Pervasiveness of the IQ rise: a cross-temporal meta-analysis

Background

Generational IQ gains in the general population (termed the Flynn effect) show an erratic pattern across different nations as well as across different domains of intelligence (fluid vs crystallized). Gains of fluid intelligence in different countries have been subject to extensive research, but less attention was directed towards gains of crystallized intelligence, probably due to evidence from the Anglo-American sphere suggesting only slight gains on this measure. In the present study, development of crystallized intelligence in the German speaking general population is assessed.

Methodology/Principal Findings

To investigate whether IQ gains for crystallized intelligence are in progress in German-speaking countries, two independent meta-analyses were performed. By means of a cited reference search in ISI Web of Science, all studies citing test manuals and review articles of two widely-used salient measures of crystallized intelligence were obtained. Additionally, the electronic database for German academic theses was searched to identify unpublished studies employing these tests. All studies reporting participants mean IQ or raw scores of at least one of the two measures were included in the present analyses, yielding over 500 studies (>1,000 samples; >45,000 individuals). We found a significant positive association between years of test performance and intelligence (1971–2007) amounting to about 3.5 IQ points per decade.

Conclusions/Significance

This study clearly demonstrates that crystallized IQ gains are substantial and of comparable strength as Flynn effects typically observed for measures of fluid intelligence in Central Europe. Since mean IQ was assessed in a large number of small, non-representative samples, our evidence suggests a remarkable robustness of these gains. Moreover, in both meta-analyses strength of gains was virtually identical. On the whole, results of the present study demonstrate a pervasive and generalizing Flynn effect in German-speaking countries.     

Fluctuating asymmetry and psychometric intelligence.

Little is known about the genetic nature of human psychometric intelligence (IQ), but it is widely assumed that IQ''s heritability is at loci for intelligence per se. We present evidence consistent with a hypothesis that interindividual IQ differences are partly due to heritable vulnerabilities to environmental sources of developmental stress, an indirect genetic mechanism for the heritability of IQ. Using fluctuating asymmetry (FA) of the body (the asymmetry resulting from errors in the development of normally symmetrical bilateral traits under stressful conditions), we estimated the relative developmental instability of 112 undergraduates and administered to them Cattell''s culture fair intelligence test (CFIT). A subsequent replication on 128 students was performed. In both samples, FA correlated negatively and significantly with CFIT scores. We propose two non-mutually exclusive physiological explanations for this correlation. First, external body FA may correlate negatively with the developmental integrity of the brain. Second, individual energy budget allocations and/or low metabolic efficiency in high-FA individuals may lower IQ scores. We review the data on IQ in light of our findings and conclude that improving developmental quality may increase average IQ in future generations.     

The evolution of brain size and intelligence in man

The brain size of hominids has increased approximately threefold during the evolution of the hominids fromAustralopithecus toHomo sapiens. It is proposed that the principal reason for this increase is that larger brains conferred greater intelligence, and greater intelligence conferred a selection advantage. A number of anthropologists have difficulty accepting this thesis because they believe that brain size is not associated with intelligence in man. Evidence is reviewed, and new evidence from two studies is presented, to show that brain size as measured by head size is positively correlated with intelligence as measured by intelligence tests. On two recent samples statistically significant correlations of .21 and .30 were obtained between estimates of brain size and IQ. It is considered that brain size is positively associated with intelligence in man and that this is the major reason for the increase in brain size of the hominids during the last 3.2 million years.     

