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Most study samples show less variability in key variables than do their source populations due most often to indirect selection into study participation associated with a wide range of personal and circumstantial characteristics. Formulas exist to correct the distortions of population-level correlations created. Formula accuracy has been tested using simulated normally distributed data, but empirical data are rarely available for testing. We did so in a rare data set in which it was possible: the 6-Day Sample, a representative subsample of 1,208 from the Scottish Mental Survey 1947 of cognitive ability in 1936-born Scottish schoolchildren (70,805). 6-Day Sample participants completed a follow-up assessment in childhood and were re-recruited for study at age 77 years. We compared full 6-Day Sample correlations of early-life variables with those of the range-restricted correlations in the later-participating subsample, before and after adjustment for direct and indirect range restriction. Results differed, especially for two highly correlated cognitive tests; neither reproduced full-sample correlations well due to small deviations from normal distribution in skew and kurtosis. Maximum likelihood estimates did little better. To assess these results' typicality, we simulated sample selection and made similar comparisons using the 42 cognitive ability tests administered to the Minnesota Study of Twins Reared Apart, with very similar results. We discuss problems in developing further adjustments to offset range-restriction distortions and possible approaches to solutions.

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The Minnesota Study of Twins Reared Apart (MISTRA) was initiated in 1979 and continued until 2000. It consisted of 139 pairs of twins who had been separated in early childhood and not re-united until adulthood, and members of their families. As part of a broader assessment, these participants completed 42 mental ability tests from three well-known test batteries. In this article, we present some background on the sample and tests and the correlation matrix of test scores, for the use of other researchers. As in all science, however, replication across samples of both tests and participants remains key to the development of ideas about mental ability.

Floyd, Shands, Rafael, Bergeron and McGrew (2009) used generalizability theory to test the reliability of general-factor loadings and to compare three different sources of error in them: the test battery size, the test battery composition, the factor-extraction technique, and their interactions. They found that their general-factor loadings were moderately to strongly dependable. We replicated the methods of Floyd et al. (2009) in a different sample of tests, from the Minnesota Study of Twins Reared Apart (MISTRA). Our first hypothesis was that, given the greater diversity of the tests in MISTRA, the general-factor loadings would be less dependable than in Floyd et al. (2009). Our second hypothesis, contrary to the positions of Floyd et al. (2009) and Jensen and Weng (1994), was that the general factors from the small, randomly-formed test batteries would differ substantively from the general factor from a well-specified hierarchical model of all available tests. Subtests from MISTRA were randomly selected to form independent and overlapping batteries of 2, 4 and 8 tests in size, and the general-factor loadings of eight probe tests were obtained in each battery by principal components analysis, principal factor analysis and maximum likelihood estimation. Results initially indicated that the general-factor loadings were unexpectedly more dependable than in Floyd et al. (2009); however, further analysis revealed that this was due to the greater diversity of our probe tests. After adjustment for this difference in diversity, and consideration of the representativeness of our probe tests versus those of Floyd et al. (2009), our first hypothesis of lower dependability was confirmed in the overlapping batteries, but not the independent ones. To test the second hypothesis, we correlated "g" factor scores from the random test batteries with "g" factor scores from the VPR model; we also calculated special coefficients of congruence on the same relation. Consistent with our second hypothesis, the general factors from small non-hierarchical models were found to not be reliable enough for the purposes of theoretical research. We discuss appropriate standards for the construction and factor analysis of intelligence test batteries. (Contains 6 tables.)

The few studies that have examined associations between measured interests and abilities have suffered from small sample sizes, restricted ranges of ability and background, preconceived groupings of interests, and measures of ability that confound general and specific cognitive abilities. In this study of 425 adults from diverse backgrounds, the authors used latent class analysis, with general intelligence and two dimensions of ability that are uncorrelated with general intelligence as covariates, to articulate eight occupational interests that could be characterized by level and profile of ability. These groups showed substantial mean differences in all covariates, and differences in the covariates had substantial effects on probabilities of interest group classification. Sex differences in ability dimensions did not, however, completely explain the sex differences in most likely interest group classification. Although socialization may explain the greater sex differences in occupational interests than abilities, biological explanations are also possible. (Contains 3 figures, 7 tables and 4 notes.)

