Human intelligence differences: towards a combined experimental-differential approach

Despite the fact that much is known about the taxonomy and predictive validity of human intelligence differences, there has been relatively little progress in understanding their cognitive bases. However, some recent firm findings mark the beginnings of a cognitive reductionism in human intelligence. Progress towards discovering 'cognitive components' that, firstly, show individual differences and, secondly, relate to psychometric intelligence differences is described here at different nominal levels of analysis: 'psychometric', 'cognitive-experimental' and 'psychophysical'. The field of intelligence differences remains a fertile yet seriously under-developed demesne in which cognitive scientists should collaborate with differential psychologists.     

Twin-singleton differences in intelligence: a meta-analysis

Since the emergence of twin studies in the 1920s, time and again the question of possible twin-singleton differences in intelligence has been posed. This study addressed the issue through a meta-analysis of published studies on this theme. Twins on the average seem to have lower IQs than singletons. The best estimate for this group difference is 4.2 IQ points (less than one-third of a standard deviation), with a great divide between study outcomes of less vs more recent birth cohorts (5.1 vs 0.5 IQ points, respectively). The evidence is based on studies from six countries (including population-based ones of entire birth cohorts), a massive database (comparisons of more than 30,000 twins with nearly 1.6 million singletons), a variety of intelligence tests, and birth cohorts spanning most of the 20th century, but, for the most part, on an age range limited to children and adolescents. The effect shows considerable between-study heterogeneity but appears robust (fail-safe N calculations), not due to influential individual studies (sensitivity analysis) or publication bias, was present since the very first published studies (cumulative meta-analysis), and appears generalizable across sex, zygosity, and other intelligence domains beyond the verbal. There are insufficient data as to whether the effect persists over the lifespan, exists as well within families, or has ceased in recent birth cohorts of highly developed countries (Denmark and The Netherlands). Likely causes of the effect appear to be prenatal and perinatal factors (reduced fetal growth and shorter gestation for twins).     

Mood regulation and emotional intelligence: individual differences

In 3 studies (Ns = 250, 83, 236), an examination of differences in how individuals experience their emotions (meta-emotion traits of clarity, attention, and intensity) led to the identification of 4 distinct types (overwhelmed, hot, cerebral, and cool). When mood was manipulated, the types differed in how they initially reacted to the emotional situation, how they regulated their mood, and how they made judgments. In particular, one type of individual (the hot type) was more reactive to emotional situations than the others. Another type of individual (the overwhelmed type) regulated mood differently than the others, which led these individuals to make judgments that were also different. Overwhelmed individuals appeared unable or unwilling to avail themselves of critical affective information.     

Emotional intelligence: a theoretical framework for individual differences in affective forecasting

Only recently have researchers begun to examine individual differences in affective forecasting. The present investigation was designed to make a theoretical contribution to this emerging literature by examining the role of emotional intelligence in affective forecasting. Emotional intelligence was hypothesized to be associated with affective forecasting accuracy, memory for emotional reactions, and subsequent improvement on an affective forecasting task involving emotionally evocative pictures. Results from two studies (N = 511) supported our hypotheses. Emotional intelligence was associated with accuracy in predicting, encoding, and consolidating emotional reactions. Furthermore, emotional intelligence was associated with greater improvement on a second affective forecasting task, with the relationship explained by basic memory processes. Implications for future research on basic and applied decision making are discussed.     

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.     

Sex differences on a measure of conformity: comments on a recent study

Reysen and Reysen's recent article made a contribution to understanding of sex differences by interpreting the results through the lens of conformity research. Reinterpretation could refer to differences in perceptual sensitivity. The results, then, are theoretically and empirically consistent with the literature.     

Cerebral lateralization and general intelligence: gender differences in a transcranial Doppler study

The present study evaluated cerebral lateralization during Raven's progressive matrices (RPM) paradigm in female and male subjects. Bilateral simultaneous transcranial Doppler (TCD) ultrasound was used to measure mean blood flow velocities (MBFV) in the right and left middle cerebral arteries (MCAs) in 24 (15 females and 9 males) right-handed normal subjects. The female subjects used a left hemisphere strategy, while males used a right hemisphere strategy to successfully solve RPM tasks. This implies that general intelligence is associated with neural systems within one hemisphere that are accessible to a variety of cognitive processes.     

Reconsolidation: a brief history, a retrieval view, and some recent issues

This review briefly traces some of the history of the phenomenon of what has come to be called "reconsolidation." The early findings of retrograde amnesia for an old but reactivated memory led to several interesting but largely behaviorally oriented studies. With only a few sporadic exceptions, research in the area languished until about 2000, when several articles caught the attention of the neuroscience community and led to a number of studies examining the phenomenon at several different levels of analysis. We consider several of the current issues generated by those studies, present a retrieval based model that may account for some findings, and indicate some possible new directions on this topic.     

Sex differences in brain activity, personality and intelligence: a test of arousability theory

This report describes empirical tests of the theory that females have higher levels of behavioural and cerebral arousal than males and that these differences cause higher neuroticism scores in females, as well as lower female psychoticism scores and lower scores for females on the Gsar factor of intelligence. Data were obtained from 76 subjects, with equal numbers of males and females and with all adult ages represented. EEG responses to auditory stimulation were quantified in terms of the ‘amplitude, ‘rate of change of amplitude and ‘sweep variability factors obtained from a principal components analysis (PCA). Gf, Gc and Gsar intelligence scores were obtained from a PCA of sub-tests from the Wechsler Adult Intelligence Scale plus Ravens Advanced Progressive Matrices test. Personality differences were evaluated using the Neuroticism and Psychoticism scales of the Eysenck Personality Questionnaire (EPQ). Bivariate hypotheses relating sex to the arousal, intelligence and personality variables were confirmed. A PCA demonstrated that sex, neuroticism, Gsar, wakefulness and high frequency EEG activity all have high loadings on a common factor. These results are consistent with expectations generated by the cerebral arousability theory of personality and intelligence.     

