A Role for the X Chromosome in Sex Differences in Variability in General Intelligence?

ABSTRACT— There is substantial evidence that males are more variable than females in general intelligence. In recent years, researchers have presented this as a reason that, although there is little, if any, mean sex difference in general intelligence, males tend to be overrepresented at both ends of its overall distribution. Part of the explanation could be the presence of genes on the X chromosome related both to syndromal disorders involving mental retardation and to population variation in general intelligence occurring normally. Genes on the X chromosome appear overrepresented among genes with known involvement in mental retardation, which is consistent with a model we developed of the population distribution of general intelligence as a mixture of two normal distributions. Using this model, we explored the expected ratios of males to females at various points in the distribution and estimated the proportion of variance in general intelligence potentially due to genes on the X chromosome. These estimates provide clues to the extent to which biologically based sex differences could be manifested in the environment as sex differences in displayed intellectual abilities. We discuss these observations in the context of sex differences in specific cognitive abilities and evolutionary theories of sexual selection.     

Not so F.A.S.T., Dr. Vickers!

Vickers and his co-workers have recently reported studies with a “frequency accrual speed test” (FAST) that was devised to assess individuals' speed or efficiency of sampling of sensory input. According to Vickers, the FAST overcomes some of the limitations of the inspection time (IT) task, a psychophysical measure that has moderate correlations with IQ scores. After several experiments, Vickers concluded that speed of sensory processing is unimportant in FAST (and IT) differences. After a critical review of the FAST's theoretical basis and empirical results, we argue that the FAST is not appropriately described as an equivalent or alternative to IT, and that results from FAST studies are not generalizable to IT. The FAST involves a substantial memory load. Moreover, no studies to date have used appropriate stimulus parameters to allow Vickers' claim that speed of processing is unimportant in FAST (and IT) variance. The FAST, we suggest, has moderate correlations with IQ-type test because it is a complex, high-level task that is poorly parameterized. Specific hypotheses are formulated based on our understanding of the nature of the FAST. For those who wish to understand the information processing bases of higher order abilities the IT task (also devised by Vickers) continues to afford a better focus of study.     

Recurrent severe hypoglycemia, intelligence, and speed of information processing

In a previous study, the cumulative effect of recurrent severe hypoglycemia on cognitive function was examined in 100 patients with insulin-treated diabetes and a significant correlation was observed between the frequency of severe hypoglycemic episodes and the difference between their current IQ and estimated premorbid IQ (Langan, Deary, Hepburn, & Frier, 1991). The study here extended this model for examination of the role of biological environmental influences on intelligence. Eighty-five of the original cohort of 100 patients were recalled and tested on Wechsler Performance IQ, a test of premorbid IQ, the Hick Reaction Time task, the Sternberg Memory Scan task and a test of Rapid Visual Information Processing (RVIP). The patients' frequency of severe hypoglycemia was assessed using a structured interview. Both IQ-decrement estimates and frequencies of severe hypoglycemia proved reliable after 18 months (correlations > .75) and the corrected correlation between these two variables was about .4. Therefore, repeated episodes of acute severe neuroglycopenia are associated with a lowering of IQ. Frequency of severe hypoglycemia was significantly related to Hick decision times and to response thresholds in the RVIP task. Factor analysis suggested that hypoglycemia affects decision or response-initiation processes rather than encoding, storage, comparison, or classification processes in short-term memory. Indices of these short-term memory processes were highly related to IQ-type test scores. This study also confirms, in a sample with a near-normal distribution of IQ, that measures of psychometric intelligence are significantly correlated with indices from elementary cognitive tests, especially RVIP task, which is based on signal-detection theory.     

125 years of intelligence in the American Journal of Psychology

A survey is made of intelligence research in the 125 years of The American Journal of Psychology. There are some major articles of note on intelligence, especially Spearman's (1904a) article that discovered general cognitive ability (g). There are some themes within intelligence on which articles appeared over the years, such as processing speed, age, and group differences. Intelligence has not been a major theme of the journal, nor has a differential approach to psychology more generally. There are periods of time--especially the 1970s--during which almost no articles appeared on intelligence. The key articles and themes on intelligence differences are discussed in detail.     