Association between individual EEG characteristics and the level of intelligence

The aim of this study was to investigate the relationship between individual electroencephalogram (EEG) characteristics in the resting state and the level of nonverbal intelligence. The study involved 77 students of Demidov Yaroslavl State University. Analysis of the relationship between IQ and spectral parameters of EEG theta, alpha, and two subbands of beta oscillations revealed that the amplitude and power of alphaband EEG oscillations and low frequency beta-band EEG oscillations were positively correlated with the performance in the nonverbal intelligence test. The variety of brain periodic regimes was assessed using the correlation dimension (CD) of EEG. The correlation dimension can be used to quantify the degree of complexity of the nonlinear dynamical system. It was found that the value of the EEG correlation dimension was positively associated with the level of intelligence. The periodicity of the EEG signal was studied using autocorrelation analysis. It was shown that the autocorrelogram duration was negatively associated with IQ and the autocorrelogram amplitude was positively associated with IQ. A regression equation for predicting the level of nonverbal intelligence based on the power of theta- and beta-band oscillations, alpha-band oscillation indexes, and the amplitude and autocorrelation characteristics of the EEG signal was obtained.     

National intelligence, suicide rate in the elderly, and a threshold intelligence for suicidality: an ecological study of 48 Eurasian countries

Across 85 countries around the world, Voracek (2004) found a significant positive relation between estimated national intelligence (IQ) and national male and female suicide rate. The relation was not attenuated when countries' per capita Gross Domestic Product (GDP) and type of national IQ estimation were statistically controlled. Independently, investigating the total national suicide rate only, Lester (2003) arrived at the same conclusion. These two findings are consistent with a corollary of de Catanzaro's (1981) evolutionary theory of human suicide, namely that a threshold intelligence is necessary for suicidality and that intelligence and suicide mortality should thus be positively related. Here, further evidence for this hypothesis is bolstered by focusing on suicide rates of the elderly. Across 48 Eurasian countries, estimated national IQ was significantly positively related to national suicide rates of people aged 65 years and over. This new ecological-level finding survived statistical controlling for a set of seven variables (type of national IQ estimation, national GDP, stableness and recency measures for suicide rates, and rates of adult literacy, urbanization and Roman Catholics), which thus are not confounding factors for the relation of intelligence and suicide mortality. Based on ecological data, the threshold IQ for suicidality is predicted to be 70 or slightly over, an estimate that is consistent with various suicidological observations.     

Effects of birth weight on later intelligence

Abstract

Identical twins are used to show the long‐term effects of birth weight when genetic and many environmental factors are controlled. Female twins between the ages of 6 and 10 are evaluated for performance IQ and other measures of activity. No relationship was found between any of these measures and birth weight except for IQ. The heavier cotwin had the higher IQ score even when the birth weight of each twin was above 2,500 grams. Birth order was not associated with intelligence within the twin pairs.     

Snijders-Oomen Nonverbal Intelligence Test: useful for the elderly?

The feasibility of the SON-R 5 1/2-17, a non-verbal intelligence test for deaf children, was investigated in a group of older adults, with a view to the future use of the test in older neurological patients. In a group of 58 healthy elderly persons intelligence was measured with the SON, the Raven Progressive Matrices and a Dutch reading test. The subjects were also asked for their subjective judgements of the tests. The SON-R 5 1/2-17 appears to be a user-friendly test. The high correlations between the subtests, and between the SON and other measures of intelligence suggest that the SON is a valid test for measuring fluid intelligence in elderly persons. The existing norms are not suitable when the SON is used in elderly people, new norms for adults should therefore be developed.     

Differential rates of secular increase of five major primary abilities

A number of studies have shown that mean intelligence levels have been increasing at a rate of around 3 IQ points per decade during the last half-century. This paper presents data for cohorts of 9-11 year olds in Northern Ireland tested in 1978 and 1988 and shows that the rates of increase differ markedly for different primary abilities. Verbal comprehension has shown virtually no increase, while there have been large increases in spatial relations and perceptual speed. Moderate increases have taken place in the numerical and reasoning primaries. The results are interpreted as supporting a nutrition theory of the secular increases in intelligence.     

Fluid intelligence and psychosocial outcome: from logical problem solving to social adaptation

Background

While fluid intelligence has proved to be central to executive functioning, logical reasoning and other frontal functions, the role of this ability in psychosocial adaptation has not been well characterized.

