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It will affect the environment of BOTH twins

Isn't that the point of the parent?

You can estimate true heritability by comparing fraternal and identical twins, both of which will have shared environment, but identical twins are identical genetically and fraternal ones are as similar as any other sibling pair.

right, the part that I missed is that they are also comparing non-twin siblings.

> You can estimate true heritability by comparing fraternal and identical twins

Can you explain this a bit more?

https://en.wikipedia.org/wiki/Twin_study

> 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 with twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior.

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