Typical intellectual engagement and cognition in the ninth decade of life: The Lothian Birth Cohort 1921

Investment traits-the tendency to seek out and engage in cognitive activity-might affect intellectual growth across the life span, specifically the development from fluid to crystallized intelligence. Here we explore how childhood IQ at age 11 years, IQ at age 79, and the investment trait Typical Intellectual Engagement (TIE) at age 81 affect the mean level and change in verbal fluency scores, used as an indicator of crystallized intelligence, across the ages 79, 83, and 87 in the Lothian Birth Cohort 1921 (maximum N = 569; Deary, Whiteman, Starr, Whalley, & Fox, 2004). A first latent growth model showed significant variance in the mean level of verbal fluency and significant decline in verbal fluency from age 79 to age 87. The rate of change was invariant across study participants in the Lothian Birth Cohort 1921. A second model found that IQ at age 11 significantly predicted IQ at age 79 (β = .66; p < .001), which in turn predicted verbal fluency and TIE in the ninth decade of life with standardized path parameters of .46 and .15 (p < .001), respectively. TIE had a significant association with verbal fluency (β = .14, p = .002); together, IQ at age 11 and 79 and TIE accounted for 25.5% of the variance in verbal fluency. A final model identified the TIE subfactor of intellectual curiosity as a significant mediator of the effect of IQ on verbal fluency; the TIE subfactors abstract thinking, reading, and problem solving showed no significant associations. In summary, TIE-in particular, intellectual curiosity-significantly mediated the effects of IQ on crystallized intelligence in old age. Because there was no significant between-subjects variance in verbal fluency trajectories in the current study, neither TIE nor IQ were associated with individual differences in cognitive decline. (PsycINFO Database Record (c) 2012 APA, all rights reserved).     

Intergenerational class mobility in Britain: A comparative look across three generations in the Lothian Birth Cohort 1936

The Lothian Birth Cohort 1936 sample is in a uniquely good position to provide relevant data on social class mobility patterns over most of the last century. These participants, with known ultimate social class attainment, took a validated mental test in the Scottish Mental Survey of 1947 and were followed up at approximately age 70. Then, besides their own age 11 mental ability data and educational and occupational attainment, they reported educational and occupational data on their parents and offspring. We constructed structural equation models of participant and offspring social class attainment and odds of social class movement, and compared results with those from the similarly recruited and assessed Lothian Birth Cohort 1921. In the 1936 cohort, relative transmission of social class from one generation to the next decreased, while relative transmission of educational attainment increased. In addition, effects of childhood IQ and educational attainment on social class decreased, apparently due to relative decrease in overall upward mobility and absolute increase in overall education level.     

Is body mass index in old age related to cognitive abilities? The Lothian Birth Cohort 1936 Study

We tested the hypothesis that the previously reported association between a higher body mass index (BMI) and poorer cognition in later adulthood is an artifact of confounding by previous cognitive ability and socioeconomic status. Participants were 1,079 adults aged about 70 years in the Lothian Birth Cohort 1936 Study, on whom there are IQ data from age 11. Cognitive outcome measures included: IQ at age 70 using the same test that was administered at age 11; composite measures of general cognitive ability (g factor), speed of information processing, and memory; and two tests of verbal ability. People classified as overweight or obese in later adulthood had significantly lower scores on tests of childhood IQ, age 70 IQ, g factor, and verbal ability. There was no significant association with processing speed or memory performance. After adjusting for childhood IQ and social class in general linear models, associations with age 70 IQ and g factor were nonsignificant or attenuated. However, throughout the models, there was a persistent (inverse) relationship between BMI and performance on the National Adult Reading Test (NART) and Wechsler Test of Adult Reading (WTAR), which remained significant after full adjustment for all sociodemographic and health covariates (for the NART, p = .025; for the WTAR, p = .011). The findings suggest that the previously reported BMI-cognition associations in later adulthood could be largely accounted for by prior ability and socioeconomic status, and by the possible influence of these factors on the adoption of health behaviors in adulthood.     

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.     

Stability and change in intelligence from age 11 to ages 70, 79, and 87: the Lothian Birth Cohorts of 1921 and 1936

Investigating the predictors of age-related cognitive change is a research priority. However, it is first necessary to discover the long-term stability of measures of cognitive ability because prior cognitive ability level might contribute to the amount of cognitive change experienced within old age. These two issues were examined in the Lothian Birth Cohorts of 1921 and 1936. Cognitive ability data were available from age 11 years when the participants completed the Moray House Test No. 12 (MHT). The Lothian Birth Cohort 1936 (LBC1936) completed the MHT a second time at age 70. The Lothian Birth Cohort 1921 (LBC1921) completed the MHT at ages 79 and 87. We examined cognitive stability and change from childhood to old age in both cohorts, and within old age in the LBC1921. Raw stability coefficients for the MHT from 11-70, 11-79, and 11-87 years were .67, .66, and .51, respectively; and larger when corrected for range restriction in the samples. Therefore, minimum estimates of the variance in later-life MHT accounted for by childhood performance on the same test ranged from 26-44%. This study also examined, in the LBC1921, whether MHT score at age 11 influenced the amount of change in MHT between ages 79 and 87. It did not. Higher intelligence from early life was apparently protective of intelligence in old age due to the stability of cognitive function across the lifespan, rather than because it slowed the decline experienced in later life.