Sex differences in intelligence and brain size: a developmental theory

The Impasse on Gender Differences in Intelligence: a Meta-Analysis on WISC Batteries

This meta-analysis reviews 79 studies (N = 46,605) that examined the existence of gender difference on intelligence in school-aged children. To do so, we limited the literature search to works that assessed the construct of intelligence through the Wechsler Intelligence Scales for Children (WISC) batteries, evaluating eventual gender differences in indices and subtests. The theoretical framework we adopted is the cross-battery approach which locates cognitive abilities into different levels, also considering the possible mediating effect of the version of the WISC being used. As for broad abilities, a notable discrepancy emerged in favour of males for visual and crystallized intelligence, while female/male differences on fluid intelligence were negligible. Conversely, females’ performance on the processing speed factor was superior. Interesting results emerged at the subtest levels, albeit with less pronounced differences in performance. Results generally showed that older versions of WISC batteries displayed larger gender differences compared to the most recent ones.

Age and Sex Invariance of the Woodcock-Johnson IV Tests of Cognitive Abilities: Evidence from Psychometric Network Modeling

The Woodcock-Johnson IV Tests of Cognitive Abilities (WJ IV COG) is a comprehensive assessment battery designed to assess broad and narrow cognitive abilities, as defined by the Cattell-Horn-Carroll (CHC) theory of intelligence. Previous studies examined the invariance of the WJ assessments across sex and age groups using factor analytic methods. Psychometric network modeling is an alternative methodology that can address both direct and indirect relationships among the observed variables. In this study, we employed psychometric network modeling to examine the invariance of the WJ IV COG across sex and age groups. Using a normative sample (n = 4212 participants) representative of the United States population, we tested the extent to which the factorial structure of the WJ IV COG aligned with CHC theory for the school-aged sample. Next, we used psychometric network modeling as a data-driven method to investigate whether the network structure of the WJ IV COG remains similar across different sex and age (age 6 to 19, inclusively) groups. Our results showed that the WJ IV COG maintained the same network structure across all age and sex groups, although the network structure at younger ages indicated weaker relationships among some subtests. Overall, the results provide construct validity evidence for the WJ IV COG, based on both theoretical and data-driven methods.

Sex specific cognitive differences in Parkinson disease

Parkinson disease (PD) is a progressive neurodegenerative disorder that is 1.5 times more common in males than in females. While motor progression tends to be more aggressive in males, little is known about sex difference in cognitive progression. We tested the hypothesis that there are sex differences in cognitive dysfunction in non-demented PD. We evaluated 84 participants (38 females) with PD and 59 controls (27 females) for demographic variables and cognitive function, including attention, working memory, executive function, and processing speed. Multivariate ANOVA revealed no significant differences between groups for demographic variables, including age, years of education, global cogntition, daytime sleepiness, predicted premorbid IQ, UPDRS score, PD phenotype, or disease duration. For cognitive variables, we found poorer performance in males versus females with PD for measures of executive function and processing speed, but no difference between male and female controls. Specifically, PD males showed greater deficits in Verbal Fluency (category fluency, category switching, and category switching accuracy), Color Word Interference (inhibition), and speed of processing (SDMT). There were no differences in measures of working memory or attention across sex and inconsistent findings for switching. Our data indicate that males with PD have significantly greater executive and processing speed impairments compared to females despite no differences in demographic variables or other measures of disease severity. Our findings are consistent with the steeper slope of disease progression reported in males with PD.

Examining Gender Differences in Neurocognitive Functioning Across Adulthood

OBJECTIVE

Previous research has shown that women have an advantage on verbal episodic memory and processing speed tasks, while men show an advantage on spatial ability measures. Previous work has also found differences in cognition across age. The current study examines gender differences in neurocognitive functioning across adulthood, whether age moderates this effect, and whether these differences remain consistent with practice across multiple testing sessions.

METHOD

Data from the Virginia Cognitive Aging Project were used, which included participants between the ages of 18 and 99 years (N = 5125). Participants completed measures assessing five cognitive domains: episodic memory, processing speed, reasoning, spatial visualization, and vocabulary.

RESULTS

Results showed that gender was significantly related to memory, speed, and spatial visualization, but not to vocabulary or reasoning. Results of invariance analyses across men and women provided evidence of configural and metric invariance, along with partial scalar invariance. Additionally, there was little evidence that age or practice influenced the gender effect on neurocognition.

