DNA markers associated with general and specific cognitive abilities

Multivariate quantitative genetic research suggests a hierarchical model of cognitive abilities where genetic effects are largely general, cutting across most cognitive abilities. Some genetic effects, however, are specific to certain cognitive abilities. These results lead to a hypothesis for molecular genetic research: Although most genes associated with one cognitive ability will be related to other cognitive abilities, some genes will be specific to a particular cognitive ability. The current research explored this hypothesis in an analysis of data in specific cognitive abilities from 86 children from 6 to 12 years of age from a larger allelic association study of general cognitive ability. Eight DNA markers were entered simultaneously in separate multiple regression analyses predicting each of four specific cognitive ability factors (Verbal, Spatial, Perceptual Speed, Memory), as well as WISC-R subtest scores. Four marjers (CTGB33, EST00083, HLA, and SOD2) showed similar effects across the cognitive ability scales, suggesting that they are related to general cognitive ability (g). These associations became negligible when the effects of ‘g’ (WISC-R IQ) were removed. Three markers (ADH5, DM, and NGFB) continued to be significantly associated with specific cognitive ability scales after the effects of ‘g’ were removed. Although preliminary, these molecular genetic results support the hierarchical model predicted by quantitative genetic research.     

Skull size and intelligence, and King Robert Bruce's IQ

An estimate of someone's IQ is a potentially informative personal datum. This study examines the association between external skull measurements and IQ scores, and uses the resulting regression equation to provide an estimate of the IQ of King Robert I of Scotland (Robert Bruce, 1274–1329). Participants were 48 relatively healthy Caucasian men (age 71–76 years) resident in Scotland. Using magnetic resonance imaging data, intracranial volume estimated from external skull length and width correlated greater than .5 (p < .001) with measured intracranial area, which correlates very highly with brain volume. IQ scores estimated from the National Adult Reading Test (NART) correlated .56 (p < .001) with measured intracranial area, and .49 (p < .01) with estimated intracranial volume based on external skull width and length. The partial correlation coefficient of this latter association was .25 (p = .09) after adjustment for measured intracranial area. Thus, actual intracranial area accounts for about 74% of the variance shared by NART and estimated intracranial volume. A cast of the skull of Robert Bruce was measured and its intracranial volume estimated. A regression equation between IQ and estimated intracranial volume, based on data from the 48 subjects, estimated the IQ of Robert Bruce at about 128 (95% confidence interval 106 to  130), i.e. almost two standard deviations above the mean. NART scores show a ceiling effect, so this estimated IQ might be an underestimate. Robert Bruce's estimated high IQ is congruent with his military, political and other intellectual achievements.     

Black–White differences on IQ and grades: The mediating role of elementary cognitive tasks

The relationship between IQ scores and elementary cognitive task (ECT) performance is well established, with variance on each largely reflecting the general factor of intelligence, or g. Also ubiquitous are Black–White mean differences on IQ and measures of academic success, like grade point average (GPA). Given C. Spearman's (Spearman, C. (1927). The Abilities of Man. New York: Macmillan) hypothesis that group differences vary directly with a test's g loading, we explored whether ECT performance could mediate Black–White IQ and GPA differences. Undergraduates (139 White and 40 Black) completed the Wonderlic Personnel Test, followed by inspection time and choice reaction time ECTs. Despite restriction of range, ECT performance completely mediated Black–White differences on IQ (d = .45). Group differences on GPA (d = .73), however, were larger and ECT performance did not mediate them. We discuss findings in light of Spearman's hypothesis.     

Inspection time and intelligence in 7-yr-old children: A follow-up

Remeasures of inspection time (IT) were obtained from 30 of the 47 children reported in Nettelbeck and Young (Personality and Individual Differences, 10, 605–614, 1989). Ages ranged from 7 yr–0 months to 7–11 and Full Scale IQ (WISC-R) ranged from 93 to 142 (mean = 116.5 SD = 12.2). Correlations between IT and the WISC-R subtests, scales and factors used in the earlier study were generally statistically significant: and similar to those found earlier. The correlation between IT and Full Scale IQ was — 0.49, with those children whose IQs were below the median score of 116 again contributing more to this outcome. Similarly, the IT-IQ correlation was higher among children showing less well-directed attention in the IT task. Once again IT correlated better with Verbal IQ, supporting the hypothesis that IT is better associated with more general intellectual functioning than with specific cognitive abilities. Cross-lagged panel correlations between IT and IQ were —0.40, whether predicting IQ from IT or the reverse. Thus, there was no evidence for a causal relationship between these variables, although both may reflect a common cognitive ability factor.     

