To the Editor. —
I was impressed with the article by Goldberg et al1 in which the authors conducted neuropsychological assessments with monozygotic twins discordant for schizophrenia. This method provides a particularly effective means of separating genetic and nongenetic factors. The article raised several questions that deserve further attention.In addition to comparisons between affected and the unaffected twins, normal monozygotic twins were compared with both sets of twins. The authors emphasize that "perhaps the most striking feature of these data were the lack of significant differences between the group of normal monozygotic twin pairs and the group of unaffected discordant twins on any neuropsychological task." On the surface, such a negative finding seems inconsistent with the notion that certain cognitive deficits in schizophrenia are vulnerability indicators because these indicators would be expected to occur disproportionately in unaffected relatives. However, the reported negative findings require a closer look.The
Green MF. Neuropsychological Performance in the Unaffected Twin. Arch Gen Psychiatry. 1992;49(3):247. doi:10.1001/archpsyc.1992.01820030079012
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