Genetic Covariation Between Brain Volumes and IQ,Reading Performance,and Processing Speed

Although there has been much interest in the relation between brain size and cognition, few studies have investigated this relation within a genetic framework and fewer still in non-adult samples. We analyzed the genetic and environmental covariance between structural MRI data from four brain regions (total brain volume, neocortex, white matter, and prefrontal cortex), and four cognitive measures (verbal IQ (VIQ), performance IQ (PIQ), reading ability, and processing speed), in a sample of 41 MZ twin pairs and 30 same-sex DZ twin pairs (mean age at cognitive test = 11.4 years; mean age at scan = 15.4 years). Multivariate Cholesky decompositions were performed with each brain volume measure entered first, followed by the four cognitive measures. Consistent with previous research, each brain and cognitive measure was found to be significantly heritable. The novel finding was the significant genetic but not environmental covariance between brain volumes and cognitive measures. Specifically, PIQ shared significant common genetic variance with all four measures of brain volume (r _g = .58–.82). In contrast, VIQ shared significant genetic influence with neocortex volume only (r _g = .58). Processing speed was significant with total brain volume (r _g = .79), neocortex (r _g = .64), and white matter (r _g = .89), but not prefrontal cortex. The only brain measure to share genetic influence with reading was total brain volume (r _g = .32), which also shared genetic influences with processing speed.     

Nonshared Environmental Influences on Sleep Quality: A Study of Monozygotic Twin Differences

Research has consistently demonstrated that environmental influences are important for explaining the variability in sleep quality observed in the general population. Although there is substantial evidence assessing associations between sleep quality and a host of environmental variables, it is possible that their effects are mediated by genetic influence. A monozygotic twin differences design was used to assess the specific contribution of nonshared environmental influences on sleep quality, whilst controlling for genetic and shared environmental effects in a sample of 380 monozygotic twins (mean age 19.8 years, SD = 1.26, range = 18–22 years). Participants completed the Pittsburgh Sleep Quality Index and questionnaires assessing several candidate “environmental” measures. When controlling for genetic and shared environmental effects, within monozygotic twin-pair differences in sleep quality were associated with within monozygotic twin-pair differences in general health for males (β = 1.56, p < 0.001) and relationship satisfaction for females (β = 1.01, p < 0.05). For the remaining environmental measures the results suggest that these seemingly “environmental” influences are actually in part dependent on genetics and/or the shared environment. These findings give insight into how specific environments affect sleep and the possible mechanisms behind these associations.     

Food Neophobia in Young Adults: Genetic Architecture and Relation to Personality,Pleasantness and Use Frequency of Foods,and Body Mass Index—A Twin Study

Food neophobia has been studied extensively in children, but its causal origins and relationship to eating behavior in adults are not well understood. We studied genetic and environmental effects on variation in food neophobia, measured using the Food Neophobia Scale, and explored associations between food neophobia and personality, pleasantness and use frequency of food groups, and body mass index in young adult twins (N = 1175, aged 20–25 years, 54.7% women). In women, additive genetic effects (heritability) accounted for 61% of variation in food neophobia, whereas in men, shared environmental effects explained 45% of the variation. Food neophobia negatively correlated with the personality trait Openness, corrected for the structural overlap (r = −0.23), and in women, these two traits had a genetic correlation (r _g = −0.39). In addition, food neophobia negatively correlated with pleasantness and use frequency of fruits and vegetables and of fish and with mean pleasantness of foods. Once evolutionarily important, food neophobia should at present be considered in nutrition counseling as a possible barrier to a balanced diet.     

Genetic Regulation of Pre-Pubertal Development of Body Mass Index: A Longitudinal Study of Japanese Twin Boys and Girls

We analyzed the genetic architecture of prepubertal development of relative weight to height in 216 monozygotic and 159 dizygotic complete Japanese twin pairs (52% girls). Ponderal index at birth (kg/m³) and body mass index (BMI, kg/m²) from 1 to 11 years of age were used. Additive genetic factors explained the major proportion (52–74%) of the variation of BMI from 1 to 11 years of age. Environmental factors common to both co-twins also showed some effect (7–28%), but at most ages this was not statistically significant. Strong genetic tracking was found for BMI from 1 to 11 years of age, but there was also evidence for a persistent effect of common environmental factors. Our results suggest that the genetic architecture of BMI development in the Japanese population is generally similar to that found in previous twin studies in Caucasian populations.     

