IQ Variations across Time,Race,and Nationality: An Artifact of Differences in Literacy Skills

Abstract

A body of data on IQ collected over 50 years has revealed that average population IQ varies across time, race, and nationality. An explanation for these differences may be that intelligence test performance requires literacy skills not present in all people to the same extent. In eight analyses, population mean full scale IQ and literacy scores yielded correlations ranging from .79 to .99. In cohort studies, significantly larger improvements in IQ occurred in the lower half of the IQ distribution, affecting the distribution variance and skewness in the predicted manner. In addition, three Verbal subscales on the WAIS show the largest Flynn effect sizes and all four Verbal subscales are among those showing the highest racial IQ differences. This pattern of findings supports the hypothesis that both secular and racial differences in intelligence test scores have an environmental explanation: secular and racial differences in IQ are an artifact of variation in literacy skills. These findings suggest that racial IQ distributions will converge if opportunities are equalized for different population groups to achieve the same high level of literacy skills. Social justice requires more effective implementation of policies and programs designed to eliminate inequities in IQ and literacy.

Get full access to this article

View all access options for this article.

References

Agassiz

E. C.

(1850) The diversity of origin of the human races. Christian Examiner, 49, 110–145.

Alfonso

V. C.

Flanagan

D. P.

, & Radwan

(2005) The impact of the Cattell—Horn—Carroll theory on test development and interpretation of cognitive and academic abilities. In Flanagan

& Harrison

P. L.

(Eds.), Contemporary intellectual assessment: theories, tests, and issues. (2nd ed.) New York: Guilford. Pp. 185–202.

Benson

(2008) Cattell-Horn-Carroll cognitive abilities and reading achievement. Journal of Psychoeducational Assessment, 26, 27–41.

Bridgeman

, & Buttram

(1975) Race differences on nonverbal analogy test performance as a function of verbal strategy training. Journal of Educational Psychology, 67, 586–590.

Brody

(1992) Intelligence. San Diego, CA: Academic Press.

Campbell

J. R.

Hombo

C. M.

, & Mazzeo

(2000) Trends in academic progress: three decades of student performance, NCES 2000-469. Washington, DC: U.S. Department of Education, Office of Educational Research and Improvement, National Center for Education Statistics, NAEP 1999.

Carroll

J. B.

(1993) Human cognitive abilities: a survey of factor-analytic studies. New York: Cambridge Univer. Press.

Colom

Lluis-Font

J. M.

, & Andrés-Pueyo

(2005) The generational intelligence gains are caused by decreasing variance in the lower half of the distribution: supporting evidence for the nutrition hypothesis. Intelligence, 33, 83–91.

Coulombe

, & Tremblay

J. F.

(2006) Literacy and growth. Topics in Macroeconomics, 6, 1–32.

10.

Dickens

W. T.

, & Flynn

J. R.

(2006) Black Americans reduce the racial IQ gap: evidence from standardization samples. Psychological Science, 17, 913–920.

11.

Dolan

C. V.

Colom

Abad

F. J.

Wicherts

J. M.

Hessen

D. J.

, & van de Sluis

(2006) Multi-group covariance and mean structure modeling of the relationship between the WAIS–III common factors and sex and educational attainment in Spain. Intelligence, 34, 193–210.

12.

Eberhardt

J. L.

(2005) Imaging race. American Psychologist, 60, 181–190.

13.

Fischer

C. S.

Hout

Jankowski

M. J.

Lucas

S. R.

Swidler

, & Voss

(1996) Inequality by design: cracking the bell curve myth. Princeton, NJ: Princeton Univer. Press.

14.

Flanagan

D. P.

, & Ortiz

S. O.

(2004) CHC theory: a taxonomy of cognitive abilities. In Watson

Steurt T.

& Skinner

C. H.

(Eds.), Encyclopedia of school psychology. New York: Kluwer Academic. Pp. 139–141.

15.

Flynn

J. R.

(1984) The mean IQ of Americans: massive gains 1932 to 1978. Psychological Bulletin, 95, 29–51.

16.

Flynn

J. R.

(1987) Massive IQ gains in 14 nations: what IQ tests really measure. Psychological Bulletin, 101, 171–191.

17.

Flynn

J. R.

(2007) What is intelligence? Cambridge, UK: Cambridge Univer. Press.

18.

Flynn

J. R.

(2009) Requiem for nutrition as the cause of IQ gains: Raven's gains in Britain 1938 to 2008. Economics and Human Biology, 7, 18–27.

19.

Gardner

(1983) Frames of mind: the theory of multiple intelligence. New York: Basic Books.