Methodology/Principal Findings

A random-probabilistic sample of 2370 secondary school students completed measures of fluid intelligence (Raven''s Progressive Matrices, RPM) and several measures of psychological adaptation: bullying (Delaware Bullying Questionnaire), domestic abuse of adolescents (Conflict Tactic Scale), drug intake (ONUDD), self-esteem (Rosenberg''s Self Esteem Scale) and the Perceived Mental Health Scale (Spanish adaptation).Lower fluid intelligence scores were associated with physical violence, both in the role of victim and victimizer. Drug intake, especially cannabis, cocaine and inhalants and lower self-esteem were also associated with lower fluid intelligence. Finally, scores on the perceived mental health assessment were better when fluid intelligence scores were higher.

Conclusions/Significance

Our results show evidence of a strong association between psychosocial adaptation and fluid intelligence, suggesting that the latter is not only central to executive functioning but also forms part of a more general capacity for adaptation to social contexts.     

Association between the CHRM2 gene and intelligence in a sample of 304 Dutch families

The CHRM2 gene is thought to be involved in neuronal excitability, synaptic plasticity and feedback regulation of acetylcholine release and has previously been implicated in higher cognitive processing. In a sample of 667 individuals from 304 families, we genotyped three single-nucleotide polymorphisms (SNPs) in the CHRM2 gene on 7q31-35. From all individuals, standardized intelligence measures were available. Using a test of within-family association, which controls for the possible effects of population stratification, a highly significant association was found between the CHRM2 gene and intelligence. The strongest association was between rs324650 and performance IQ (PIQ), where the T allele was associated with an increase of 4.6 PIQ points. In parallel with a large family-based association, we observed an attenuated - although still significant - population-based association, illustrating that population stratification may decrease our chances of detecting allele-trait associations. Such a mechanism has been predicted earlier, and this article is one of the first to empirically show that family-based association methods are not only needed to guard against false positives, but are also invaluable in guarding against false negatives.     

Learning, working memory, and intelligence revisited

Based on early findings showing low correlations between intelligence test scores and learning on laboratory tasks, psychologists typically have dismissed the role of learning in intelligence and emphasized the role of working memory instead. In 2006, however, B.A. Williams developed a verbal learning task inspired by three-term reinforcement contingencies and reported unexpectedly high correlations between this task and Raven's Advanced Progressive Matrices (RAPM) scores [Williams, B.A., Pearlberg, S.L., 2006. Learning of three-term contingencies correlates with Raven scores, but not with measures of cognitive processing. Intelligence 34, 177-191]. The present study replicated this finding: Performance on the three-term learning task explained almost 25% of the variance in RAPM scores. Adding complex verbal working memory span, measured using the operation span task, did not improve prediction. Notably, this was not due to a lack of correlation between complex working memory span and RAPM scores. Rather, it occurred because most of the variance captured by the complex working memory span was already accounted for by the three-term learning task. Taken together with the findings of Williams and Pearlberg, the present results make a strong case for the role of learning in performance on intelligence tests.     

The census and selective service

Abstract

A number of studies are reviewed to show that the mean level of measured intelligence in Scotland is approximately three to four IQ points lower than that in England. These results are a striking contrast with another set of findings showing that in the nineteenth century Scotland had a greater proportional output of people of intellectual distinction than did England. Evidence on Scottish‐English differences in education, parental attention, and per capita income suggests that none of these factors can explain the difference in mean IQ. It is suggested that the principal cause of the reduced mean intelligence in Scotland lies in the selective emigration of the more intelligent, and evidence is reviewed to show that this emigration has occurred for at least a century.     

