Twin Study

Twin studies reveal the absolute and relative importance of environmental and genetic influences on individual's in a sample. Twin research is considered a key tool in behavioral genetics and in content fields, from biology to psychology. Twin studies are one part of a broader field – 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 study of varying family environments (across pairs) and widely differing genotypes: 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. 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. Discordance in MZ twins provides a powerful window into environmental effects (see below).
The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins (they are for most traits) this implicates their higher genetic similarity as a cause of variance in the behavior. 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