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SHORT REPORTS |
Department of Psychiatry, Division of Psychological Medicine, Section of General Psychiatry, Institute of Psychiatry, Kings College London, UK
Correspondence: Dr Eugenia Kravariti, Institute of Psychiatry, Department of Psychiatry, Division of Psychological Medicine, Box 63, De Crespigny Park, London SE5 8AF, UK. Tel: +44 20 7848 0331, fax: +44 20 7701 9041, e-mail: e.kravariti{at}iop.kcl.ac.uk
Declaration of interest None. Funding detailed in Acknowledgements.
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ABSTRACT |
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INTRODUCTION |
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METHOD |
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Parental socio-economic status at birth was assessed using the Standard Occupational Classification (Office of Population Censuses and Surveys, 1990) and handedness was determined using the Annett Scale (Annett, 1970) right-handed, n=97; left-handed, n=11. Participants were assessed using the Schedule for Affective Disorders and Schizophrenia Lifetime Version (Spitzer & Endicott, 1978), supplemented with additional clinical information to enable DSMIV (American Psychiatric Association, 1994) diagnoses to be made. All of the relatives were well at the time of assessment, but 19 relatives had fulfilled the criteria for major depressive, dysthymic, panic, anxiety or phobic disorder at some time in their life. To reach diagnoses for those individuals who were not directly assessed, a structured family history was taken from the most reliable informants in each family, using the Family History Research Diagnostic Criteria (Endicott et al, 1975) or, more recently, the Family Interview for Genetic Studies (Nurnberger et al, 1994), supplemented by clinical notes.
Genetic liability to schizophrenia was represented by a continuous quantitative measure which has been described in detail by McDonald et al (2004). Briefly, the calculation of this scale is based upon normal distribution theory and assumes a polygenic multifactorial liability threshold model of schizophrenia. Patients, relatives with schizophrenia-spectrum disorders, and unaffected relatives were initially assigned imputed liabilities within the normal distribution (2.78, 2.08 and 70.08 respectively), based upon the population prevalence of these disorders. For each family, imputed liabilities for all adult members were then adjusted to account for family size, age, affected/unaffected status and genetic relatedness as far as second degree from the index patient. Higher scores on this scale reflect higher presumed genetic liability to schizophrenia.
VerbalSpatial Contrast IQ
A short form of the Wechsler Adult Intelligence Scale Revised
(WAISR; Wechsler,
1981), consisting of vocabulary, comprehension, similarities,
block design and object assembly sub-test, was administered to all
participants. Employing the formulas provided by Canavan and Beckmann
(1993), we estimated scores
(with population mean=100, s.d.=15) on two orthogonal factors derived from a
principal-component analysis of the WAISR, namely General Ability IQ
(equivalent to Wechslers Full-Scale IQ; mean=100.1, s.d.=14.6) and
VerbalSpatial Contrast IQ (mean=95.6; s.d.=13.8). The latter is an
index of asymmetry of brain function and can produce markedly different
characterisations of ability compared with Verbal IQ (VIQ) and Performance IQ
(PIQ) (Canavan et al,
1986). Scores below 100 indicate asymmetry in favour of verbal
skills, whereas scores above 100 indicate asymmetry in favour of spatial
skills (Canavan & Beckmann,
1993).
Data analysis
The data were analysed using Stata version 7.0 for Windows. The association
of genetic liability with VerbalSpatial Contrast IQ was examined using
multiple regression analysis, adjusting for characteristics that showed
significant associations with VerbalSpatial Contrast IQ in preliminary
simple regression analyses (age, gender, number of years of education and
parental socio-economic status, but not handedness), and also adjusting for
General Ability IQ (larger differences between verbal and performance skills
are more common at the higher IQ levels
(Iverson et al, 2001),
although in our sample General Ability IQ and VerbalSpatial Contrast IQ
were non-significantly, albeit positively correlated (P40.1)). To
take into account the lack of independence of observations among related
participants, we used multilevel modelling, incorporating a robust estimator
for the variances of the regression coefficient estimates. The analysis was
repeated after excluding participants with any lifetime psychiatric diagnosis,
and those at the extremes (520 or 460, n=19) of the age distribution
(in whom the interpretation of the component scores requires some caution;
Canavan et al, 1986).
The analysis was also repeated separately for male and female participants
(because of the skewed gender distribution).
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RESULTS |
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DISCUSSION CUSSION |
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ACKNOWLEDGMENTS |
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REFERENCES |
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Amminger, G. P., Schlögelhofer, M., Lehner, T., et al (2000) Premorbid performance IQ deficit in schizophrenia. Acta Psychiatrica Scandinavica, 102, 414 422.[CrossRef][Medline]
Annett, M. (1970) A classification of hand preference by association analysis. British Journal of Psychology, 61, 303 321.[Medline]
Canavan, A. G. M. & Beckmann, J. (1993) Deriving principal-component IQ scores from the WAIS R. British Journal of Clinical Psychology, 32, 81 86.
Canavan, A. G. M., Dunn, G. & McMillan,T. M. (1986) Principal components of the WAIS R. British Journal of Clinical Psychology, 25, 81 86.
Endicott, J., Andreasen, N. & Spitzer, R. (1975) Family History Research Diagnostic Criteria. New York: Biometrics Research Division, New York State Psychiatric Institute.
Gottesman, I. I. & Gould, T. D. (2003) The
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McDonald, C., Bullmore, E. T., Sham, P., et al
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Nurnberger, J., Blehar, M., Kaufmann, C., et al (1994) Diagnostic interview for genetic studies. Rationale, unique features, and training. NIMH Genetics Initiative. Archives of General Psychiatry, 51, 849 859.[Abstract]
Office of Population Censuses and Surveys (1990) Standard Occupational Classification. Volume 3. London: Stationery Office.
Purcell, D. W., Lewine, R. R., Caudle, J., et al (1998) Sex differences in verbal IQperformance IQ discrepancies among probands with schizophrenia and normal volunteers. Journal of Abnormal Psychology, 107, 161 165.[CrossRef][Medline]
Spitzer, R. & Endicott, J. (1978) Schedule for Affective Disorders and Schizophrenia Lifetime Version. New York: New York State Psychiatric Institute.
Wechsler, D. (1981) Manual for Wechsler Adult Intelligence Scale Revised. San Antonio, TX: Psychological Corporation.
Received for publication December 27, 2004. Revision received April 12, 2005. Accepted for publication May 6, 2005.
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