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Twin studies reveal the absolute and relative importance of environmental and genetic influences on individuals in a sample. Twin research is considered a key tool in behavioral genetics and in content fields, from biology to psychology. Twin studies are part of the methods used in behavior genetics, which includes all data that are genetically informative – siblings, adoptees, pedigree data etc. Twins are a valuable source for observation because they allow the study of varying family environments (across pairs) and widely differing genetic makeup: "identical" or monozygotic (MZ) twins share nearly 100% of their genes, which means that most differences between the twins (such as height, susceptibility to boredom, intelligence, depression, etc.) is due to experiences that one twin has but not the other twin. "Fraternal" or dizygotic (DZ) twins share only about 50% of their genes. Thus powerful tests of the effects of genes can be made. Twins share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) by virtue of being born in the same time and place. The presence of a given genetic trait in only one member of a pair of identical twins (called discordance) provides a powerful window into environmental effects. The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins (which is found for most traits), this implicates that genes play an important role in these traits. By comparing many hundreds of families of twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior. Modern twin studies have shown that almost all traits are in part influenced by genetic differences, with some characteristics showing a strong influence (e.g. height), others an intermediate level (e.g. personality traits) and some more complex heritabilities, with evidence for different genes affecting different aspects of the trait — as in the case of autism. ==History== Twins have been of interest to scholars since early civilization, including the early physician Hippocrates (5th century BCE), who attributed similar diseases in twins to shared material circumstances, and the stoic philosopher Posidonius (1st century BCE), who attributed such similarities to shared astrological circumstances.〔Cicero, De Divinatione, (On Divination), ii. 42〕 More recent study is from Sir Francis Galton's pioneering use of twins to study the role of genes and environment on human development and behavior. Galton, however, was unaware of the difference between identical and DZ twins. This factor was still not understood when the first study using psychological tests was conducted by Edward Thorndike (1905) using fifty pairs of twins. This paper was an early statement of the hypothesis that family effects decline with age. His study compared twin pairs age 9-10 and 13-14 to normal siblings born within a few years of one another. Thorndike incorrectly reasoned that his data supported for there being one, not two, twin types. This mistake was repeated by Ronald Fisher (1919), who argued :"''The preponderance of twins of like sex, does indeed become a new problem, because it has been formerly believed to be due to the proportion of identical twins. So far as I am aware, however, no attempt has been made to show that twins are sufficiently alike to be regarded as identical really exist in sufficient numbers to explain the proportion of twins of like sex''." An early, and perhaps first, study understanding the distinction is from the German geneticist Hermann Werner Siemens in 1924. Chief among Siemens' innovations was the ''polysymptomatic similarity diagnosis''. This allowed him to account for the oversight that had stumped Fisher, and was a staple in twin research prior to the advent of molecular markers. Wilhelm Weinberg and colleagues in 1910 used the identical-DZ distinction to calculate respective rates from the ratios of same- and opposite-sex twins in a maternity population. They partitioned co-variation amongst relatives into genetic and environmental elements, anticipating the later work of Fisher and Wright, including the effect of dominance on similarity of relatives, and beginning the first classic-twin studies. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Twin study」の詳細全文を読む スポンサード リンク
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