Is the Flynn effect on g?: A meta-analysis

Black/White differences in mean IQ have been clearly shown to strongly correlate with g loadings, so large group differences on subtests of high cognitive complexity and small group differences on subtests of low cognitive complexity. IQ scores have been increasing over the last half century, a phenomenon known as the Flynn effect. Flynn effect gains are predominantly driven by environmental factors. Might these factors also be responsible for group differences in intelligence? The empirical studies on whether the pattern of Flynn effect gains is the same as the pattern of group differences yield conflicting findings. A psychometric meta-analysis on all studies with seven or more subtests reporting correlations between g loadings and standardized score gains was carried out, based on 5 papers, yielding 11 data points (total N = 16,663). It yielded a true correlation of − .38, and none of the variance between the studies could be attributed to moderators. It appears that the Flynn effect and group differences have different causes. Suggestions for future research are discussed.     

What has caused the Flynn effect? Secular increases in the Development Quotients of infants

Results of five studies show that during the second half of the twentieth century there were increases in the Development Quotients (DQs) of infants in the first two years of life. These gains were obtained for the Bayley Scales in the United States and Australia, and for the Griffiths Test in Britain. The average of 19 data points is a DQ gain of approximately 3.7 DQ points per decade. Similar gains of approximately 3.9 IQ points per decade have been present among preschool children aged 4–6 years. These gains are about the same as the IQ gains of school age students and adults on the Wechsler and Binet tests. This suggests that the same factor has been responsible for all these secular gains. This rules out improvements in education, greater test sophistication, etc. and most of the other factors that have been proposed to explain the Flynn effect. It is proposed that the most probable factor has been improvements in pre-natal and early post-natal nutrition.     

The Flynn effect in South Africa

This is a study of secular score gains in South Africa. The findings are based on representative samples from datasets utilized in norm studies of popular mainstream intelligence batteries such as the WAIS as well as widely used test batteries which were locally developed and normed in South Africa. Flynn effects were computed in three ways. First, studies where two different groups take the same test, with several years in between, using representative or comparable samples were used. Second, studies where the same group takes two different test batteries at a specific time were used. Third, the score differences between English- and Afrikaans-speaking Whites in South Africa in the 20th century were compared. The Flynn effect in White groups in South Africa is somewhat smaller than the Flynn effect in Western, industrialized countries (total N = 6534), and the Flynn effect in Indian groups is substantially smaller (total N = 682). Non-verbal IQ scores surpassed increases in verbal IQ scores. The findings from English- and Afrikaans-speaking Whites evidence a leveling out of differences in score gains over the 20th century (total N = 79,310). A meta-regression analysis showed no clear support for the moderators a) method used for computing the Flynn effect gain, b) type of test battery, c) time span, d) quality of the sample, and e) average age of sample.     

The secular rise in IQs in the Netherlands: Is the Flynn effect on g?

IQ scores have been increasing over the last half century, a phenomenon known as the Flynn effect. In this study, we focused on the question to what extent these secular gains are on the g factor. Two IQ batteries: the Interest-School achievement-Intelligence Test (ISI) and the Groningen Final Examination Primary Education (GALO) yielded small and modest negative correlations between standardized gains and g loadings. As these studies employ large samples this suggests that the combined literature now shows a modest negative relationship between d (the secular change in test score) and g.     

Heterosis doesn't cause the Flynn effect: a critical examination of Mingroni (2007)

Mingroni (2007) proposed that heterosis or hybrid vigor may be the principal driver of the Flynn effect--the tendency for IQ scores to increase at a rate of approximately 3 points per decade. This model was presented as a resolution to the IQ paradox--the observation that IQ scores have been increasing despite their high adult heritability--on the basis that substantial changes in IQ can only be accounted for by changes in underlying genetic factors. It is here argued that this model is predicated upon a misconception of the Flynn effect, which is most pronounced on the least g-loaded components of cognitive ability tests and is uncorrelated with genetic effects such as inbreeding depression scores (which are correlated with the g loadings of tests). Evidence supportive of the recently proposed life history model of the Flynn effect is presented. In the discussion, other theoretical objections to the heterosis model are also considered. On this basis, it is concluded that the Flynn effect is strongly entwined with developmental status and that heterosis cannot be its principal cause.     

The Flynn effect,group differences,and g loadings

Flynn effect gains are predominantly driven by environmental factors. Might these factors also be responsible for group differences in intelligence? Group differences in intelligence have been clearly shown to strongly correlate with g loadings. The empirical studies on whether the pattern of Flynn effect gains is the same as the pattern of group differences yield conflicting findings. We present new evidence on the topic using a number of datasets from the US and the Netherlands. Score gains and g loadings showed a small negative average correlation. The general picture is now that there is a small, negative correlation between g loadings and Flynn effect gains. It appears that the Flynn effect and group differences have different causes.     