What doesn't kill you makes you dumber. Strengthening the link between infectious disease, intelligence and personality

A causal link between childhood exposure to disease and the development of intelligence would have major implications for public health and international development programmes.The idea that infectious disease during childhood affects the developing brain to impact intelligence has been around for decades. Recent evidence from more rigorous studies, which have controlled carefully for other factors such as nutrition and education, has strengthened the case. If these new epidemiological and molecular studies really do confirm a clear link between childhood infection and intelligence, the consequences for health policy and development assistance could be profound. The results could mandate an increased focus not only on eradicating or controlling infectious diseases, but also on reducing their impact on children in the absence of cures or vaccines.If these … studies do show a clear link between childhood infection and intelligence, the consequences for health policy and development assistance could be profoundYet, even in the light of new evidence, it is hard to unravel causes from effects, and the debate continues over which diseases are most responsible, along with the precise physiological and molecular mechanisms involved. There is no shortage of theories to explain why infectious disease seems to have so profound an effect on intelligence, and, as a result, on the intellectual and economic performance of whole nations or regions. The stage is set for more studies to drill down into neurological and cognitive mechanisms: to explain why the prevalence of infectious disease is a reliable predictor of intelligence at the population level; to differentiate between the impact of various pathogens; and to identify the evolutionary rationale of these links. There is also mounting evidence that some parasites can alter their host''s personality through mechanisms evolved to modify their host''s behaviour to their own advantage, which could explain environmental risk factors for mental disorders, such as schizophrenia.After a few intermittent references earlier last century, the US economist Andrew Kamarck made the first attempt to link infectious disease with a nation''s performance during the 1970s [1]. Kamarck identified the interplay of three factors—temperature, humidity and infectious diseases—on the economic performance of nations through their impact on vitality and intellectual attainment.Somewhat surprisingly, both economists and biologists neglected Kamarck''s findings that link infectious disease and either intelligence or performance at a population level for another three decades. Eventually, Christopher Eppig and colleagues at the University of New Mexico in Albuquerque, USA, published a seminal paper in 2010, which reported a strong correlation between the prevalence of infectious disease in a country and intelligence as measured by supposedly culturally independent IQ tests [2]. In the past, other environmental factors, such as average temperature, have been shown to affect intelligence, but in Eppig''s study, infectious disease seems to trump these. The country with the highest average IQ of all, Singapore, is hot and humid, but has the world''s lowest rates of infectious disease largely because of excellent healthcare.The Eppig study also offers a plausible explanation for the so-called Flynn Effect, named after the political scientist James Flynn who described and promoted the apparent sustained and significant increase in average intelligence in many developed nations during the past half-century or more [3]. “Our research suggests that infectious disease may be the most important factor influencing IQ,” Eppig confirmed. “Infectious disease has the strongest correlation with average IQ, and the largest independent contribution when other factors are controlled.” The researchers found that the correlation between average IQ and infectious disease at the cross-national level is between −0.76 and −0.82; 0 would equate to no correlation and −1 would be total correlation. The results indicate a high degree of correlation, and, just as importantly, the study determined that the probability of this correlation having occurred by chance was incredibly low.…both economists and biologists neglected Kamarck''s findings that link infectious disease and either intelligence or performance at a population level for another three decadesThe intelligence scores were largely taken from an earlier study conducted by UK psychologist Richard Lynn and Finnish political scientist Jaan Mikk, which analysed IQ scores from 113 countries [4], and the data on infectious diseases were provided by the World Health Organization. Given that both sets of data were openly available, it was easy for other groups to perform their own analyses to either corroborate or refute Eppig''s findings. Chris Hassall and colleagues at Carleton University in Ottawa, Canada, have done just such a follow up to identify or eliminate any statistical artefacts that might weaken or cast doubt on the findings [5]. One of the significant possible artefacts for which Hassall controlled is a phenomenon known as autocorrelation, which is the tendency for two sets of data to seem to be linked just because they have similar spatial patterns of variation. “Having