In a recent paper, Johnson, Bouchard, Krueger, McGue, and Gottesman (2004) addressed a long-standing debate in psychology by demonstrating that the g factors derived from three test batteries administered to a single group of individuals were completely correlated. This finding provided evidence for the existence of a unitary higher-level general intelligence construct whose measurement is not dependent on the specific abilities assessed. In the current study we constructively replicated this finding utilizing five test batteries. The replication is important because there were substantial differences in both the sample and the batteries administered from those in the original study. The current sample consisted of 500 Dutch seamen of very similar age and somewhat truncated range of ability. The batteries they completed included many tests of perceptual ability and dexterity, and few verbally oriented tests. With the exception of the "g" correlations involving the Cattell Culture Fair Test, which consists of just four matrix reasoning tasks of very similar methodology, all of the "g" correlations were at least 0.95. The lowest "g" correlation was 0.77. We discuss the implications of this finding.

Fluctuating asymmetry (FA) is the non-pathological left-right asymmetry of body traits that are usually left-right symmetrical, such as eye breadths and elbow to wrist lengths in humans, but which can be affected by developmental stressors. It is generally considered throughout biology to be an indicator of developmental instability and thus of lack of overall biological fitness. Several investigators have proposed that deficiencies in general intelligence ("g") may be indicators of the same kind of instability in human brain development. If so, FA and "g" should be negatively correlated. Moreover, because "g" shows substantial genetic influences, FA should also show genetic influences, and the two sets of genetic influences should be correlated. We investigated these propositions in a sample of 263 adults that included 88 pairs of twins. Results indicated genetic influences on FA, but FA and "g" were not correlated at either the observed or genetic levels.

Recent work with the 42 mental ability tests administered to participants of the Minnesota Study of Twins Reared Apart (MISTRA) has suggested that there are important dimensions of mental ability that function independently of "g". Two of these dimensions, rotation-verbal and focus-diffusion, appear to involve trade-offs: greater residual rotation ability implies less residual verbal ability and vice-versa, and the focus-diffusion dimension functions similarly. These two uncorrelated dimensions also show strong sex differences. Individuals lying at different positions along these dimensions may have brains that differ structurally and/or functionally, leading to differences in the ways they approach mental ability tasks. If so, we should expect a lack of factorial invariance in test scores across groups of individuals lying at different positions on the two dimensions, indicating that the tests do not measure the same constructs in the same ways in the different groups. This study demonstrated such a lack of factorial invariance in each of the three mental ability batteries included in the MISTRA assessment. The Wechsler Adult Intelligence Scale can be scored to correlate 0.7 with each of the two dimensions. We propose that use of these scores may help to clarify brain-mapping studies relating brain structure and function and to facilitate understanding of sex differences in mental abilities.

Empirical data suggest that there is at most a very small sex difference in general mental ability, but men clearly perform better on visuospatial tasks while women clearly perform better on tests of verbal usage and perceptual speed. In this study, we integrated these overall findings with predictions based on the Verbal-Perceptual-Rotation (VPR) model ([Johnson, W., and Bouchard, T. J. (2005a). Constructive replication of the visual-perceptual-image rotation (VPR) model in Thurstone's (1941) battery of 60 tests of mental ability. Intelligence, 33, 417-430.; Johnson, W., and Bouchard, T. J. (2005b). The structure of human intelligence: It's verbal, perceptual, and image rotation (VPR), not fluid and crystallized. Intelligence, 33. 393-416.]) of the structure of mental abilities. We examined the structure of abilities after removing the effects of general intelligence, identifying three underlying dimensions termed rotation-verbal, focus-diffusion, and memory. Substantial sex differences appeared to lie along all three dimensions, with men more likely to be positioned towards the rotation and focus poles of those dimensions, and women displaying generally greater memory. At the level of specific ability tests, there were greater sex differences in residual than full test scores, providing evidence that general intelligence serves as an all-purpose problem solving ability that masks sex differences in more specialized abilities. The residual ability factors we identified showed strong genetic influences comparable to those for full abilities, indicating that the residual abilities have some basis in brain structure and function.