CONCLUSIONS

Consistent with the previous research, these results suggest that there is a female advantage in episodic memory and processing speed, and a male advantage in spatial visualization. Gender was shown to influence cognition similarly across adulthood. Furthermore, the influence of gender remained the same across three sessions, which is consistent with the previous work that has shown that training does not differentially impact performance on spatial ability measures for females compared to males.

Sex Similarities and Differences in Intelligence in Children Aged Two to Eight: Analysis of SON-R 2-8 Scores

The aim of this study was to investigate sex similarities and differences in visuospatial and fluid abilities and IQ scores based on those abilities in children aged two to eight. Standardization data from the Snijders-Oomen Nonverbal Intelligence Test for Children aged 2-8 (SON-R 2-8) were used. A representative sample composed of 965 children from the Netherlands and 762 children from Germany was examined. Small but significant mean sex differences favoring girls were observed until age four. At ages six and seven, boys achieved similar cognitive development levels to girls regarding all abilities assessed and outperformed girls on the Mosaics subtest measuring visuospatial cognition. Boys also displayed higher variability rates in performance. The distribution of IQ scores, with the overrepresentation of girls scoring above mean and the overrepresentation of boys scoring below mean in early childhood, altered with age towards parity between the sexes. The results suggest that girls tend to mature earlier with respect to cognitive abilities. During the course of development, however, the differences between girls and boys may become negligible.

Growth in early life and physical and intellectual development at school age: a cohort study

The associations between growth during early life and subsequent cognitive development and physical outcomes are not widely known in low-resource settings. We examined postnatal weight and height gain through early life and related these measurements to the nutritional status and intellectual development of the same children when they were between 7 and 9 years old. Mothers had enrolled in an randomised controlled trial to evaluate the effect of prenatal micronutrient supplementation on birth weight. Their children were born in 2004, their height and weight were measured at 6, 12, 18 and 24 months of age and were followed up between October 2012 and September 2013 (at ages 7-9 years, n 650). Height-for-age, weight-for-age and BMI-for-age were used to describe the nutritional status, and the Wechsler Intelligence Scale for Children fourth edition was used to measure the intellectual function. Multilevel linear and logistic modelling was used to estimate the association between early growth and subsequent growth and intellectual function. After adjustment, weight gain from 6 to 12 months of age was associated with Full-scale Intelligence Quotient, Verbal Comprehension Index, Working Memory Index and Perceptual Reasoning Index. Weight gain during early life was associated with subsequent nutritional status. For every 1 kg increase in weight during the 0- to 6-month period, the OR for underweight, thinness and stunting at 7-9 years of age were 0·19 (95 % CI 0·09, 0·37), 0·34 (95 % CI 0·19, 0·59) and 0·40 (95 % CI 0·19, 0·83), respectively. Weight gain during the periods of 6-12 months of age and 18-24 months of age was also associated with a lower risk of being underweight. Weight gain during early life was associated with better growth outcomes and improved intellectual development in young school-aged children.

Psychometric Investigation of the Raven's Colored Progressive Matrices Test in a Sample of Preschool Children

The present study investigated the psychometric properties of the Raven's Colored Progressive Matrices (CPM) test in a sample of preschoolers from Brazil (n = 582; age: mean = 57 months, SD = 7 months; 46% female). We investigated the plausibility of unidimensionality of the items (confirmatory factor analysis) and differential item functioning (DIF) for sex and age (multiple indicators multiple causes method). We tested four unidimensional models and the one with the best-fit index was a reduced form of the Raven's CPM. The DIF analysis was carried out with the reduced form of the test. A few items presented DIF (two for sex and one for age), confirming that the Raven's CPM items are mostly measurement invariant. There was no effect of sex on the general factor, but increasing age was associated with higher values of the g factor. Future research should indicate if the reduced form is suitable for evaluating the general ability of preschoolers.

