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May 1997

Smooth Pursuit Eye Tracking in Twins: A Critical Commentary

Author Affiliations

Department of Psychology Harvard University 33 Kirkland St Cambridge, MA 02138
Department of Psychiatry Harvard Medical School McLean Hospital Belmont, Mass
School of Orthoptics Latrobe University Bundoora, Victoria, Australia

Arch Gen Psychiatry. 1997;54(5):429-431. doi:10.1001/archpsyc.1997.01830170047007

LITMAN ET al1 conducted a study of smooth pursuit eye movements in monozygotic (MZ) twins discordant for schizophrenia. Although their data show a high concordance for eye tracking among pairs of clinically discordant twins with schizophrenia, confirming previous twin studies of eye tracking,2,3 they conclude that eye tracking dysfunction (ETD) is an outcome of having schizophrenia but does not represent a potential genetic indicator. We believe that this is an unwarranted conclusion and address this issue and other serious flaws in their design and interpretation.

SAMPLE AND SAMPLE SIZE  Litman et al compared 12 pairs of MZ twins discordant for schizophrenia with 12 pairs of MZ "normal" twins. Small samples permit nonrandom errors to exert a distorting effect on the data and, as we shall demonstrate, such distorting effects did occur in this particular study.Included in the group of twins discordant for schizophrenia are 2 probands who

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