Twin Study
Twin studies (studies involving twins) help disentangle the relative importance of environmental and genetic influences on individual traits and behaviors. Twin research is considered a key tool in behavioral genetics and related fields. Alternate groups that can also be studied based on facts about genetic similarity include full siblings in general and adoptees.
Twins are a valuable source for observation due to their genotypes and family environments tending to be similar. More specifically, monozygotic (MZ) or "identical" twins, share nearly 100% of their genetic polymorphisms, which means that most variation in pairs' traits (measured height, susceptibility to boredom, intelligence, depression, etc.) is due to their unique experiences. Dizygotic (DZ) or "fraternal" twins share only about 50% of their polymorphisms. DZ twins are helpful to study because they tend to 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 classical twin design compares the similarity of MZ (identical) and DZ (fraternal) twins. MZ twins are almost always more similar than DZ twins due to higher genetic similarity coupled with the same amount of environmental similarity. By comparing many hundreds of families of twins, researchers can then understand more about the complementary roles of genetic effects, shared environment, and unique environment in shaping people.
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 - as in the case of autism.
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