reanalysed the data, I am fairly convinced that there is a strong correlation between the health impacts of parasites and IQ,” Hassall confirmed. In fact, Eppig himself suggested that Hassall''s results were stronger than his own. “They found that, when controlling for spatial autocorrelation, infectious disease was an even better predictor of average national IQ than our own analysis had found,” he commented.Meanwhile, Eppig has published another study analysing the correlation between disease and intelligence within a single country [6]. He chose the USA because there is good data available for individual states, with sufficient variation across the country as a whole to provide the necessary range of data. This study was conducted partly in response to criticism of the first one on the grounds that national differences in culture and education might not have been fully filtered out. By studying just one country with a significant degree of cultural and educational harmony, Eppig hoped to provide an even more convincing case for the link between infectious disease and IQ.According to Michael Woodley, who has been studying the link between infectious disease and intelligence at the University of Surrey, UK, the correlation found in the US study is not as strong, but is still significant. “They found a weaker set of relationships, but infectious disease was still a potent predictor of cross state variance in IQ,” Woodley said, but added that these studies beg the question of cause and effect. “The question is, have they found evidence that infectious disease has a causal influence on IQ, or is it the case that cross national patterns of IQ affect disease ecology?” he explained. The suggestion is that intelligence itself can affect the prevalence of disease. “My cautious take is that it''s a bit of both.”Hassall conceded that his and the other studies have only identified a correlation between infectious disease and intelligence, albeit a strong one, and not a causal link. But he added that there were plausible underlying physiological explanations for the link, although as yet there is no definitive proof for any.“We can only speculate about the possible causal links,” agreed Joachim Kurtz, a group leader whose lab works on animal evolutionary ecology at the Westfälische Wilhelms–Universität Münster in Germany. “There are at least two non-exclusive possibilities: firstly, given that the brain needs a lot of energy, the energetic costs of parasitic infection and immune defense may provide a mechanistic explanation for the correlation […] a second, slightly frightening and more direct possibility is that parasite manipulation might make hosts stupid.”The first possibility could be caused by the need to reroute energy from the brain to repair tissue damaged by parasites, or by energy lost through malnutrition as a result of diarrhoea, vomiting, or diminished absorption through the digestive tract. It could also result from the parasite accessing cellular or macromolecular resources at the expense of the developing brain, or by the energy cost of maintaining a heightened immune response. All these factors might decrease the energy and nutrients available to the developing brain and cause reduced cognitive capability.The second possibility cited by Kurtz might involve direct damage to, or alteration of, neurological mechanisms, perhaps deliberately engineered by the parasite for its advantage. The case of rabies is an extreme example of an infection in which the parasite, a virus infecting nerve cells and causing acute encephalitis, changes its host''s behaviour to increase the chance of its spreading, in this case causing the host to bite others and spread the virus through saliva.But there is growing evidence that parasites causing chronic infections can alter behaviour in more subtle ways to increase the chance of transmissionBut there is growing evidence that parasites causing chronic infections can alter behaviour in more subtle ways to increase the chance of transmission. Kurtz cited the case of the protozoa Toxoplasma gondii, referring to a recent paper by Czech parasitologist Jaroslav Flegr from Charles University, Prague, which found that infection can trigger various psychiatric and neurological diseases, including schizophrenia, in people with genetic predispositions [7].“Dozens of studies published in the past 20 years clearly show that toxoplasmosis is responsible for a large number of cases of schizophrenia,” Flegr noted. “Recent results, some of them published by our group, show that toxoplasmosis-associated schizophrenia has more severe clinical symptoms than other kinds of schizophrenia.” Such symptoms were associated with noticeable changes in brain morphology and included impaired reaction times as well as personality changes, Flegr added. Together, these changes were found to increase the risk not just of suicide but also accidental injury or death [8,9]. Adding these factors together, Flegr estimated that latent toxoplasmosis is indirectly responsible for more than one million deaths per year, which would make it the world''s second most dangerous protozoan parasite after malaria, albeit indirectly killing its victims.In the case of T. gondii the same ‘chicken and egg'' question arises of whether infection causes the psychiatric disorders, or whether psychiatric disorders make infection more likely. According to Flegr, there