Behavioural-genetic studies have shown that as adopted children grow into adulthood, the correlation between their IQ scores and those of their biologically unrelated family members diminishes to zero.

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This is a well-established finding in behavioural-genetic research. As adopted individuals become adults, their IQ scores become more and more similar to those of their biologically related family members, and less similar to those of their adoptive family members. By adulthood, the correlation between IQ scores of adopted individuals and their adoptive family members is close to zero.

Behavioural-Genetic Studies Have Shown That as Adopted Children Grow into Adulthood