Heritability, Assortative Mating and Gender Differences in Violent Crime: Results from a Total Population Sample Using Twin, Adoption, and Sibling Models

Research addressing genetic and environmental determinants to antisocial behaviour suggests substantial variability across studies. Likewise, evidence for etiologic gender differences is mixed, and estimates might be biased due to assortative mating. We used longitudinal Swedish total population registers to estimate the heritability of objectively measured violent offending (convictions) in classic twin (N = 36,877 pairs), adoptee-parent (N = 5,068 pairs), adoptee-sibling (N = 10,610 pairs), and sibling designs (N = 1,521,066 pairs). Type and degree of assortative mating were calculated from comparisons between spouses of siblings and half-siblings, and across consecutive spouses. Heritability estimates for the liability of violent offending agreed with previously reported heritability for self-reported antisocial behaviour. While the sibling model yielded estimates similar to the twin model (A ≈ 55%, C ≈ 13%), adoptee-models appeared to underestimate familial effects (A ≈ 20–30%, C ≈ 0%). Assortative mating was moderate to strong (r _spouse = 0.4), appeared to result from both phenotypic assortment and social homogamy, but had only minor effect on variance components. Finally, we found significant gender differences in the etiology of violent crime.     

Heritability and Longitudinal Stability of Schizotypal Traits During Adolescence

The study investigated the genetic and environmental etiology of schizotypal personality traits in a non-selected sample of adolescent twins, measured on two occasions between the ages of 11 and 16 years old. The 22-item Schizotypal Personality Questionnaire- Child version (SPQ-C) was found to be factorially similar to the adult version of this instrument, with three underlying factors (Cognitive-Perceptual, Interpersonal-Affective, and Disorganization). Each factor was heritable at age 11–13 years (h ² = 42–53%) and 14–16 years old (h ² = 38–57%). Additive genetic and unique environmental influences for these three dimensions of schizotypal personality acted in part through a single common latent factor, with additional genetic effects specific to both Interpersonal-Affective and Disorganization subscales at each occasion. The longitudinal correlation between the latent schizotypy factor was r = 0.58, and genetic influences explained most of the stability in this latent factor over time (81%). These longitudinal data demonstrate significant genetic variance in schizotypal traits, with moderate stability between early to middle adolescence. In addition to common influences between the two assessments, there were new genetic and non-shared environmental effects that played a role at the later assessment, indicating significant change in schizotypal traits and their etiologies throughout adolescence.     

Internalizing Behavior in Adolescent Girls Affects Parental Emotional Overinvolvement: A Cross-lagged Twin Study

The aim of this study was to examine the direction and the etiology of the association between different parenting styles (parental emotional overinvolvement [EOI] and parental criticism) and internalizing behavior from adolescence to early adulthood. A longitudinal genetically informative cross-lagged design was applied to a population-based sample of Swedish twins contacted at age 16–17 (n = 2369) and at age 19–20 (n = 1705). Sex-limitation modelling revealed different effects for boys and girls. For girls, genetic influences on internalizing problems at age 16–17 independently explained 2.7% of the heritability in parental EOI at age 19–20. These results suggest that emotionally overinvolved and self-sacrificing parental behavior stems in part from daughters (but not sons) genetic predisposition for internalizing behavior. These findings highlight the importance of genetically influenced child-driven effects underlying the parenting-internalizing association, and clarify that the role of such effects may differ depending on sex, type of parenting and developmental period.     