is molecular evidence to support the hypothesis that infection causes psychiatric disorders. “It has been known for a long time that toxoplasmosis increases the concentration of dopamine in the infected brain,” he said. “In 2009 it was shown that the genome of Toxoplasma contains genes for two rate-limiting enzymes for synthesis of dopamine in the brain tissue [10]. Another study then demonstrated that large amounts of this neurotransmitter are synthesized in cysts of Toxoplasma in the brains of infected laboratory animals [11].”There has been growing evidence that such disruption in dopamine production does increase the risk of developing schizophrenia [12]. Flegr speculated that this manipulation of the host''s neurotransmitter production, primarily an increase in dopamine combined with a decrease in serotonin, has its roots in animal evolution. “At least some of the changes are most probably results of manipulative activity of the parasite aimed at increasing efficiency of transmission from an intermediate animal to definitive host by predation,” he explained. “Some are probably just side effects of chronic disease.”…Flegr estimated that latent toxoplasmosis is indirectly responsible for more than one million deaths per year […] the world''s second most dangerous protozoan parasite…Although infection by T. gondii is particularly common in Africa and South America, Flegr noted that it also has a high incidence in cooler and drier regions, being associated with the consumption of raw vegetables and raw meat. The latter factor perhaps explains its high prevalence in France and Germany, where 40–50% of the population are infected, compared with less than 20% in the UK and USA. These are large figures nonetheless, so the recent findings highlight the urgency of further research to understand the genetic risk factors that predispose infected individuals to neurological illness.“…our hypothesis predicts that the infections that cause the greatest amount of energy to be diverted away from the brain will have the largest detrimental effect…”When it comes to the less clearly defined issue of intelligence, researchers are just beginning to identify candidate genes in the host. Among the best known is microcephalin, a gene known to regulate brain size, but the precise role of which in intelligence has yet to be explained. However Heiner Rindermann from the Institute of Psychology at Chemnitz, Germany, has found evidence that high levels of microcephalin within a population seem to be associated with low levels of disease and higher intelligence [13]. “Microcephalin does not predict IQ at the individual level, but it does at ecological scales,” Rindermann said. The reason the operation of microcephalin can only be seen at the population level, he explained, is that it does not provide any physiological protection against disease but does make people more sensitive to dirt and more likely to indulge in hygienic behaviour, which affects all people in the vicinity.“The role of infectious disease burdens as the principal mediator of this ecological relationship suggests that populations exhibiting high levels of microcephalin were better able to cope with historical disease burdens,” Rindermann reasoned. “We believe that microcephalin might have encoded for disgust sensitivity, hence more sensitive populations transitioning out of the hunter-gatherer mode of subsistence and into the agrarian one could have carried on growing such that the frequency of IQ-enhancing mutations could have increased via runaway gene-culture co-evolution.”This three-way link between microcephalin, disease and intelligence remains speculative, but the overall association between infectious parasites and broad cognitive behaviour is increasingly well established. It is not yet clear, though, which diseases are the main culprits, with a few exceptions such as T. gondii for psychotic disorders. Differentiating between the different pathogens is one of the main targets for research in the field, according to Eppig. “We have not done empirical work on this question yet, although we have a project in the works, but our hypothesis predicts that the infections that cause the greatest amount of energy to be diverted away from the brain will have the largest detrimental effect,” he said. “This means that long-term, chronic, infections are more likely to have a greater detrimental effect on the brain than short-term infections. In particular, we predict that parasites causing diarrheal diseases, malaria and tuberculosis, to name a few, will have the largest effect.”However, Woodley commented there is evidence that sexually transmitted diseases rather than diseases of the intestinal or respiratory tracts have the largest impact on intelligence. But these diseases are often chronic, although Woodley suggested that the correlation could simply result from people with higher IQs being less likely to catch them.All this research paints an increasingly detailed picture of how infectious diseases and the development of intelligence are linked; but there is clearly much more to be done to unravel the underlying mechanisms. The evidence already accumulated indicates that continuing efforts to eradicate disease in the developing world should be increased. However, as Hassall pointed out, the societal case for doing that stands on its own and does not need to be associated with intelligence.     

