 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 â 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. intelligence quotient) and some more complex heritabilities, with evidence for different genes affecting different aspects of the trait â 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, (
Twin_studies