Evidence for a Positive Ecological Correlation of Regional Intelligence and Suicide Mortality in the United States during the Early 20th Century

Several contemporary cross-national and intranational geographic studies have reported positive ecological (group-level) associations of intelligence and suicide mortality. These findings are consistent with facts from suicide research and with an evolutionary view of suicidal behavior. The present research extended these accounts cross-temporally. Analysis of E. L. Thorndike's state-level personal quality scores and standardized birth rates of eminent persons, taken as proxy variables for regional intelligence, along with historical state suicide rates (1913–1924 and 1928–1932) showed that intelligence and suicide mortality across the USA were already clearly positively related during the early 20th century, suggesting time stability of the effect. Within the USA, the effect is possibly due to state differences in the ethnic composition, which correspond to both suicide rates and intelligence proxies. It is argued that the most parsimonious interpretation of these ecological findings remains that they indeed reflect individual-level effects, that a positive link between intelligence and suicide is entirely compatible with positive overall links between intelligence and health and longevity, and that the ultimate explanative background for the positive link between intelligence and suicide may be provided through the framework of Rushton's differential K theory.

SEX DIFFERENCES ON THE WISC-III AMONG CHILDREN IN SUDAN AND THE UNITED STATES

The sex differences on the WISC-III are reported for the thirteen subtests, the Verbal and Performance IQs, the four Index IQs and the Full Scale IQs in Sudan and the United States. The sex differences are closely similar in the two samples with a correlation of 0.878 (p<0.001) for the thirteen subtests. Males obtained significantly higher Full Scale IQs in the two samples of 0.23d and 0.11d, respectively.

Self-Perception of Intelligence in Male and Female Undergraduates in Old and New Welsh Universities

In 1991 John Major's UK government announced that the binary divide in UK higher education was to be phased out as polytechnics would be given permission by the Privy Council to apply for university status. But has the binary divide really ceased to exist in higher education? Given that the ‘old’ universities typically ask for higher A-level grades than the ‘new’ ones we might ask: do students at new universities perceive themselves as being less able than those at old universities? In addition to the possibility of differences between institutions we might also ask do the sexes differ in their self-perceptions of intelligence? Over the last 25 years, a number of studies have demonstrated a robust gender difference in self-estimation of intelligence, with female undergraduates consistently producing lower ratings of their own intelligence than their male counterparts (see for example Hogan, 1978; Higgins, 1987 and Furnham, 2000, 2001). Does this situation still prevail in today's universities where more women than men now enter higher education? Finally, given the rapid rise in the proportion of the population entering higher education during the 1990s we might ask whether this rise has had an effect on the self-perception of intelligence in students. The current study was designed to throw some light on all three of these questions by simply asking undergraduate samples at an old and a new Welsh university what score they think they would achieve on an IQ test. The findings suggest that female undergraduates still rate themselves less highly than males, that students attending new universities perceive themselves as being less intelligent than those studying at old universities and finally, that during the 1990s there was a general fall in self-estimates of IQ amongst university students.

Interactions between Sex, Socioeconomic Level, and Children's Cognitive Performance

This study assesses the interactions between sex, socioeconomic level, and children’s cognitive performance. Cognitive performance was measured for a sample of 453 Portuguese children, aged between 4 and 10 years, with 218 boys and 235 girls; verbal and nonverbal cognitive ability and intelligence quotient were measured by the Cognitive Skills Scale for Children. Multivariate analysis of variance assessed the effects of sex and family’s socioeconomic level on intelligence quotient. A statistically significant interaction between sex and socioeconomic level was observed for nonverbal intelligence quotient, total intelligence quotient, and two subtests. Socioeconomic level had more influence than sex on most of the cognitive tests.

Sex differences in the intellectual functioning of early school-aged children in rural China

Background

Gender disparities in China are concentrated in poor rural areas and among poor households. The difference in intelligence between boys and girls is less clear in rural China. The purpose of this paper was to assess sex differences in the intellectual function of early school-aged children in rural China.

Methods

One thousand seven hundred forty four early school-aged offspring of women who had participated in a prenatal supplementation trial with different combinations of micronutrients and continued to reside in two rural counties in China were followed. We measured their Full-Scale IQ (FSIQ), Verbal Comprehension Index (VCI), Working Memory Index (WMI), Perceptual Reasoning Index (PRI) and Processing Speed Index (PSI) using the Wechsler Intelligence Scale for Children (WISC-IV). Multilevel analyses were used to assess sex differences in intellectual functioning in 7-10-year-old children in rural China.