The nature and evolution of intelligence in orangutans (Pongo pygmaeus)

Orangutans share many intellectual qualities with African great apes and humans, likely because of their recent common ancestry. They may also show unique intellectual adaptations because of their long evolutionary divergence from the African lineage. This paper assesses orangutan intelligence in light of this evolutionary history. Evidence derives from observations of juvenile ex-captive orangutans reintroduced to free forest life by the Wanariset Orangutan Reintroduction Project, East Kalimantan, Indonesia. The intellectual qualities shared by great apes and humans point to a distinct “great ape” intelligence with hierarchization as a pivotal cognitive mechanism. Evolutionary reconstructions jibe with this view and suggest that technically difficult foods may have been key selection pressures. Orangutans should then show hierarchical intelligence when obtaining difficult foods. Evidence on ex-captive orangutans' techniques for processing difficult foods concurs. Intellectual qualities distinct to orangutans may owe to arboreal travel pressures; in particular arboreality may aggravate foraging problems. Evidence confirms that ex-captive orangutans' techniques for accessing difficult foods located arboreally are intellectually complex—i.e. they show hierarchization. These findings suggest other factors probably important to understanding great ape and orangutan forms of intelligence and their evolutionary origins.     

Secular rates and correlates for gastroschisis in California (1968-1977)

This study reports on an epidemiological investigation of gastroschisis using birth certificate data from California for the period of 1968-1977. Gastroschisis has been reported to be on the increase in a number of countries. The distribution of the 166 California gastroschisis cases reported during this time period indicated a clear upward secular trend (P less than .001) with the rate per 1,000 increasing from .006 in 1968 to .089 by 1977. This secular trend was observed in every maternal age group and for gravidity level 1. Gastroschisis also occurred more frequently among younger mothers (P less than .001) and low gravidity mothers (P less than .001). Omphalocele, another eventration defect, was also examined but did not display an upward secular trend.     

A Potential Spatial Working Memory Training Task to Improve Both Episodic Memory and Fluid Intelligence

One current challenge in cognitive training is to create a training regime that benefits multiple cognitive domains, including episodic memory, without relying on a large battery of tasks, which can be time-consuming and difficult to learn. By giving careful consideration to the neural correlates underlying episodic and working memory, we devised a computerized working memory training task in which neurologically healthy participants were required to monitor and detect repetitions in two streams of spatial information (spatial location and scene identity) presented simultaneously (i.e. a dual n-back paradigm). Participants’ episodic memory abilities were assessed before and after training using two object and scene recognition memory tasks incorporating memory confidence judgments. Furthermore, to determine the generalizability of the effects of training, we also assessed fluid intelligence using a matrix reasoning task. By examining the difference between pre- and post-training performance (i.e. gain scores), we found that the trainers, compared to non-trainers, exhibited a significant improvement in fluid intelligence after 20 days. Interestingly, pre-training fluid intelligence performance, but not training task improvement, was a significant predictor of post-training fluid intelligence improvement, with lower pre-training fluid intelligence associated with greater post-training gain. Crucially, trainers who improved the most on the training task also showed an improvement in recognition memory as captured by d-prime scores and estimates of recollection and familiarity memory. Training task improvement was a significant predictor of gains in recognition and familiarity memory performance, with greater training improvement leading to more marked gains. In contrast, lower pre-training recollection memory scores, and not training task improvement, led to greater recollection memory performance after training. Our findings demonstrate that practice on a single working memory task can potentially improve aspects of both episodic memory and fluid intelligence, and that an extensive training regime with multiple tasks may not be necessary.     