Brain mechanisms and intelligence. Psychometric g and executive function

Sternberg [Sternberg, R.J. (1985). Beyond IQ: a triarchic theory of human intelligence. New York: Cambridge Univ. Press.] has proposed that the general intelligence, or the g factor, obtained when batteries of mental tests are factor analyzed, is a reflection of the fact that executive functions (EF) are common to all cognitive tests. Three lines of evidence that fail to support Sternberg's formulation are presented. First, in animal problem solving studies, there is only a modest degree of overlap between brain structures that are critical for g, and brain structures that have been identified as the rodent EF system. Second, children with attention deficit-hyperactivity disorder (ADHD), characterized by EF dysfunction, do not have IQ scores that are lower, on average, than children in the test standardization populations. Third, human frontal lobe patients often have clear EF deficits, but IQ (a next-best estimate of g) may be preserved. These findings cast serious doubt on the plausibility of Sternberg's formulation. Clarifying the distinction between psychometric g and EF can be important for understanding the differences between practical and psychometric intelligence.     

Finding the g-factor in brain structure using the method of correlated vectors

It is unclear whether brain mechanisms underlying human intelligence are distributed throughout the brain or mainly concentrated in the frontal lobes. Data are inconsistent possibly due, at least in part, to the different ways the construct of intelligence is measured. Here we apply the method of correlated vectors to determine how the general factor of intelligence (g) is related to regional gray matter and white matter volumes. This is a re-analysis of an earlier study showing regional gray matter and white matter volume is correlated to Full Scale IQ (FSIQ). However, it is well-known that FSIQ taps several cognitive abilities and skills in addition to g. The results now show that the g factor accounts for several but not all FSIQ/gray matter correlations distributed throughout the brain and these areas may differ for young and older adults.     

Identifying g: A review of current factor analytic practices in the science of mental abilities

Factor analysis is arguably one of the most important tools in the science of mental abilities. While many studies have been conducted to make recommendations regarding “best practices” concerning its use, it is unknown the degree to which contemporary ability researchers abide by those standards. The current study sought to evaluate the typical practices of contemporary ability researchers. We analyzed articles reporting factor analyses of cognitive ability tests administered to adult samples over a 12 year period. Results suggest that, in aggregate, the science of mental abilities seems to be doing well with respect to the issues of sample size, number of indicators (relative to number of factors) and breadth of indicators. Further, our results suggest that the majority of ability researchers are using methods of factor analysis that allow for the identification of a g factor. However, 14.57% failed to use a method that allowed a common factor to emerge. These results provide insights regarding the methodological quality of the science of mental abilities, and will hopefully encourage further “introspective” research into the science of mental abilities.     

Black–White differences on the g-factor in South Africa: a “Jensen Effect” on the Wechsler Intelligence Scale for Children — revised

A test is made to determine whether South African Black–White differences on various tests of cognitive performance are like the Black–White differences in the United States in being positively associated with a test's g loadings, where g is the general factor of intelligence. Data are analyzed from Skuy, Schutte, Fridjhon and O'Carroll [Skuy, M., Schutte, E., Fridjhon, P., & O'Carroll, S. (2001). Suitability of published neuropsychological test norms for urban African secondary school strudents in South Africa. Personality and Individual Differences, 30, 1413–1425) of 154 13- to 15-year-old secondary school students in Soweto, Johannesburg, on the Wechsler Intelligence Scale for Children–Revised (WISC-R). The more highly correlated a sub-test was with g, the more it predicted the African–White difference (r =0.77, p=0.05). The effect remained even when the Vocabulary sub-test was excluded or when g was extracted from the Black rather than from the White standardization sample (r=0.60, P<0.05), as it did as well if Spearman's rho was used instead of Pearson's r (g from Whites=0.74, g from Blacks=0.74, P< 0.005). Understanding observed Black–White differences around the world requires new research on the nature and nurture of g.     

Is age kinder to the initially more able?: differential ageing of a verbal ability in the Healthy Old People in Edinburgh study

Studies that examine whether there is differential age-related decline in intelligence test scores according to initial cognitive status, social class and education are reviewed. We tested the differential ageing hypothesis which states that decline in ability scores over time will be lower in those with superior mental ability, more education and higher social class at the time of original testing. 387 healthy old people were tested at baseline and followed up 4 years later. The National Adult Reading Test (NART) was administered on both occasions. NART-estimated IQ fell by a mean of 2.1 points over 4 years (p<0.001). Subjects in higher social class categories had higher NART scores overall (p<0.001), and had smaller NART decrements over 4 years. Ex-professional and -secretarial groups fell by a mean of 1.4 points in this period, versus 3.2 points for skilled manual, semi-skilled and unskilled groups. In addition, those who fell more in NART were older (p=0.001), were less educated (p<0.001), and had lower NART (p<0.001) scores at baseline. In summary, those with higher baseline ability, in higher social class groups, with more education and who are younger are relatively protected from decline in this verbal ability with age. Possible mechanisms of differential decline in this verbal intelligence with age are discussed.     