Item-response theory modeling of IQ gains (the Flynn effect) on crystallized intelligence: Rodgers' hypothesis yes,Brand's hypothesis perhaps

Potential explanations for generational intelligence test score gains continue to be subject to intense debate and scrutiny in the scientific community. However, the explanatory value of some of the proposed causes remains difficult to determine, since only little empirical evidence is available. To clarify the role of two scarcely investigated theories accounting for the Flynn effect, this study set out to examine the role of changing test-taking behavior (Brand's hypothesis) and of a narrowing of the IQ ability distribution (Rodgers' hypothesis). Archival records of crystallized intelligence test performance over a time-span of 17 years of a large number of psychiatric inpatients and outpatients in Austria were investigated (N = 5445; 1978–94). This sample was particularly suitable to investigate our hypotheses since participants were under no pressure to perform which makes observed changes in test taking behavior attributable to personal style and ability rather than differential performance in pressure situations. Analytical approaches of both classical test theory and item response theory (IRT) yielded gains of 1.0 to 2.4 IQ points per decade. Test-taking behavior indicative of guessing and decreasing population IQ variability appeared to contribute both to IQ test score gains. IRT-based analyses showed that gains were largely preserved when controlling for highest educational qualification, while the test instrument showed measurement invariance between cohorts. However, IRT-based results also suggested that changes in test-taking behavior might not necessarily reflect increased guessing, but item drift instead. In all, this evidence emphasizes better performance of individuals of the lower tail of the IQ ability distribution in more recent years as one important contributing factor for generational IQ test score gains.     

Secular declines in Spearman's g: some evidence from the United States

The Flynn Effect (J. Flynn, 1987) refers to the apparent increases in intelligence quotient (IQ) observed over the past few decades. A related phenomenon is that the variance in test scores accounted for by Spearman's g (C. Spearman, 1904) varies according to IQ level. That is, g accounts for less variance in high IQ groups than in low IQ groups. Spearman termed this variant aspect of g the "law of diminishing returns." This study extends prior research on the Flynn Effect and the law of diminishing returns by examining changes in the statistical importance of Spearman's g that may accompany secular increases in IQ. Based on the standardization data from the United States versions of the Wechsler scales (i.e., Wechsler Preschool and Primary Scales of Intelligence, D. Wechsler, 1967; Wechsler Intelligence Scale for Children, D. Wechsler, 1949; Wechsler Adult Intelligence Scale, D. Wechsler, 1955), this study indicated that, in most cases, the statistical significance of Spearman's g has indeed declined over the past several years. A. R. Jensen (1998) suggested that the components of the Flynn Effect warrant special investigation. The present study addresses this issue, in part by analyzing the Verbal and Performance subscales of the Wechsler scales. Additionally, this study further confirms the law of diminishing returns as applicable to different ages and time periods.     

Secular Declines in Spearman's g: Some Evidence from the United States

The Flynn Effect (J. Flynn, 1987) refers to the apparent increases in intelligence quotient (IQ) observed over the past few decades. A related phenomenon is that the variance in test scores accounted for by Spearman's g (C. Spearman, 1904) varies according to IQ level. That is, g accounts for less variance in high IQ groups than in low IQ groups. Spearman termed this variant aspect of g the “law of diminishing returns.” This study extends prior research on the Flynn Effect and the law of diminishing returns by examining changes in the statistical importance of Spearman's g that may accompany secular increases in IQ. Based on the standardization data from the United States versions of the Wechsler scales (i.e., Wechsler Preschool and Primary Scales of Intelligence, D. Wechsler, 1967; Wechsler Intelligence Scale for Children, D. Wechsler, 1949; Wechsler Adult Intelligence Scale, D. Wechsler, 1955), this study indicated that, in most cases, the statistical significance of Spearman's g has indeed declined over the past several years. A. R. Jensen (1998) suggested that the components of the Flynn Effect warrant special investigation. The present study addresses this issue, in part by analyzing the Verbal and Performance subscales of the Wechsler scales. Additionally, this study further confirms the law of diminishing returns as applicable to different ages and time periods.     