Lifetime exposure to environmental lead and children's intelligence at 11-13 years: the Port Pirie cohort study.

OBJECTIVE--To examine the association between environmental exposure to lead and children''s intelligence at age 11-13 years, and to assess the implications of exposure in the first seven years of life for later childhood development. DESIGN--Prospective cohort study. SUBJECTS--375 children born in or around the lead smelting town of Port Pirie, Australia, between 1979 and 1982. MAIN OUTCOME MEASURE--Children''s intelligence quotient (IQ) measured at 11-13 years of age. RESULTS--IQ was inversely associated with both antenatal and postnatal blood lead concentrations. Verbal, performance, and full scale IQ were inversely related to blood lead concentration with no apparent threshold. Multivariate analyses indicated that after adjustment for a wide range of confounders, the postnatal blood lead concentrations (particularly within the age range 15 months to 7 years) exhibited inverse associations with IQ. Strong associations with IQ were observed for lifetime average blood lead concentrations at various ages. The expected mean full scale IQ declined by 3.0 points (95% confidence interval 0.07 to 5.93) for an increase in lifetime average blood lead concentration from 0.48 to 0.96 mumol/l (10 to 20 micrograms/dl). CONCLUSION--Exposure to environmental lead during the first seven years of life is associated with cognitive deficits that seem to persist into later childhood.     

Association of chronotype and social jetlag with human non-verbal intelligence

The aim of this study was to evaluate the effects of chronotype and social jetlag (SJL) on intelligence. Subjects were aged 14–25 years (n = 1008). A significant effect of intelligence on academic performance, as measured by the Raven’s Standard Progressive Matrices test, was found (F_2,917 = 11.75, P < 0.0001, η² = 0.03). When SJL was less than 2 hours, the intelligence of people with late chronotype was found to be higher than that of subjects with early and intermediate chronotypes (F_2,305 = 3.12, P < 0.05, η² = 0.02). A negative effect of SJL on the results of intelligence testing was noted only in subjects with late chronotype (F_2,536 = 2.61, P < 0.05, η² = 0.02). Our data suggest that people with late chronotype have a higher level of intelligence, but these advantages disappear when SJL ≥2 hours.     

Group differences in mean intelligence for the Dutch and Third World immigrants.

Evidence from eleven samples indicates that the mean IQ of third world immigrants in the Netherlands is lower than the Dutch mean by approximately one standard deviation for Surinamese and Antillians, and by approximately one and a half standard deviations for Turks and Moroccans. Since IQ tests provide the best prediction of success in school and organizations, it could be that the immigrants' lower mean IQ is an important factor in their low status on the Dutch labour market. The IQs of second-generation immigrants are rising.     

Social intelligence, human intelligence and niche construction

This paper is about the evolution of hominin intelligence. I agree with defenders of the social intelligence hypothesis in thinking that externalist models of hominin intelligence are not plausible: such models cannot explain the unique cognition and cooperation explosion in our lineage, for changes in the external environment (e.g. increasing environmental unpredictability) affect many lineages. Both the social intelligence hypothesis and the social intelligence-ecological complexity hybrid I outline here are niche construction models. Hominin evolution is hominin response to selective environments that earlier hominins have made. In contrast to social intelligence models, I argue that hominins have both created and responded to a unique foraging mode; a mode that is both social in itself and which has further effects on hominin social environments. In contrast to some social intelligence models, on this view, hominin encounters with their ecological environments continue to have profound selective effects. However, though the ecological environment selects, it does not select on its own. Accidents and their consequences, differential success and failure, result from the combination of the ecological environment an agent faces and the social features that enhance some opportunities and suppress others and that exacerbate some dangers and lessen others. Individuals do not face the ecological filters on their environment alone, but with others, and with the technology, information and misinformation that their social world provides.