As Dr Green implies, the interpretation of the comparison of unaffected and normal monozygotic twins is complex and open to a number of interpretations. We have examined the issue in more detail in an unpublished manuscript. To summarize our conclusion, it appears that the emotional burden of having an ill cotwin, differences in cooperation and anxiety level, and small differences in parental socioeconomic status might have contributed to the effect sizes that we observed. How- ever, as Dr Green correctly notes, differences were consistently in the predicted direction (unaffected twins performed worse than normal twins). Moreover, they were most marked for those key variables that yielded prominent differences in the contrast between the unaffected and affected groups. This pattern is not inconsistent with the notion that there are cognitive markers for the genetic component of schizophrenia.1-3 In fact, the socioemotional and the cognitive marker viewpoints are not mutually
Goldberg TE, Torrey EF, Weinberger DR. Neuropsychological Performance in the Unaffected Twin-Reply. Arch Gen Psychiatry. 1992;49(3):247. doi:10.1001/archpsyc.1992.01820030079013
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