Resolving the debate over birth order, family size, and intelligence

Hundreds of research articles have addressed the relationship between birth order and intelligence. Virtually all have used cross-sectional data, which are fundamentally flawed in the assessment of within-family (including birth order) processes. Although within-family models have been based on patterns in cross-sectional data, a number of equally plausible between-family explanations also exist. Within-family (preferably intact-family) data are prerequisite for separating within- and between-family causal processes. This observation reframes an old issue in a way that can be easily addressed by studying graphical patterns. Sibling data from the National Longitudinal Survey of Youth are evaluated, and the results are compared with those from other studies using within-family data. It appears that although low-IQ parents have been making large families, large families do not make low-IQ children in modern U.S. society. The apparent relation between birth order and intelligence has been a methodological illusion.     

Family size and intelligence revisited: the role of emotional intelligence.

Studies examining the link between family size and intelligence have consistently found a negative relationship. Children born into larger families tend to score lower on intelligence tests than children raised in smaller families. One recurrent but unexplained finding is that the relation between intelligence and number of siblings is consistently significant for verbal intelligence but inconsistent for nonverbal intelligence. Here, we conceptualize emotional intelligence as one facet of nonverbal intelligence. The research develops a measure of emotional intelligence and uses it to test the hypothesis that emotional intelligence is positively correlated with family size. The results, based upon a sample of graduate students, support the hypothesized relationship. Implications for the study of family size and intelligence, for refining the conceptualizations and measures of nonverbal intelligence, and for leadership theory, are discussed.     

A differential paradox: the controversy surrounding the Scottish mental surveys of intelligence and family size

In 1947, the Scottish Council for Research in Education and the Population Investigation Committee conducted a survey of Scottish schoolchildren, exploring the relations between tested intelligence and fertility. The survey was not only significant for its size, measuring the IQ of all 11-year-olds at school on the day of testing, some 80,805 children, but also because it was a repeat survey. Its purpose was to establish whether the intelligence of the population had declined because of the negative correlation between IQ and family size. The paper will explore how the impetus for the 1947 survey came from attempts to revive the fortunes of the eugenics movement, based upon the interdisciplinary study of population. While most expected the study to provide evidence of a decline in intelligence, it revealed an increase. This was in spite of a continuing process of differential fertility. This paper will explore the influence of these results, described as a "paradox," upon the future development of the eugenics movement and the sciences of population. While for many, the results were seen to have completely, and thankfully, undermined eugenic fears of degeneration, the supposed "resolution" of the paradox in 1962 provided the basis of a meritocratic and optimistic "new eugenics" that sought to reunite social and biological scientists concerned with human betterment in Britain and the United States.     

The relationship between baseline pupil size and intelligence

Pupil dilations of the eye are known to correspond to central cognitive processes. However, the relationship between pupil size and individual differences in cognitive ability is not as well studied. A peculiar finding that has cropped up in this research is that those high on cognitive ability have a larger pupil size, even during a passive baseline condition. Yet these findings were incidental and lacked a clear explanation. Therefore, in the present series of studies we systematically investigated whether pupil size during a passive baseline is associated with individual differences in working memory capacity and fluid intelligence. Across three studies we consistently found that baseline pupil size is, in fact, related to cognitive ability. We showed that this relationship could not be explained by differences in mental effort, and that the effect of working memory capacity and fluid intelligence on pupil size persisted even after 23 sessions and taking into account the effect of novelty or familiarity with the environment. We also accounted for potential confounding variables such as; age, ethnicity, and drug substances. Lastly, we found that it is fluid intelligence, more so than working memory capacity, which is related to baseline pupil size. In order to provide an explanation and suggestions for future research, we also consider our findings in the context of the underlying neural mechanisms involved.     

