Because individuals cannot be randomly assigned to different environmental or genetic backgrounds, family, adoption, and twin studies take advantage of different naturally occurring events to estimate the extent to which a trait or disorder is due to genetic or environmental influences. In the family study design the rate of a disorder is compared among the biological relatives of individuals with and without the disorder. If the disorder occurs more often in the family members of individuals with the disorder, this suggests that familial factors increase risk for the disorder.
Previous family studies have found that most psychological traits and disorders are significantly familial. However, because a disorder could run in families because of either genetic influences or shared environmental factors, adoption and twin studies are necessary to disentangle their relative contributions.
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