Multivariate genetic analysis of Wechsler Intelligence Scale for Children--Revised (WISC-R) factors

Applying Biometric Growth Curve Models to Developmental Synchronies in Cognitive Development: The Louisville Twin Study

Biometric latent growth curve models were applied to data from the LTS in order to replicate and extend Wilson's (Child Dev 54:298-316, 1983) findings. Assessments of cognitive development were available from 8 measurement occasions covering the period 4-15 years for 1032 individuals. Latent growth curve models were fit to percent correct for 7 subscales: information, similarities, arithmetic, vocabulary, comprehension, picture completion, and block design. Models were fit separately to WPPSI (ages 4-6 years) and WISC-R (ages 7-15). Results indicated the expected increases in heritability in younger childhood, and plateaus in heritability as children reached age 10 years. Heritability of change, per se (slope estimates), varied dramatically across domains. Significant genetic influences on slope parameters that were independent of initial levels of performance were found for only information and picture completion subscales. Thus evidence for both genetic continuity and genetic innovation in the development of cognitive abilities in childhood were found.

Comparing the Developmental Genetics of Cognition and Personality over the Life Span

Empirical studies of cognitive ability and personality have tended to operate in isolation of one another. We suggest that returning to a unified approach to considering the development of individual differences in both cognition and personality can enrich our understanding of human development. We draw on previous meta-analyses of longitudinal, behavior genetic studies of cognition and personality across the life span, focusing particular attention on age trends in heritability and differential stability. Both cognition and personality are moderately heritable and exhibit large increases in stability with age; however, marked differences are evident. First, the heritability of cognition increases substantially with child age, while the heritability of personality decreases modestly with age. Second, increasing stability of cognition with age is overwhelmingly mediated by genetic factors, whereas increasing stability of personality with age is entirely mediated by environmental factors. Third, the maturational time-course of stability differs: Stability of cognition nears its asymptote by the end of the first decade of life, whereas stability of personality takes three decades to near its asymptote. We discuss how proximal gene-environment dynamics, developmental processes, broad social contexts, and evolutionary pressures may intersect to give rise to these divergent patterns.

Genetic and environmental influences on analogical and categorical verbal and spatial reasoning in 12-year old twins

Research on the genetic influences on different abstract reasoning skills (fluid intelligence) and their interrelation (especially in childhood/adolescence) has been sparse. A novel cognitive test battery, the Verbal and Spatial Reasoning test for Children (VESPARCH 1), consisting of four matched (in terms of test-procedure and design) subtests assessing verbal [analogical (VA) and categorical (VC)] and spatial [analogical (SA) and categorical (SC)] reasoning, was administered to a population based sample of 12-year old twins (169 pairs). Multivariate analysis was conducted to explore the genetic relationship between the four cognitive sub-domains. Heritabilities were 0.62 (VA), 0.49 (VC), 0.52 (SA), and 0.20 (SC). Genetic influences were due to one common factor with no specific genetic influences. This shared genetic factor also explained almost the entire covariance between the domains, as environmental variance was largely specific to each subtest. The finding of no genetic influences specific to each subtest may be due to the uniquely matched design of the VESPARCH 1, reducing confoundment of different test modalities used in conventional tests. For future research or when interpreting previous studies, our findings highlight the importance of taking such potential artefacts (i.e. different test modalities for different sub-domains) into account when exploring the relationship between cognitive sub-domains.

Phenotypic complexity, measurement bias, and poor phenotypic resolution contribute to the missing heritability problem in genetic association studies

BACKGROUND

The variance explained by genetic variants as identified in (genome-wide) genetic association studies is typically small compared to family-based heritability estimates. Explanations of this 'missing heritability' have been mainly genetic, such as genetic heterogeneity and complex (epi-)genetic mechanisms.

METHODOLOGY

We used comprehensive simulation studies to show that three phenotypic measurement issues also provide viable explanations of the missing heritability: phenotypic complexity, measurement bias, and phenotypic resolution. We identify the circumstances in which the use of phenotypic sum-scores and the presence of measurement bias lower the power to detect genetic variants. In addition, we show how the differential resolution of psychometric instruments (i.e., whether the instrument includes items that resolve individual differences in the normal range or in the clinical range of a phenotype) affects the power to detect genetic variants.