A Bivariate Twin Study of Regional Brain Volumes and Verbal and Nonverbal Intellectual Skills During Childhood and Adolescence

Twin studies indicate that both intelligence and brain structure are moderately to highly heritable. Recent bivariate studies of adult twins also suggest that intelligence and brain morphometry are influenced by shared genetic factors. The current study examines shared genetic and environmental factors between brain morphometry and intelligence in a sample of children and adolescents (twins, twin siblings, and singletons; n = 649, ages 4–19). To extend previous studies, brain morphometric data were parsed into subregions (lobar gray/white matter volumes, caudate nucleus, lateral ventricles) and intelligence into verbal and nonverbal skills (Wechsler Vocabulary and Block Design subtests). Phenotypic relationships between brain volumes and intelligence were small. Verbal skills shared unique environmental effects with gray matter volumes while nonverbal skills shared genetic effects with both global and regional gray and white matter. These results suggest that distinct mechanisms contribute to the small phenotypic relationships between brain volumes and verbal versus nonverbal intelligence.     

Apolipoprotein E is not Related to Memory Abilities at 70 Years of Age

APOE e4-related memory deficits were reported in a normal population aged between 50 and 60 when controlling for general cognitive ability in early adulthood. This extended findings of APOE e4 effects on cognitive ability in 60–80-year-olds to a younger group and confirmed that this effect relates to changes in memory ability with age. The present study tests the association of APOE e4 variation with verbal and spatial memory in a sample of 70-year-olds both adjusted and non-adjusted for childhood and adult general cognitive ability. The 1,013 participants comprise surviving members of the 1947 Scottish Mental Survey resident in the Lothian area of Scotland. They were tested on general cognitive ability at age 11 years and followed up at about age 70 with tests of verbal (immediate and delayed) and spatial memory. General linear models were used to test the association between variation in the APOE polymorphism (e4 presence vs. absence) and memory measures. Of the eight measures tested, Spatial span forward was significantly associated with APOE e4 variation (P = 0.04) when adjusting for IQ, whereas Logical memory immediate was associated with APOE e4 variation (P = 0.04) in the analysis not controlling for IQ. Neither of these tests was significant when a correction for multiple testing was applied. APOE e4 does not influence memory abilities in a normal population of 70-year-olds. Edited by Chandra Reynolds.     

Genetic and Environmental Multidimensionality of Well- and Ill-Being in Middle Aged Twin Men

The goals of the study were to determine the extent to which the underlying structure of different types of well-being was multidimensional and whether well- and ill-being were influenced by similar or different genetic and environmental factors. Participants were 1226 male twins ages 51–60, from the Vietnam Era Twin Study of Aging. Measures included: psychological well-being, Multidimensional Personality Questionnaire Well-Being scale (MPQWB), life satisfaction, self-esteem, and depressive symptoms. A two-orthogonal-factor common pathway model fit the data well. Psychological well-being and self-esteem loaded most strongly on Factor 1, which was highly heritable (h² = .79). Life satisfaction loaded most strongly on Factor 2, which was only moderately heritable (h² = .32). Only MPQWB had measure-specific genetic influences. Depressive symptoms loaded on both factors, and only depressive symptoms had measure-specific common environmental influences. All measures had specific unique environmental influences. Results indicate that well-being is genetically and environmentally multidimensional and that ill-being has partial overlap with both latent factors.     

Genetic Influences on Change in BMI from Middle to Old Age: A 29-Year Follow-up Study of Twin Sisters

Weight gain through middle age is a common phenomenon that increases the risk for different types of metabolic diseases and functional limitations later in life. This study examined genetic and environmental influences on the evolution of body mass index (BMI) in women from middle to old age. BMI was evaluated in 102 monozygotic and 114 dizygotic pairs of twin sisters from the year 1975, when they were 42.6 ± 3.4 years-old, and thereafter in 1981, 1990, 2001 and 2004, in a total 29-year follow-up period. We examined genetic and environmental influences explaining BMI overall level and its rate of change using a latent growth modeling approach. The results showed that mean (±SD) BMI increased from 24.1 ± 3.1 to 28.2 ± 5.1 kg/m² during the 29-year period. The heritability of BMI showed a consistent increment across occasions, from 54% in 1975, to 72% in 2004. Genetic influences accounted for both overall BMI level (60%) and BMI rate of change (64%). Genetic and environmental correlations between BMI level and rate of change were: r_g = 0.40 and r_e = −0.24, respectively. We conclude that in relatively healthy women, genes affecting level of BMI may differ from those affecting change in BMI with age. These results provide a basis for identifying genetic variants for change in BMI. Edited by Danielle Posthuma.     

