brought up in different environments. These curiosities are getting rarer. Until the 1960s, twins offered for adoption in the West were often separated at birth, on the argument that two babies would be too much for one mother. That no longer happens

"Almost 150 years ago, English scientist Francis Galton coined the phrase "nature versus nurture" - and proposed that research on twins could resolve the debate. Genetics have long seemed to weigh heavily in favor of the role of nature in shaping the people we become. But even identical twins are different to varying degrees, and some researchers believe those differences suggest a third influence at work, called epigenetics. Peter Miller of National Geographic Magazine wrote the magazine's January cover story, "A Thing or Two About Twins." Miller explains how scientists are expanding the field of epigenetics with research on twins."