title: Measurement confounding affects the extent to which verbal IQ explains social gradients in mortality.
abstract: BACKGROUND
  IQ is thought to explain social gradients in mortality. IQ scores are based roughly equally on Verbal IQ (VIQ) and Performance IQ tests. VIQ tests, however, are suspected to confound true verbal ability with socioeconomic status (SES), raising the possibility that associations between SES and IQ scores might be overestimated. We examined, first, whether two of the most common types of VIQ tests exhibited differential item functioning (DIF) favouring persons of higher SES and/or majority race/ethnicity. Second, we assessed what impact, if any, this had on estimates of the extent to which VIQ explains social gradients in mortality.
  
  
  METHODS
  Data from the General Social Survey-National Death Index cohort, a US population representative dataset, was used. Item response theory models queried social-factor DIF on the Thorndike Verbal Intelligence Scale and Wechsler Adult Intelligence Scales, Revised Similarities test. Cox models examined mortality associations among SES and VIQ scores corrected and uncorrected for DIF.
  
  
  RESULTS
  When uncorrected for DIF, VIQ was correlated with income, education, occupational prestige and race, with correlation coefficients ranging between |0.12| and |0.43|. After correcting for DIF, correlations ranged from |0.06| to |0.16|. Uncorrected VIQ scores explained 11-40% of the Relative Index of Inequalities in mortality for social factors, while DIF-corrected scores explained 2-29%.
  
  
  CONCLUSIONS
  Two of the common forms of VIQ tests appear confound verbal intelligence with SES. Since these tests appear in most IQ batteries, circumspection may be warranted in estimating the amount of social inequalities in mortality attributable to IQ.
authors.affiliation: Department of Psychiatry, University of Rochester Medical Center, Rochester, New York, USA.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Chapman
authors.affiliation: Department of Family Medicine, University of Rochester Medical Center, Center for Communication and Disparities Research, Rochester, New York, USA Department of Public Health Sciences, University of Rochester Medical Center, Rochester, New York, USA.
authors.forename: Kevin
authors.initials: K
authors.lastname: Fiscella
authors.affiliation: Department of Psychiatry, University of Rochester Medical Center, Rochester, New York, USA Department of Family Medicine, University of Rochester Medical Center, Center for Communication and Disparities Research, Rochester, New York, USA.
authors.forename: Paul
authors.initials: P
authors.lastname: Duberstein
authors.affiliation: Department of Society, Human Development, and Health, Harvard University School of Public Health, Boston, Massachusetts, USA.
authors.forename: Ichiro
authors.initials: I
authors.lastname: Kawachi
authors.affiliation: Department of Health Management and Policy, Columbia University, Mailman School of Public Health, New York, New York, USA.
authors.forename: Peter
authors.initials: P
authors.lastname: Muennig
mesh_terms: D000328:Adult; D015986:Confounding Factors, Epidemiologic; D005260:Female; D006801:Humans; D007360:Intelligence; D007361:Intelligence Tests; D008297:Male; D008875:Middle Aged; D009026:Mortality; D016016:Proportional Hazards Models; D012923:Social Class; D011795:Surveys and Questionnaires; D014481:United States / Q000453:epidemiology; D014705:Verbal Behavior
keywords: Cognition; Mortality; Social Epidemiology
references.citation: Am J Epidemiol. 2010 Feb 15;171(4):399-406
references.pmid: 20080810
references.citation: Soc Sci Med. 1997 Mar;44(6):757-71
references.pmid: 9080560
references.citation: Am J Epidemiol. 2005 May 1;161(9):831-9
references.pmid: 15840615
references.citation: J Genet Psychol. 1972 Dec;121(2d Half):179-86
references.pmid: 4650422
references.citation: J Int Neuropsychol Soc. 2002 Mar;8(3):341-8
references.pmid: 11939693
references.citation: Psychol Sci Public Interest. 2010 Aug;11(2):53-79
references.pmid: 26168413
references.citation: Proc Natl Acad Sci U S A. 2011 May 10;108(19):7716-20
references.pmid: 21518867
references.citation: Behav Brain Sci. 2001 Dec;24(6):1109-1110
references.pmid: 18241401
references.citation: Perspect Psychol Sci. 2007 Dec;2(4):313-45
references.pmid: 26151971
references.citation: Lancet. 2005 May 21-27;365(9473):1765-6
references.pmid: 15910942
references.citation: J Epidemiol Community Health. 2007 Nov;61(11):997-1003
references.pmid: 17933959
references.citation: Arch Clin Neuropsychol. 2011 Aug;26(5):396-404
references.pmid: 21636602
references.citation: J Epidemiol Community Health. 2014 Aug;68(8):728-33
references.pmid: 24729404
references.citation: J Gerontol B Psychol Sci Soc Sci. 2002 Nov;57(6):P548-58
references.pmid: 12426438
references.citation: BMC Res Notes. 2011 Oct 06;4:385
references.pmid: 21978529
references.citation: J Clin Exp Neuropsychol. 2004 Oct;26(7):903-20
references.pmid: 15742541
references.citation: Int J Epidemiol. 2011 Jun;40(3):626-44
references.pmid: 21037248
references.citation: Am J Epidemiol. 2009 Apr 1;169(7):901-8
references.pmid: 19251755
references.citation: Am J Epidemiol. 1997 Jan 1;145(1):72-80
references.pmid: 8982025
references.citation: Stat Med. 2004 Dec 30;23(24):3803-20
references.pmid: 15580597
references.citation: Am J Public Health. 1989 Mar;79(3):340-9
references.pmid: 2916724
references.citation: J Pers Soc Psychol. 1995 Nov;69(5):797-811
references.pmid: 7473032
references.citation: Am Psychol. 2012 Feb-Mar;67(2):130-59
references.pmid: 22233090
references.citation: Am J Epidemiol. 2009 Mar 1;169(5):606-15
references.pmid: 19147741
references.citation: J Epidemiol Community Health. 2007 Mar;61(3):241-7
references.pmid: 17325403
references.citation: Exp Aging Res. 2008 Jan-Mar;34(1):80-100
references.pmid: 18189169
references.citation: Biostatistics. 2006 Apr;7(2):213-24
references.pmid: 16192414
references.citation: Int J Epidemiol. 2010 Oct;39(5):1362-9
references.pmid: 20504860
references.citation: Epidemiol Perspect Innov. 2009 Sep 04;6:4
references.pmid: 19732410
references.citation: Am J Epidemiol. 1994 Sep 15;140(6):489-99
references.pmid: 8067342
references.citation: Psychol Methods. 2003 Jun;8(2):185-205
references.pmid: 12924814
references.citation: J Pers Soc Psychol. 2004 Jan;86(1):174-99
references.pmid: 14717635