In two recent papers, Johnson & Bouchard [Johnson, W., & Bouchard, T. J., Jr. (2005a). The structure of human intelligence: It is verbal, perceptual, and image rotation (VPR) , not fluid and crystallized. "Intelligence," 33, 393-416, Johnson W., & Bouchard, T. J., Jr. (2005b). Constructive replication of the visual perceptual-image rotation model in Thurstone's (1941) battery of 60 tests of mental ability. "Intelligence," 33, 417-430.] have evaluated the relative descriptive accuracies of the Cattell-Horn fluid-crystallized model and the Vernon verbal-perceptual model of the structure of human intelligence. In all three samples evaluated in those papers, the Vernon model provided a measurably more accurate description of the data. In addition, descriptive accuracy could be significantly improved with the use of a four-stratum model with a "g" factor at the top of the hierarchy and three factors at the third stratum. The model accords similar importance to verbal, perceptual, and image rotation abilities in the hierarchy; thus, Johnson and Bouchard termed it the Verbal-Perceptual-Image Rotation (VPR) model. In the current study, we constructively replicated the model comparisons and development of the VPR model using the data matrix of 46 mental ability tests published by de Wolff and Buiten [de Wolff, Ch. J., & Buiten (1963). Een factoranalyse van vier testbatterijen [A factor analysis of four test batteries]. "Nederlands Tijdschrift voor de Psychologie," 18, 220-239.]. The sample consisted of 500 professional seamen of the Royal Dutch Navy.

In previous papers [Johnson, W., & Bouchard Jr., T. J. (2005a). Constructive Replication of the Visual-Perceptual-Image Rotation (VPR) Model in Thurstone's (1941) Battery of 60 Tests of Mental Ability. Intelligence, 33, 417-430.] [Johnson, W., & Bouchard Jr., T. J. (2005b). The Structure of Human Intelligence: It's Verbal, perceptual, and image rotation (VPR), not Fluid and Crystallized. Intelligence, 33, 393-416.] we have proposed the Verbal, perceptual, and image rotation (VPR) model of the structure of mental abilities. The VPR model is hierarchical, with a g factor that contributes strongly to broad verbal, perceptual, and image rotation abilities, which in turn contribute to 8 more specialized abilities. The verbal and perceptual abilities, though separable, are highly correlated, as are the perceptual and mental rotation abilities. The verbal and mental rotation abilities are much less correlated. In this study we used the twin sample in the Minnesota Study of Twins Reared Apart to estimate the genetic and environmental influences and the correlations among them at each order of the VPR model. Genetic influences accounted for 67-79% of the variance throughout the model, with the exception of the second-stratum Content Memory factor, which showed 33% genetic influence. These influences could not be attributed to assessed similarity of rearing environment. Genetic correlations closely mirrored the phenotypic correlations. Together, these findings substantiate the theory that the entire structure of mental abilities is strongly influenced by genes.

The concept of a general intelligence factor or "g" is controversial in psychology. Although the controversy swirls at many levels, one of the most important involves "g's" identification and measurement in a group of individuals. If "g" is actually predictive of a range of intellectual performances, the factor identified in one battery of mental ability tests should be closely related to that identified in another dissimilar aggregation of abilities. We addressed the extent to which this prediction was true using three mental ability batteries administered to a heterogeneous sample of 436 adults. Though the particular tasks used in the batteries reflected varying conceptions of the range of human intellectual performance, the "g" factors identified by the batteries were completely correlated (correlations were 0.99, 0.99, and 1.00). This provides further evidence for the existence of a higher-level "g" factor and suggests that its measurement is not dependent on the use of specific mental ability tasks.

A battery of "g" loaded mental ability tests was administered to 274 Bedouin children and adolescents, full and half siblings from 106 families. Results demonstrate internal validity for the tests in this cultural context and support the idea that shared family environment is a strong determinant of sibling similarity for children in the given age range. (SLD)

Described is an ongoing study of monozygotic and dizygotic twins, separated in infancy and reared apart. Discussed are the procedures and results of this study with interpretations of the results. The effects of parenting, education, and other interventions are considered. (CW)

Provides a comprehensive contemporary summary of the world literature on IQ correlations between relatives. A profile of average correlations consistent with a polygenic mode of inheritence is revealed from a summary of 111 studies, although a marked degree of heterogeneity of the correlations within familial groupings is also found. (CS)