Results

Boys’ adjusted mean FSIQ score was 0.97 points higher (95 % CI: -2.22 − 0.28) than that of girls. Girls obtained higher mean WMI and PSI scores, with 1.32 points (95 % CI: 0.14 − 2.51) and 3.10 points (95 % CI: 1.82–4.38) higher adjusted means, respectively. Boys’ adjusted mean VCI and PRI scores were significantly higher than those of girls, and the mean differences were 2.44 points (95 % CI: 0.95 − 3.94) and 3.68 points (95 % CI: 2.36 − 5.01), respectively.

Conclusions

There is no evidence to suggest sex differences in the general intelligence of early school-aged children in rural China. However, a difference in general intelligence between 10-year-old boys and girls was evident. Girls and boys in rural China tended to show different specific cognitive abilities.

Sex Differences in Variability in General Intelligence: A New Look at the Old Question

The idea that general intelligence may be more variable in males than in females has a long history. In recent years it has been presented as a reason that there is little, if any, mean sex difference in general intelligence, yet males tend to be overrepresented at both the top and bottom ends of its overall, presumably normal, distribution. Clear analysis of the actual distribution of general intelligence based on large and appropriately population-representative samples is rare, however. Using two population-wide surveys of general intelligence in 11-year-olds in Scotland, we showed that there were substantial departures from normality in the distribution, with less variability in the higher range than in the lower. Despite mean IQ-scale scores of 100, modal scores were about 105. Even above modal level, males showed more variability than females. This is consistent with a model of the population distribution of general intelligence as a mixture of two essentially normal distributions, one reflecting normal variation in general intelligence and one refecting normal variation in effects of genetic and environmental conditions involving mental retardation. Though present at the high end of the distribution, sex differences in variability did not appear to account for sex differences in high-level achievement.

Chinese sex differences in intelligence: Some new evidence

Sex differences on the WISC-R in Chinese children were examined in a sample of 788 aged 12 years. Boys obtained a higher mean full scale IQ than girls of 3.75 IQ points, a higher performance IQ of 4.20 IQ points, and a higher verbal IQ of 2.40 IQ points. Boys obtained significantly higher means on the information, picture arrangement, picture completion, block design, and object assembly subtests, while girls obtained a significantly higher mean on coding. The results were in general similar to the sex differences in the United States standardisation sample of the WISC-R. Boys showed greater variability than girls.

Gender differences in intellectual performance persist at the limits of individual capabilities

Males predominate at the top in chess, and chess is a useful domain to investigate possible causes of gender differences in high achievement. Opportunity, interest and extent of practice can be controlled for. Organized chess has objective performance measures, extensive longitudinal population-level data and little gatekeeper influence. Previous studies of gender differences in chess performance have not controlled adequately for females on average playing fewer rated games and dropping out at higher rates. The present study did so by examining performance of international chess players at asymptote and over equal numbers of rated games. Males still were very disproportionately represented at the top. Top female players showed signs of having less natural talent for chess than top males, such as taking more rated games to gain the grandmaster title. The hypothesis that males predominate because many more males play chess was tested by comparing gender performance differences in nations with varying percentages of female players. In well-practised participants, gender performance differences stayed constant even when the average national percentage of female international players increased from 4.2% to 32.3%. In Georgia, where women are encouraged strongly to play chess and females constitute nearly 32% of international players, gender performance differences are still sizeable. Males on average may have some innate advantages in developing and exercising chess skill.

Factor structure and sex differences on the Wechsler Preschool and Primary Scale of Intelligence in China, Japan and United States

This study presents data on the factor structure of the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) and sex and cultural differences in WPPSI test scores among 5- and 6-year-olds from China, Japan, and the United States. Results show the presence of a verbal and nonverbal factor structure across all three countries. Sex differences on the 10 subtests were generally consistent, with a male advantage on a subtest of spatial abilities (Mazes). Males in the Chinese sample obtained significantly higher Full Scale IQ scores than females and had lower variability in their test scores. These observations were not present in the Japan and United States samples. Mean Full Scale IQ score in the Chinese sample was 104.1, representing a 4-point increase from 1988 to 2004.

Gender differences in variance and means on the Naglieri Non-verbal Ability Test: data from the Philippines

BACKGROUND

Research on gender differences in intelligence has focused mostly on samples from Western countries and empirical evidence on gender differences from Southeast Asia is relatively sparse.

AIMS

This article presents results on gender differences in variance and means on a non-verbal intelligence test using a national sample of public school students from the Philippines.

