What they were looking for, if you go back in the record, was always that elusive direct genetic evidence. For decades researchers in behavioral genetics had performed kinship studies, research involving adoptees and identical twins, to determine the degree to which genes might influence behavioral traits such as intelligence, perseverance, extraversion, and more. Identical twins share more or less 100% of their DNA, with some complications, while ordinary siblings share an estimated 50% of their genetic code, with even more complications, and half-siblings share ~25%…. These estimates of genetic similarity allowed researchers to crudely match that estimated similarity with quantitative behavioral variables like IQ. Identical twins who were raised in different environments permitted a glimpse at how their nearly-identical DNA resulted in similarity under remarkably different circumstances, while the differences between adopted children and their siblings who were the biological progeny of the parents permitted similar questions through a different lens.
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