CONCLUSION

We conclude that careful phenotypic data modelling can improve the genetic signal, and thus the statistical power to identify genetic variants by 20-99%.

Genome-wide scan of IQ finds significant linkage to a quantitative trait locus on 2q

A genome-wide linkage scan of 795 microsatellite markers (761 autosomal, 34 X chromosome) was performed on Multidimensional Aptitude Battery subtests and verbal, performance and full scale scores, the WAIS-R Digit Symbol subtest, and two word-recognition tests (Schonell Graded Word Reading Test, Cambridge Contextual Reading Test) highly predictive of IQ. The sample included 361 families comprising 2-5 siblings who ranged in age from 15.7 to 22.2 years; genotype, but not phenotype, data were available for 81% of parents. A variance components analysis which controlled for age and sex effects showed significant linkage for the Cambridge reading test and performance IQ to the same region on chromosome 2, with respective LOD scores of 4.15 and 3.68. Suggestive linkage (LOD score>2.2) for various measures was further supported on chromosomes 6, 7, 11, 14, 21 and 22. Where location of linkage peaks converged for IQ subtests within the same scale, the overall scale score provided increased evidence for linkage to that region over any individual subtest. Association studies of candidate genes, particularly those involved in neural transmission and development, will be directed to genes located under the linkage peaks identified in this study.

Head size and intelligence, learning, nutritional status and brain development

This multifactorial study investigates the interrelationships between head circumference (HC) and intellectual quotient (IQ), learning, nutritional status and brain development in Chilean school-age children graduating from high school, of both sexes and with high and low IQ and socio-economic strata (SES). The sample consisted of 96 right-handed healthy students (mean age 18.0 +/- 0.9 years) born at term. HC was measured both in the children and their parents and was expressed as Z-score (Z-HC). In children, IQ was determined by means of the Wechsler Intelligence Scale for Adults-Revised (WAIS-R), scholastic achievement (SA) through the standard Spanish language and mathematics tests and the academic aptitude test (AAT) score, nutritional status was assessed through anthropometric indicators, brain development was determined by magnetic resonance imaging (MRI) and SES applying the Graffar modified method. Results showed that microcephalic children (Z-HC < or = 2 S.D.) had significantly lower values mainly for brain volume (BV), parental Z-HC, IQ, SA, AAT, birth length (BL) and a significantly higher incidence of undernutrition in the first year of life compared with their macrocephalic peers (Z-HC > 2S.D.). Multiple regression analysis revealed that BV, parental Z-HC and BL were the independent variables with the greatest explanatory power for child's Z-HC variance (r(2) = 0.727). These findings confirm the hypothesis formulated in this study: (1) independently of age, sex and SES, brain parameters, parental HC and prenatal nutritional indicators are the most important independent variables that determine HC and (2) microcephalic children present multiple disorders not only related to BV but also to IQ, SA and nutritional background.

Genetic and environmental mediation of the relationship between language and nonverbal impairment in 4-year-old twins

This study of 4-year-old twins investigated the genetic and environmental origins of comorbidity between language impairment and nonverbal ability by testing the extent to which language impairment in one twin predicted nonverbal ability in the co-twin. Impairment of language ability was defined as scores below the 15th percentile on a general language scale derived from a battery of diverse language tests. Four hundred thirty-six children, members of 160 monozygotic (MZ) and 131 same-sex dizygotic (DZ) twin pairs, were identified as language impaired. Language-impaired probands also suffered significant impairments in nonverbal ability. DeFries-Fulker extremes analysis showed evidence for substantial genetic mediation of the phenotypic relationship between language impairment and poor nonverbal ability in that language problems in one twin predicted poor nonverbal ability in the co-twin, much more so for MZ twins than for DZ twins. This finding held even when we excluded those children with language impairment whose nonverbal score indicated general cognitive delay. These results point to a general genetic factor that includes both language and nonverbal problems.

Genetic influences on childhood competencies: a twin study

OBJECTIVE

To estimate genetic, environmental, and rater contrast influences on parental reports of Activities, Social, School, and Total Competence scales of the Child Behavior Checklist (CBCL).

METHOD

Parents of 492 twin pairs aged 8-12 years completed CBCLs. Genetic, shared and unique environmental, and rater bias effects were estimated for the Activities, Social, School, and Total Competence scales. Data on boys and girls were analyzed separately.