Individual differences in intelligence: A reappraisal of the confluence model

The confluence model posits that an individual's intellectual development is a function of the intellectual environment he or she is exposed to in the home. Families with many children spaced close together are presumed to have intellectual environments inferior to those of families with few children spaced far apart. To date, the major support for the confluence model has come from analyses of the family size-birth order means of large aggregate data sets. Analyses of the individual scores of some of those same aggregate data sets, however, do not substantiate the confluence model. Explained variance for the aggregate (90%+) reduce to less than 3% in the individual analyses. A recommendation is made to shift the focus of the confluence model from family configuration variables to family interaction varibles utilizing longitudinal data from individuals.     

Null sex differences in general intelligence: evidence from the WAIS-III

There is an increasing number of studies claiming that the sex differences in general intelligence are "real." The empirical evidence is based on the summation of the standardized sex differences in several cognitive batteries. However, the scientific construct of general ability rests on the correlations among test scores, rather than on their summation. The latter (ability in general) is an arbitrary variable, not a scientific construct. General ability is not a function of any particular cognitive test, but a source of variance evidenced by the correlation between several diverse tests, each of which reflects general ability (g) to some extent, but also group factors and test specificity. Because there are important educational, economic, and social consequences of a group difference in general ability, it is especially germane to evaluate the possibility of an average sex difference in its proxy measures, such as IQ. The Spanish standardization of the WAIS-III is analyzed in the present study. The sample was made up of 703 females and 666 males, aged 15-94, drawn as a representative sample of the population in terms of educational level and geographical location. Although a male advantage of 3.6 IQ points is observed, the difference is in "ability in general," not in "general ability" (g). Given that the main ingredient of the strong association between IQ and a broad range of social correlates is g, and given that there is no sex difference in g, then the average IQ sex-difference favoring males must be attributed to specific group factors and test specificity.     

Racial differences in intelligence: The importance of the executive system

The race-IQ controversy arose at a time when psychometric views of intelligence dominated. Little attention was paid to more process-oriented models in spite of the fact they provide alternative perspectives on the causes of individual differences in problem-solving. We hypothesized that much of the IQ spread commonly observed between black and white children can be attributed to differences in components of their executive systems, including the knowledge base, control processes, and metacognitive states. To test this possibility, black and white children who differed significantly on fluid and crystallized intelligence were tested on multiple tasks reflecting components of the executive systems as well as on perceptual efficiency tasks. Striking group differences were observed in metamemory, stragegy use, and general knowledge, but few reliable differences were found in perceptual efficiency. Regression analyses showed that different factors predicted fluid and crystallized intelligence, with metamemory predicting the latter but not the former. An implication of these findings has potential educational significance: training directed at executive skills, introduce at an early age, might elevate learning and problem-solving skills in black children, thereby reducing racial differences in crystallized intelligence.     

Group differences in intelligence and speed of information-processing

Three samples of different levels of mental ability, but each of average or above- average intelligence, were administered a battery of reaction time test which measure the speed with which persons can execute various elementary cognitive processes. Discriminant analyses revealed significant differences between the groups in reaction times and intraindividual standard deviations, and univariate analyses indicated that higher ability groups tended to obtain significantly faster mean reaction times and to show significantly less intraindividual variability than lower ability groups. Groups which differed the most in mental ability differed to the greatest extent on the more complex reaction time tests but this pattern was reversed for groups which were relatively close in mean IQ. Generally, the results support the hypothesis that speed- of-processing is an important factor unferlying intelligence, but a number of puzzling findings indicate the need for continued research in this area.     

Individual differences, intelligence, and behavior analysis

Despite its avowed goal of understanding individual behavior, the field of behavior analysis has largely ignored the determinants of consistent differences in level of performance among individuals. The present article discusses major findings in the study of individual differences in intelligence from the conceptual framework of a functional analysis of behavior. In addition to general intelligence, we discuss three other major aspects of behavior in which individuals differ: speed of processing, working memory, and the learning of three-term contingencies. Despite recent progress in our understanding of the relations among these aspects of behavior, numerous issues remain unresolved. Researchers need to determine which learning tasks predict individual differences in intelligence and which do not, and then identify the specific characteristics of these tasks that make such prediction possible.     

Sex differences in the structure of intelligence: A confirmatory factor analysis

The LISREL model of Jöreskog and Sörbom was used to perform a comparative factor analysis on intelligence data from four sex by generation groups. The covariance matrices for 10 psychometric ability measures were used to isolate Spatial and Verbal Intelligence factors. Results from a series of factor analysis models indicated substantial invariance in the factor pattern and factor covariance matrices, although the hypothesis of complete invariance in factor pattern was rejected. The groups did differ in observed and unique variances. The results were basically consistent with the hypothesis that males and females have similar intellectual structure, which would have been obscured had standardized factor analysis been employed. There were also sex and generation differences in the Spatial and Verbal factor means.     

Sex differences in self-estimates of lay dimensions of intelligence

260 participants rated themselves on 12 items that made up the three types of intelligence as noted by Sternberg, et al. in 1981. There were sex differences on two of the three standardized scores with men rating themselves higher than women on practical and verbal intelligence. This confirms previous studies of sex differences in the ratings of over-all (g) and multiple intelligences.