Over the past century, several large studies have been published exploring IQ differences between monozygotic (MZ) twins (TRA). Overall, this work reported that MZ differed in TRA IQ by ~8.0 points and demonstrated an intraclass correlation (ICC) of ~0.75. Unfortunately, this prior research has been largely mixed: it averages data from dozens of TRA pairs without taking into account important life experiences, including education (which has shown a causal effect on IQ performance). In this paper, we collected data from every available TRA case published in the academic literature over the past century that included both individual IQ and biographical data. This data set (which we believe represents the entire non-amalgamated TRA field) contains 87 pairs. For our analysis, we divided these pairs into three groups: similar, somewhat different, and very different schooling. The analysis shows that schooling differences have a significant impact not only on the absolute IQ difference between TRA pairs (5.8, 12.1 and 15.1 points, respectively) but also on the ICC (0.87, 0.80, 0.56, respectively). These findings raise an important question regarding the historical use of the ICC as a measure of genetic influence on IQ and other psychological traits. It is recommended that the field of TRA studies focus on individual pairs rather than groups and that researchers share individual data from TRA pairs included in historical aggregate analyses.
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