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
Holzman PS, Levy DL, Matthysse SW, Abel LA. Smooth Pursuit Eye Tracking in TwinsA Critical Commentary. Arch Gen Psychiatry. 1997;54(5):429-431. doi:10.1001/archpsyc.1997.01830170047007