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

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

On intelligence education and religious beliefs

A number of authors have suggested that education mediates the negative effect of intelligence on religiosity. However, there is very little direct evidence for this mediation, and the indirect evidence is contradictory. The results of the current paper suggest that, by and large, education does not mediate the effect of intelligence on religiosity. However, the results also suggest that since education has a positive effect on religiosity when religious background is strong and a negative effect when religious background is weak, and since intelligence has a positive effect on education, the negative effect of intelligence on religiosity is stronger when religious background is strong than when it is weak. We examine this mediated moderation model in two large, nationally representative, databases.     

Intelligence, birth order, and family size

The analysis of the National Child Development Study in the United Kingdom (n = 17,419) replicates some earlier findings and shows that genuine within-family data are not necessary to make the apparent birth-order effect on intelligence disappear. Birth order is not associated with intelligence in between-family data once the number of siblings is statistically controlled. The analyses support the admixture hypothesis, which avers that the apparent birth-order effect on intelligence is an artifact of family size, and cast doubt on the confluence and resource dilution models, both of which claim that birth order has a causal influence on children's cognitive development. The analyses suggest that birth order has no genuine causal effect on general intelligence.     

Race, brain size, and intelligence: another reply to Cernovsky

Five sets of observations require explanation. Firstly, within both black and white populations there is a small positive correlation between IQ and brain size whether or not there is control for body size. Secondly, racial differences in average brain size occur such that Mongoloids greater than Caucasoids greater than Negroids especially with control for body size. Thirdly, mammals with larger brains relative to their bodies perform better on learning tasks than mammals with smaller brains relative to their bodies. Fourthly, average racial differences on measures of cognitive performance parallel the differences in average brain size. Fifthly, average racial differences on numerous other traits parallel the differences in intelligence and brain size. It is incumbent on scientists to explain these facts.     

Race, brain size, and intelligence: a reply to Cernovsky

Cernovsky's 1990 critique of my work on the relation between brain size and IQ inadequately presents my position. I did not address the issue of sex differences in brain size nor did I conclude that "women are less intelligent than men" (p. 337). In the autopsy study cited by Cernovsky, it was concluded that, when body size is controlled, the male-female difference in brain size is removed but the black-white difference in brain size remains. Cernovsky also ignores much additional data, including that Mongoloid populations have larger and heavier brains than Caucasoids. Here, I review evidence on the relation between (a) brain size and race and (b) brain size and intelligence. Data are also tabulated for personality, speed of maturation, and sexuality, on all of which the Caucasoid average consistently falls between those of Mongoloids and Negroids. This ordering may be explained by a gene-based evolutionary theory of r/K reproductive strategies in which Mongoloids are more K-selected than Caucasoids and Caucasoids more than Negroids.     

Developmental correlates of emotional intelligence: Temperament,family environment and childhood trauma

This study examined temperament, environmental factors (family environment and childhood trauma), and the interaction between them as developmental correlates of ability and trait emotional intelligence (EI). Using 97 university students, correlational analyses revealed that temperament characteristics were related to trait but not ability EI. Components of family environment and childhood trauma were not significantly related to ability or trait EI. Multiple regression analyses confirmed the importance of temperament in predicting trait EI, but provided little support for the role of environmental factors or their interactions with temperament in predicting either type of EI.     

Inspection time and measured intelligence.

Ten subjects with WAIS Full Scale IQ scores ranging from 119 to 47 were required to discriminate between two lines of markedly different length, exposed in random order for ten different durations, by pressing one of two keys. Estimates of inspection time gamma, the rate at which sensory input is accumulated and passed to subsequent decision processes, were calculated directly from the psychometric functions obtained gamma was found to correlate negatively with subtests contributing to Performance IQ. There was a substantial degree of consistency between estimates of gamma on two different occasions and training did not appear to affect the index, although differences between the two estimates increased as the value of the first estimate increased. Mean overall response latency did not correlate significantly with intelligence and measures obtained from retarded subjects were very like those reported in similar experimental situations utilizing normal subjects. This unexpected finding is interpreted as suggesting that, in this situation, retarded subjects have been prepared to respond on the basis of less evidence than is usually required by intellectually normal subjects.