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"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."

"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."