20.

Gardner

(1999) Intelligence reframed: multiple intelligences for the 21st century. New York: Basic Books.

21.

Gelade

G. A.

(2008) The geography of IQ. Intelligence, 36, 495–501.

22.

Gould

S. J.

(1981) The mismeasure of man. New York: Norton.

23.

Greenfield

P. M.

(1998) The cultural evolution of IQ. In Neisser

(Ed.), The rising curve: long-term gains in IQ and related measures. Washington, DC: American Psychological Association. Pp. 81–123.

24.

Herrnstein

R. J.

, & Murray

(1994) The bell curve: intelligence and class structure in American life. New York: Free Press.

25.

Horn

J. L.

, & Noll

(1997) Human cognitive capabilities: Gf–Gc theory. In Flanagan

Genshaf

J. L.

, & Harrison

P. L.

(Eds.), Contemporary intellectual assessment: theories, tests, and issues. New York: Guilford. Pp. 53–91.

26.

Huang

M-H.

, & Hauser

R. M.

(1998) Trends in Black–White test score differentials: II. The WORDSUM Vocabulary Test. In Neisser

(Ed.), The rising curve: long-term gains in IQ and related measures. Washington, DC: American Psychological Association. Pp. 303–332.

27.

Hunt

, & Wittmann

(2006) National intelligence and national prosperity. Intelligence, 36, 1–9.

28.

Jensen

A. R.

(1998) The g factor. Westport, CT: Praeger.

29.

Kilburn

M. R.

Hanser

L. M.

, & Klerman

J. A.

(1998) Estimating AFQT scores for National Educational Longitudinal Study (NELS) respondents. Santa Monica, CA: RAND Distribution Services.

30.

Kirsch

I. S.

Jungeblut

Jenkins

, & Kolstad

(1993) Adult literacy in America: a first look at the results of the National Adult Literacy Survey. Princeton, NJ: Educational Testing Service.

31.

Lynn

(1998) In support of nutrition theory. In Neisser

(Ed.), The rising curve: long-term gains in IQ and related measures. Washington, DC: American Psychological Association. Pp. 207–215.

32.

Lynn

(2006) Race differences in intelligence: an evolutionary analysis. Augusta, GA: Washington Summit.

33.

Lynn

(2008) The global bell curve. Augusta, GA: Washington Summit.

34.

Lynn

, & Mikk

(2009) National IQs predict educational attainment in math, reading and science across 56 nations. Intelligence, 37, 305–310.

35.

Lynn

, & Vanhanen

(2002) IQ and the wealth of nations. Westport, CT: Praeger.

36.

Lynn

, & Vanhanen

(2006) IQ and global inequality. Augusta, GA: Washington Summit.

37.

Martorell

(1998) Nutrition and the worldwide rise in IQs. In Neisser

(Ed.), The rising curve: long-term gains in IQ and related measures. Washington, DC: American Psychological Association. Pp. 183–206.

38.

McGrew

K. S.

(2005) The Cattell-Horn-Carroll theory of cognitive abilities: past, present, and future. In Flanagan

D. P.

& Harrison

P. L.

(Eds.), Contemporary intellectual assessment: theories, tests, and issues. (2nd ed.) New York: Guilford. Pp. 136–182.

39.

McGrew

K. S.

, & Flanagan

D. P.

(1998) The Intelligence Test Desk Reference (ITDR): Gf–Gc cross-battery assessment. New York: The Guilford Press.

40.

Mingroni

M. A.

(2004) The secular rise in IQ: giving heterosis a closer look. Intelligence, 32, 65–83.

41.

Murray

(2006) Changes over time in the Black–White difference on mental tests: evidence from the children of the 1979 cohort of the National Longitudinal Survey of Youth. Intelligence, 34, 527–540.

42.

National Assessment of Adult Literacy. (2004) A first look at the literacy of America's adults in the 21st century. Washington, DC: U.S. Department of Education, Institute of Education Sciences, NCES 2006–470.

43.

National Center for Education Statistics. (1993) 120 years of American education: a statistical portrait. ( Snyder

, Ed.) Washington, DC: U.S. Department of Education, Institute of Education Sciences, NCES 1993.

44.

Neisser

(1998) The rising curve: long-term gains in IQ and related measures. Washington, DC: American Psychological Association.

45.

Nettelbeck

, & Wilson

(2004) The Flynn effect: smarter not faster. Intelligence, 32, 85–93.

46.

Nott

J. C.

, & Gliddon

J. R.

(1854) Types of mankind. Philadelphia: Lippincot, Grambo.

