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Douglas_Knight comments on [Question] Adoption and twin studies confounders - Less Wrong
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One confounder for twins is that they were in resource competition when they were in their mom's womb. This environmental effect probably causes twins to appear more different and results in us, when using them to study genetic influence, underestimating the role of genetics.
Why would this make them more different? Do you think one twin wins a resource competition and the other loses? Why do you think that?
No, resource competition is an environmental influence that twins share and it makes them similar to each other and different from single births. For example, it suppresses IQ. The most extreme similarity is that it makes them more likely to miscarry, especially boys, but this confounds other measurements.
I don't understand your questions. Why would it not make them more different? The competition introduces another source of variability: what fraction of the resources a particular fetus gets. A singleton has no such randomness, since it just gets 100%, there's no competition.