RESULTS

Moderate genetic influences were found only for the School scale (60%-76%), while shared environment accounted for most of the variance in Activities, Social, and Total Competence scales. Gender differences are reported. Similar to a prior twin study of CBCL problem syndromes, there was no evidence of rater bias.

CONCLUSIONS

Estimates of genetic influence on these child competence domains were high for School Competence, while social competence and activity competence evidenced higher levels of shared environmental influences. Organization and wording of CBCL items may avoid rater biases in reporting. These findings have implications for interventions to improve school, social, and activities competence.

The structure of language abilities at 4 years: a twin study

Normal language development was studied in 310 pairs of 4-year-old twins born in the United Kingdom in 1994. Twins were assessed individually in their homes on a diverse battery of language and nonverbal measures. Rotated factor analyses indicated the presence of a general Language factor (L) as well as a general Nonverbal (NV) factor. Moderate genetic influence was found for both L and NV abilities. Bivariate genetic analysis estimated a genetic correlation of .63 between L and NV abilities, implying that over half of the genetic influence on L overlaps with genetic influence on NV. These results suggest that at age 4, genetic influences on individual differences in language overlap substantially with genetic influences on individual differences in other cognitive abilities, although perhaps less so than later in development.

Nutritional status, brain development and scholastic achievement of Chilean high-school graduates from high and low intellectual quotient and socio-economic status

The objective of the present study was to investigate the inter-relationships between nutritional status (past and current nutrition), brain development, and scholastic achievement (SA) of Chilean high-school graduates from high and low intellectual quotient (IQ) and socio-economic status (SES) (mean age 18.0 (SD 0.9) years). Results showed that independently of SES, high-school graduates with similar IQ have similar nutritional, brain development and SA variables. Multiple regression analysis between child IQ (dependent variable) and age, sex, SES, brain volume (BV), undernutrition during the first year of life, paternal and maternal IQ (independent variables) revealed that maternal IQ (P<0.0001), BV (P<00387) and severe undernutrition during the first year of life (P<0.0486), were the independent variables with the greatest explanatory power for child IQ variance (r2 0.707), without interaction with age, sex or SES. Child IQ (P<0.0001) was the only independent variable that explained both SA variance (r2 0.848) and academic aptitude test variance (r2 0.876) without interaction with age, sex or SES. These results confirm the hypotheses formulated for this study that: (1) independently of SES, high-school graduates with similar IQ have similar variables of nutritional status, brain development and SA; (2) past nutritional status, brain development, child IQ and SA are strongly and significantly inter-related. These findings are relevant in explaining the complex interactions between variables that affect IQ and SA and can be useful for nutritional and educational planning.

A study of genetic and environmental effects on the co-occurrence of problem behaviors in three-year-old twins

The authors examined the genetic and environmental causes of the co-occurrence of problem behaviors in children. The analyses involved mother and father ratings of Oppositional, Withdrawn/Depressed, Aggressive, Anxious, Overactive, and Sleep Problems in 446 monozygotic and 912 dizygotic pairs of 3-year-old twins. Genetic factors contributed on average .150 (37.3%), shared environment .206 (51.2%), and nonshared environment .046 (11.4%) to the phenotypic correlations between the syndromes. Genetic and environmental factors caused different groupings. Internalizing and Externalizing groupings were indicative of nonshared environmental factors; clusters of problem behaviors with either the Aggressive or Anxious symptoms were most suggestive of genetic factors, and high scores on all syndromes indicated shared environmental influences.

The independent prediction of general intelligence by elementary cognitive tasks: genetic and environmental influences

Current theories of intelligence have, in some cases, begun to include elementary cognitive tasks. Behavioral genetic studies of intelligence have not taken these theories into account. The current study includes 135 MZ and 128 DZ twin pairs from the Western Reserve Twin Project. The 11 WISC-R subtests as well as 6 elementary cognitive tasks were employed. Using a Schmid-Leiman (1957) transformation, analyses indicate a four-group factor model, supported by a second-order general factor at both phenotypic and biometric levels. Results indicate that the general factor, group factors, and specific residuals are necessary when examining additive genetic variance. Common environmental variance can be collapsed into a single general factor.