In the Netherlands the anti-Flynn effect is a Jensen effect

In this study, 63 observations of secular IQ changes (both Flynn and anti-Flynn effects) are collected from three demographically diverse studies of the Dutch population for the period 1975–2005 (representing the 1950–1990 birth cohorts), along with data on g loadings and subtest reliabilities. The method of correlated vectors is used to explore the association between Flynn and anti-Flynn effect magnitudes, both independently and together, and the g loadings of subtests. Despite a positive vector correlation the Flynn effects are not associated with the Jensen effect (r = .307, ns, N = 36), however the anti-Flynn effects are (r = .406, P = .05, N = 27). Combined, the vector correlation becomes negative but non-significant (r = −.111, ns, N = 63). Declines due to the anti-Flynn effect are estimated at −4.515 points per decade, whereas gains due to the Flynn effect are estimated at 2.175 points per decade. The N-weighted net of these is a loss of −1.350 points per decade, suggesting an overall tendency towards decreasing IQ in the Netherlands with respect to these cohorts. The Jensen effect on the anti-Flynn effect suggests that it may be related to bio-demographic changes within the Netherlands which have reduced ‘genetic-g’, despite the presence of large, parallel gains on subtests that may be relatively more sensitive to cultural-environmental improvements.     

An estimate of the Flynn Effect: changes in IQ and subtest gains of 10-yr-old French children between 1965 and 1988

The significant increase in IQ in the industrialised countries during recent decades, known as the eponymous Flynn Effect, tends to be given numerous explanations (progress in schooling, television, nutrition, etc.). One way of studying this effect consists of assessing in which subtests the change in IQ takes place. Using the comparison among 120,000 French children on an intelligence test in 1965 and the results of 8640 children on the same test in 1988, Greenfield's hypothesis is discussed: that television and other media using graphical representations have induced an improvement in spatial and visual capacities to the detriment of capacities relating to vocabulary. The results are compatible with Greenfield's hypothesis, although other explanations are still possible.     

Secular IQ increases by epigenesis? The hypothesis of cognitive genotype optimization

The short timescale of massive secular IQ gains ("Flynn Effect") is inconsistent with positive selection of a recent gene mutation, but other genetic mechanisms are possible. Principles of evolutionary psychology, combined with secular trends, suggest an epigenetic explanation: the Cognitive Genome Optimization Hypothesis. Per life-history theory, favorable secular trends may change the phenotypic expression of the genotype which controls the neurophysiology of problem solving. The hypothesis posits two intermediate steps between reliable nutrition (the starting point) and higher IQs (ending point): (1) Earlier cognitive maturation and (2) further calibration of cognitive function by reliable social resources (cultural complexity, mandatory education). Unlike earlier generations, more resources can be deployed to cognitive maturation than to physical survival, and more time is available to calibrate cognitive processing into the upper end of the trait value range for intelligence. The secular trend of earlier puberty timing is critical: data show an association between puberty and higher IQ.     

Using Score Equating and Measurement Invariance to Examine the Flynn Effect in the Wechsler Adult Intelligence Scale

The Flynn effect (FE; i.e., increase in mean IQ scores over time) is commonly viewed as reflecting population shifts in intelligence, despite the fact that most FE studies have not investigated the assumption of score comparability. Consequently, the extent to which these mean differences in IQ scores reflect population shifts in cognitive abilities versus changes in the instruments used to measure these abilities is unclear. In this study, we used modern psychometric tools to examine the FE. First, we equated raw scores for each common subtest to be on the same scale across instruments. This enabled the combination of scores from all three instruments into one of 13 age groups before converting raw scores into Z scores. Second, using age-based standardized scores for standardization samples, we examined measurement invariance across the second (revised), third, and fourth editions of the Wechsler Adult Intelligence Scale. Results indicate that while scores were equivalent across the third and fourth editions, they were not equivalent across the second and third editions. Results suggest that there is some evidence for an increase in intelligence, but also call into question many published FE findings as presuming the instruments' scores are invariant when this assumption is not warranted.     

The Flynn effect and U.S. policies: the impact of rising IQ scores on American society via mental retardation diagnoses

Over the last century, IQ scores have been steadily rising, a phenomenon dubbed the Flynn effect. Because of the Flynn effect, IQ tests are periodically renormed, making them harder. Given that eligibility for mental retardation (MR) services relies heavily on IQ scores, renormed tests could have a significant impact on MR placements. In longitudinal IQ records from 9 sites around the country, students in the borderline and mild MR range lost an average of 5.6 points when retested on a renormed test and were more likely to be classified MR compared with peers retested on the same test. The magnitude of the effect is large and affects national policies on education, social security, the death penalty, and the military. This paper reports the perceptions of professionals as they relate to IQ score fluctuations in normal, borderline, and/or MR populations.     