47.

Perie

, & Moran

(2005) NAEP 2004 trends in academic progress: three decades of student performance in reading and mathematics. Washington, DC: U.S. Department of Education, Institute of Education Sciences, National Center for Education Statistics, NCES 2005–464.

48.

Rindermann

(2007a) Relevance of education and intelligence at the national level for the economic welfare of people. Intelligence, 36, 127–142.

49.

Rindermann

(2007b) The g factor of international cognitive ability comparisons: the homogeneity of results in PISA, TIMSS, PIRLS, and IQ tests across nations. European Journal of Personality, 21, 667–706.

50.

Rushton

J. P.

, & Jensen

A. R.

(2005) Thirty years of research on race differences in cognitive ability. Psychology, Public Policy, and Law, 11, 235–294.

51.

Rushton

J. P.

, & Jensen

A. R.

(2006) The totality of available evidence shows the race IQ gap still remains. Psychological Science, 17, 921–922.

52.

Rushton

J. P.

, & Rushton

E. W.

(2003) Brain size, IQ, and racial group differences: evidence from musculoskeletal traits. Intelligence, 31, 139–155.

53.

Shaw

Greenstein

Lerch

Clasen

Lenroot

Gogtay

Evans

Rapoport

, & Giedd

(2006) Intellectual ability and cortical development in children and adolescents. Nature, 440, 676–679.

54.

Sigman

, & Whaley

S. E.

(1998) The role of nutrition in the development of intelligence. In Neisser

(Ed.), The rising curve: long-term gains in IQ and related measures. Washington, DC: American Psychological Association. Pp. 155–182.

55.

Spearman

C. S.

(1904) General intelligence objectively determined and measured. American Journal of Psychology, 15, 201–293.

56.

Spearman

C. S.

(1927) The abilities of man. New York: Macmillan.

57.

Sternberg

R. J.

(1980) Sketch of a componential subtheory of human intelligence. Behavioral & Brain Sciences, 3, 573–584.

58.

Sternberg

R. J.

(1999) Successful intelligence. New York: Plume.

59.

Sticht

T. G.

Caylor

J. S.

Kern

R. P.

, & Fox

L. C.

(1972) Project REALISTIC: determination of adult functional literacy skill levels. Reading Research Quarterly, 7, 424–465.

60.

Sundet

J. M.

Barlaug

D. G.

, & Torjussen

T. M.

(2004) The end of the Flynn effect? A study of secular trends in mean intelligence test scores of Norwegian conscripts during half a century. Intelligence, 32, 349–362.

61.

Taylor

E. K.

(1947) Tables for the determination of the significance of skewness and of the significance of the difference in the skewness of two independent distributions. Psychometrika, 12, 111–125.

62.

Teasdale

T. W.

, & Owen

D. R.

(2005) A long-term rise and recent decline in intelligence test performance: the Flynn effect in reverse. Personality and Individual Differences, 39, 837–843.

63.

Tiegs

E. W.

, & Clark

W. W.

(1959) Survey of Reading Achievement: junior high level (grades 7-9). Monterey, CA: California Test Bureau.

64.

United Nations Development Program. (2006) United Nations Human Development Report: beyond scarcity: power, poverty and global water crisis. Basingstoke, UK: Palgrave Macmillan.

65.

Vernon

P. A.

Wickett

J. A.

Bazana

, & Stelmack

R. M.

(2000) The neuropsychology and psychophysiology of human intelligence. In Sternberg

R. J.

(Ed.), Handbook of intelligence. Cambridge, UK: Cambridge Univer. Press. Pp. 245–264.

66.

Wechsler

(1949) The Wechsler Intelligence Scale for Children. New York: Psychological Corp.

67.

Wechsler

(1955) The Wechsler Adult Intelligence Scale. New York: Psychological Corp.

68.

Wicherts

J. M.

Dolan

C. V.

Hessen

D. J.

Oosterveld

van Baal

G. C. M.

Boomsma

D. I.

, & Span

M. M.

(2004) Are intelligence tests measurement invariant over time? Investigating the nature of the Flynn effect. Intelligence, 32, 509–537.

69.

Williams

W. M.

(1998) Are we raising smarter children today? School- and home-related influences on IQ. In Neisser

(Ed.), The rising curve: long-term gains in IQ and related measures. Washington, DC: American Psychological Association. Pp. 125–154.

70.

Yerkes

R. M.

(Ed.) (1921) Psychological examining in the United States Army. Memoirs of the National Academy of Sciences, 15, 1–8.