SAMPLE

More than 2,700 sixth graders from public schools across the country were tested with the Naglieri Non-verbal Ability Test (NNAT).

METHODS

Variance ratios (VRs) and log-transformed VRs were computed. Proportion ratios for each of the ability levels were also calculated and a chi-square goodness-of-fit test was performed. An analysis of variance was performed to determine the overall gender difference in mean scores as well as within each of three age subgroups.

RESULTS

Our data show non-existent or trivial gender difference in mean scores. However, the tails of the distributions show differences between the males and females, with greater variability among males in the upper half of the distribution and greater variability among females in the lower half of the distribution. Descriptions of the results and their implications are discussed.

CONCLUSIONS

Results on mean score differences support the hypothesis that there are no significant gender differences in cognitive ability. The unusual results regarding differences in variance and the male-female proportion in the tails require more complex investigations.

The brain as a distributed intelligent processing system: an EEG study

Background

Various neuroimaging studies, both structural and functional, have provided support for the proposal that a distributed brain network is likely to be the neural basis of intelligence. The theory of Distributed Intelligent Processing Systems (DIPS), first developed in the field of Artificial Intelligence, was proposed to adequately model distributed neural intelligent processing. In addition, the neural efficiency hypothesis suggests that individuals with higher intelligence display more focused cortical activation during cognitive performance, resulting in lower total brain activation when compared with individuals who have lower intelligence. This may be understood as a property of the DIPS.

Methodology and Principal Findings

In our study, a new EEG brain mapping technique, based on the neural efficiency hypothesis and the notion of the brain as a Distributed Intelligence Processing System, was used to investigate the correlations between IQ evaluated with WAIS (Whechsler Adult Intelligence Scale) and WISC (Wechsler Intelligence Scale for Children), and the brain activity associated with visual and verbal processing, in order to test the validity of a distributed neural basis for intelligence.

Conclusion

The present results support these claims and the neural efficiency hypothesis.

Sex differences in means and variability on the progressive matrices in university students: A meta-analysis

A meta-analysis is presented of 22 studies of sex differences in university students of means and variances on the Progressive Matrices. The results disconfirm the frequent assertion that there is no sex difference in the mean but that males have greater variability. To the contrary, the results showed that males obtained a higher mean than females by between .22d and .33d, the equivalent of 3.3 and 5.0 IQ conventional points, respectively. In the 8 studies of the SPM for which standard deviations were available, females showed significantly greater variability (F(882,656) = 1.20, p < .02), whilst in the 10 studies of the APM there was no significant difference in variability (F(3344,5660) = 1.00, p > .05).

INTELLIGENCE IN TAIWAN: PROGRESSIVE MATRICES MEANS AND SEX DIFFERENCES IN MEANS AND VARIANCES FOR 6- TO 17-YEAR-OLDS

Data for Raven's Progressive Matrices are reported for a sample of 6290 6- to 17-year-olds in Taiwan. The Taiwanese obtained a mean IQ of 109.5, in relation to a British mean of 100. There was no difference in mean scores of boys and girls at age 7 years. At age 10 years girls obtained significantly higher scores than boys, and at ages 13 and 16 years boys obtained significantly higher scores than girls. There was no sex difference in variance at age 7 years. At ages 10, 13 and 16 years variance was significantly greater in boys.

Further analyses of the effects of practice, dropout, sex, socio-economic advantage, and recruitment cohort differences during the University of Manchester longitudinal study of cognitive change in old age

A sample of 4,314 volunteers who, when first recruited, were aged from 41 to 93 years were quadrennially tested from 2 to 4 occasions during the next 4 to 20 years on the Cattell Culture Fair intelligence test, 2 tests of information-processing speed, the Wechsler Adult Intelligence Scale (WAIS) vocabulary test, and 3 memory tests. After significant effects of practice, sex, demographics, socio-economic advantage, and recruitment cohort had been identified and considered, performance on all tests declined with age. These age-related declines accelerated for the Cattell and WAIS, 2 tests of information speed, and 2 of the memory tests. For all tests individuals’ trajectories of age-related change diverged with increasing age but, unexpectedly, were not affected by demographic factors. Practice gains from an initial experience of the cognitive tests remained undiminished as the interval before the second experience increased from 4 to 8 + years.