An increase of intelligence in China 1986–2012

The Flynn effect has been widely researched in Western and European nations, while it has been comparatively understudied in Asian countries. This study examines possible Flynn effects in China from 1985 to 86 and to 2011–12. Results are reported for an IQ increase among 12 year olds on the Full Scale IQ WISC-R (Wechsler Intelligence Scale for Children-Revised) of 6.19 IQ points, a gain on the Performance IQ of 6.55 IQ points, and a gain on the Verbal IQ of 1.91 IQ points.     

Fluid intelligence but not vocabulary has increased in Britain, 1979–2008

New standardizations of the Coloured and Standard Progressive Matrices in Britain in 2007 and 2008 show that on the Coloured Progressive Matrices the intelligence of 4 to 11 year olds increased over the years 1982–2007 by 8 IQ points, representing a gain of 3.2 IQ points a decade, and on the Standard Progressive Matrices the intelligence of 7 to 15 year olds increased over the years 1979–2008 by 6.2 IQ points, representing a gain of 1.85 IQ points a decade. IQ gains were greater among those at the low ability level. New standardizations of the Crichton Vocabulary Scale and the Mill Hill Vocabulary Scale in Britain in 2007 and 2008 show a marginal decline on the Crichton Vocabulary Scale among 4 to 11 year olds, and a marginal increase on the Mill Hill Vocabulary Scale among 7 to 15 year olds, suggesting there has been little change in vocabulary over the same time periods. These results are more consistent with the improvements in nutrition theory than with the improvements in education and greater cognitive stimulation theories of the secular gains in intelligence.     

Ability differentials between nations are unlikely to disappear

Comments on the original article, "Intelligence: New findings and theoretical developments," by R. E. Nisbett, J. Aronson, C. Blair, W. Dickens, J. Flynn, D. F. Halpern, and E. Turkheimer (see record 2011-30298-001). This comment challenges Nisbett et al's argument that Flynn effect gains will eliminate cross-national IQ inequalities "by the end of the 21st century and falsify the hypothesis that some nations lack the intelligence to fully industrialize" (p. 140). The present authors find that this optimism is not justified by the evidence. In Europe and the United States, Flynn effects are indeed rare in cohorts born after about 1980. Furthermore, it is necessary to distinguish between accelerated childhood development and higher adult intelligence. (PsycINFO Database Record (c) 2012 APA, all rights reserved).     

A critique of the Flynn Effect: massive IQ gains, methodological artifacts, or both?

The Flynn Effect proposed by Flynn (1984; 1987) is reviewed and evaluated. Even in the presence of a skeptical and critical scrutiny of the effect, it appears that there is more than just methodological artifact to be explained. But the acceptance of the effect has been too quick. The proper explanations for the effect will not be meaningful until the nature of the effect is much better understood than it is now. Six questions are raised that have not been adequately answered. Two criticisms of the logic underlying the Flynn Effect are presented — one showing that even if IQ and SAT are highly correlated, their secular means will not necessarily track one another; the second showing that results by Flynn, (1984) are as consistent with a changing IQ variance as with a changing mean. The second of these is empirically evaluated with a re-analysis of a subset of the sources of Flynn's original 1984 data. Finally, 10 research strategies and designs are suggested that would help us better understand the effect. The critique is developed with the goal of clarifying the nature, meaning and causes of the Flynn Effect. The author hopes that this critique will stimulate both healthy skepticism about the Flynn Effect and careful research into its actual causes.     

Comparability of IQ scores over time

This study investigates the comparability of IQ scores. Three cohorts (1933/36, 1997/98, 2006) of Estonian students (N = 2173) are compared using the Estonian National Intelligence Test. After 72 years the secular rise of the IQ test scores is.79 SD. The mean .16 SD increase in the last 8 years suggests a rapid increase of the Flynn Effect (FE) in Estonia. The measurement is not strictly invariant, which means that the IQ scores of different cohorts are not directly comparable. Less than perfect comparability of test scores is caused by at least two factors: time between measurements and societal/educational changes between cohorts. As was to be expected, the meaning of subtests and the meaning of the g score have changed over time.     

Expanding variance and the case of historical changes in IQ means: a critique of Dickens and Flynn (2001)

The Flynn effect is the rise in mean IQ scores during the 20th century, amounting to about 0.33 IQ points per year. Many theoretical explanations have been proposed, though none are universally accepted. W. Dickens and J. R. Flynn's (2001) new approach explains the large IQ changes by means of recursive models of IQ growth. A salient feature of their models is that IQ phenotypes and their supportive environments are correlated; in addition, environmental effects can rebound on phenotypic IQ to increase or lower IQ. In this critique, the authors examine an empirical challenge to their models, which typically imply large changes in IQ variance. However, the historical rise in IQ mean level has not been accompanied by substantial variance changes, a finding inconsistent with the properties of the proposed model.