The effect of IQ level on the degree of cognitive deterioration experienced during acute hypoglycemia in normal humans

Acute hypoglycemia provokes a deterioration in cognitive function both in normal individuals and patients with Type 1 diabetes. A large interindividual variation is observed in the magnitude of the cognitive dysfunction observed during hypoglycemia, the reasons for which are not apparent. This study examines whether IQ level exerts a differential effect on the impairment of cognitive performance induced during acute hypoglycemia. Twenty-four nondiabetic participants were divided into high and average IQ groups according to their results on the Alice Heim 4 test and the National Adult Reading Test. Cognitive function was assessed during hypoglycemia using the following cognitive test battery: Paced Auditory Serial Addition Test (PASAT), Rapid Visual Information Processing (RVIP), Trail-Making B (TMB), Digit Symbol Substitution Test (DSST), and Four-Choice Reaction Time (CRT). In Condition A (the placebo condition), the participants' blood glucose was maintained at 4.5 mmol/1 throughout. On two occasions (Conditions B and C), the blood glucose was stabilized at 4.5 mmol/1 for 30 min, lowered to 2.5 mmol/1 (hypoglycemia) for 60 min, and restored to 4.5 mmol/1 for 30 min. Under each condition, the cognitive test battery was performed immediately after stabilization of blood glucose at 4.5 mmol/1, and the battery was repeated as follows: Condition A—after a further 40 min of euglycemia; Condition B—after 5 min of hypoglycemia; Condition C—after 40 min of hypoglycemia. Multivariate analysis of variance demonstrated a detoriation in cognitive performance as a result of hypoglycemia irrespective of IQ group (p< .005). Acute hypoglycemia induced a significant deterioration in cognitive function in all tests except TMB (p< .05). No overall effect of IQ on deterioration in cognitive performance could be ascertained, although univariate analysis of variance revealed an IQ effect on two of the test: The avergae IQ group deteriorated significantly less than the higher IQ group during hypoglycemia in the 4-s PASAT task (p = .03) and tended to have higher false alarm rates in the RVIP (p = .06). In conclusion, individuals with a higher IQ do not appear to be protected from tge adverse effects of acute hypoglycemia on cognitive function.     

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.     

A smarter brain is associated with stronger neural interaction in healthy young females: A resting EEG coherence study

General intelligence, the g factor, is a major issue in psychology and neuroscience. However, the neural mechanism of the g factor is still not clear. It is suggested that the g factor should be non-modular (a property across the brain) and show good colinearity with various cognitive tests. This study examines the hypothesis that functional connectivity may be a good candidate for the g factor. We recorded resting state eyes-closed EEG signals in 184 healthy young females. Coherence values of 38 selected channel pairs across delta, theta, alpha, beta and gamma frequencies were correlated with six intelligence quotient (IQ) subtests, including symbol search, block design, object assembly, digit span, similarity and arithmetic. A three-stage analytic flow was constructed to delineate common (g factor) and unique neural components of intelligence. It is noticed that the coherence pattern demonstrates good correlation with five of the IQ subtests (except symbol search) and non-modularity in the brain. Our commonality analyses support connectivity strength in the brain as a good indicator of the g factor. For the digit span and arithmetic tests, the uniqueness analyses provide left-lateralized topography relevant to the operation of working memory. Performance on the arithmetic test is further correlated with strengths at left temporo-parietal and bilateral temporal connections. All the significant correlations are positive, indicating that the stronger the connectivity strengths, the higher the intelligence. Our analyses conclude that a smarter brain is associated with stronger interaction in the central nervous system. The implication and why the symbol search does not show parallel results are discussed.     

Are Shunt Revisions Associated with IQ in Congenital Hydrocephalus? A Meta -Analysis

Although it is generally acknowledged that shunt revisions are associated with reductions in cognitive functions in individuals with congenital hydrocephalus, the literature yields mixed results and is inconclusive. The current study used meta-analytic methods to empirically synthesize studies addressing the association of shunt revisions and IQ in individuals with congenital hydrocephalus. Six studies and three in-house datasets yielded 11 independent samples for meta-analysis. Groups representing lower and higher numbers of shunt revisions were coded to generate effect sizes for differences in IQ scores. Mean effect size across studies was statistically significant, but small (Hedges’ g = 0.25, p < 0.001, 95 % CI [0.08, 0.43]) with more shunt revisions associated with lower IQ scores. Results show an association of lower IQ and more shunt revisions of about 3 IQ points, a small effect, but within the error of measurement associated with IQ tests. Although clinical significance of this effect is not clear, results suggest that repeated shunt revisions because of shunt failure is associated with a reduction in cognitive functions.     

Intelligence, reaction times, and peripheral nerve conduction velocity

Two studies (with sample sizes of 85 and 88) are reported that investigated relationships among measures of intelligence, speed of information processing, and peripheral nerve conduction velocity (NCV). In both studies, NCV was significantly correlated with IQ scores (rs = .42 and .48) and with reaction times (RTs; rs = −.28 and −.18): Thus, faster NCV was associated with higher IQ scores and faster speed of processing. In both studies, NCV and RTs contributed significantly, in combination, to the prediction of fullscale IQ (shrunken multiple Rs = .53 and .57), but the expected pattern of causal relationships between the variables was not borne out. The results are interpreted in terms of a “neural efficiency” model of intelligence, which has recieved support from other studies of physiological correlates of human intelligence.     

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.     

Population sex differences in IQ at age 11: the Scottish mental survey 1932

There is uncertainty whether the sexes differ with respect to their mean levels and variabilities in mental ability test scores. Here we describe the cognitive ability distribution in 80,000+ children—almost everyone born in Scotland in 1921—tested at age 11 in 1932. There were no significant mean differences in cognitive test scores between boys and girls, but there was a highly significant difference in their standard deviations (P<.001). Boys were over-represented at the low and high extremes of cognitive ability. These findings, the first to be presented from a whole population, might in part explain such cognitive outcomes as the slight excess of men achieving first class university degrees, and the excess of males with learning difficulties.     

Estimating the strength of genetic selection against heritable g in a sample of 3520 Americans,sourced from MIDUS II

The relationship between IQ and completed fertility among a sample of 3520 Americans from MIDUS II (1960's birth cohorts) is examined using a common factor comprised of eight cognitive ability measures, in order to determine the rate of phenotypic IQ loss due to genetic selection. Negative correlations are present in both the male and female subsamples, and are associated with a predicted loss in heritable g (g.h) of − .262 points per decade, increasing to − 1.072 points when the additive effect of mutation accumulation is considered. The ability–fertility associations showed Jensen effects at the level of the whole sample (.167), and also separately for each sex (.185 and .147 for the females and males respectively). The magnitude of the expected g.h loss in this cohort due to selection is comparable to that derived from a meta-analysis of disattenuated decadal g.h declines from eight US studies (− .44 points per decade; N = 127,389). There is a Flynn effect in the US amounting to gains of 3.6 points per decade, which are concentrated on more environmentally plastic and specialized sources of ability variance (s.e) suggesting co-occurrent socio-ecological specialization with respect to narrower cognitive abilities in the present cohort.     

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.     

Investigating cognitive transfer within the framework of music practice: genetic pleiotropy rather than causality

The idea of far transfer effects in the cognitive sciences has received much attention in recent years. One domain where far transfer effects have frequently been reported is music education, with the prevailing idea that music practice entails an increase in cognitive ability (IQ). While cross‐sectional studies consistently find significant associations between music practice and IQ, randomized controlled trials, however, report mixed results. An alternative to the hypothesis of cognitive transfer effects is that some underlying factors, such as shared genes, influence practice behaviour and IQ causing associations on the phenotypic level. Here we explored the hypothesis of far transfer within the framework of music practice. A co‐twin control design combined with classical twin‐modelling based on a sample of more than 10,500 twins was used to explore causal associations between music practice and IQ as well as underlying genetic and environmental influences. As expected, phenotypic associations were moderate (r = 0.11 and r = 0.10 for males and females, respectively). However, the relationship disappeared when controlling for genetic and shared environmental influences using the co‐twin control method, indicating that a highly practiced twin did not have higher IQ than the untrained co‐twin. In line with that finding, the relationship between practice and IQ was mostly due to shared genetic influences. Findings strongly suggest that associations between music practice and IQ in the general population are non‐causal in nature. The implications of the present findings for research on plasticity, modularity, and transfer are discussed.     

Expanding the g-nexus: Further evidence regarding the relations among national IQ, religiosity and national health outcomes

The current study seeks to better understand how religiosity and health are positioned within the g-nexus. Specifically, the degree to which differences in average IQ across nations is associated with differences in national religiosity (i.e., belief rate) and national health statistics independent of differences in national wealth is examined. Consistent with expectations, results show that, independent of national wealth and belief rate, IQ has a positive influence on national health as indicated by fertility rate, infant mortality rate, maternal mortality rate, and deaths due to HIV/AIDS, and life expectancy. Additionally, as hypothesized, IQ and belief rate interacted to influence reproductive health (i.e., fertility rate, infant- and maternal mortality). Specifically, high IQ acts as a buffer against the negative effects of belief rate; when IQ is high belief rate has no effect, but when IQ is low belief rate has a strong negative effect. The pattern of findings from this study, combined with previous research, serve to confirm that general cognitive ability (i.e., the g-factor) is an important and central node within a larger nexus of psychological and social variables. Theoretical and epidemiological implications are discussed.