Differential Genetic Etiology of Reading Difficulties as a Function of IQ: An Update

In order to test the hypothesis that the genetic etiology of reading disability differs as a function of IQ, composite reading performance data from 308 pairs of identical (monozygotic, MZ) twins and 440 pairs of fraternal (dizygotic, DZ) twins (254 same-sex and 186 opposite-sex) in which at least one member of each pair was classified as reading-disabled were subjected to multiple regression analysis (DeFries and Fulker, Behav Genet 15:467–473, 1985; Acta Genet Med Gemellol 37:205–216, 1988). In the total sample, heritability of the group deficit in reading performance (h_g²) was .61 (±.06). However, results of fitting an extended regression model to reading performance and IQ data suggested that the genetic etiology of reading disability differs as a linear function of IQ (p ≤ .04). When the basic regression model was fitted separately to data from twin pairs with Wechsler (Examiner's manual: Wechsler intelligence scale for children—revised, 1974; Examiner's manual: Wechsler adult intelligence scale—revised, 1981) Full Scale IQ scores in the upper and lower 25% of the sample, resulting estimates of h_g² were .75 (±.12) and .50 (±.10), respectively (p ≤ .045). These results suggest that reading difficulties in children with a higher IQ are due substantially to genetic influences and may require intensive remediation efforts.     

Endophenotypes in a Dynamically Connected Brain

We examined the longitudinal genetic architecture of three parameters of functional brain connectivity. One parameter described overall connectivity (synchronization likelihood, SL). The two others were derived from graph theory and described local (clustering coefficient, CC) and global (average path length, L) aspects of connectivity. We measured resting state EEG in 1,438 subjects from four age groups of about 16, 18, 25 and 50 years. Developmental curves for SL and L indicate that connectivity is more random at adolescence and old age, and more structured in middle-aged adulthood. Individual variation in SL and L were moderately to highly heritable at each age (SL: 40–82%; L: 29–63%). Genetic factors underlying these phenotypes overlapped. CC was also heritable (25–49%) but showed no systematic overlap with SL and L. SL, CC, and L in the alpha band showed high phenotypic and genetic stability from 16 to 25 years. Heritability for parameters in the beta band was lower, and less stable across ages, but genetic stability was high. We conclude that the connectivity parameters SL, CC, and L in the alpha band show the hallmarks of a good endophenotype for behavior and developmental disorders.     

Environmental Effects Exceed Genetic Effects on Perceived Intensity and Pleasantness of Several Odors: A Three-Population Twin Study

Human genes encoding odorant receptors have been identified, but the contribution of genetic effects to total variation in specific odor perceptions is largely unknown. We estimated the relative contributions of genetic and environmental effects to variation in the perceived intensity and pleasantness of cinnamon, chocolate, turpentine, and isovaleric acid (sweaty) odors by quantitative genetic modeling of odor rating data from 856 twin individuals (including 83 complete monozygotic and 275 dizygotic twin pairs) aged 10–60 years (44% males and 56% females) from Australia, Denmark, and Finland. Results from fitting univariate models including components for additive genetic (A), shared environmental (C), and non-shared environmental (E) effects to the data implied that non-shared environmental effects account for the most variation in ratings of individual odors while genetic effects play only a minor role. Multivariate independent pathway model revealed a modest but significant common additive genetic component for intensity ratings, explaining 18% of the total variation. The results promote the importance of inter-individual variation in odor exposures and olfactory plasticity to odor perception. Edited by Deborah Finkel.     

Genetic and Environmental Causes of Variation in Trait Resilience in Young People

The aim of this multi-informant twin study was to determine the relative role of genetic and environmental factors in explaining variation in trait resilience in adolescents. Participants were consenting families (N = 2,638 twins in 1,394 families), from seven national cohorts (age 12–18 years, both sexes) of monozygotic and dizygotic twins reared together. Questionnaire data on the adolescents’ Ego-resilience (ER89) was collected from mothers, fathers and twins, and analysed by means of multivariate genetic modelling. Variance in trait resilience was best represented in an ADE common pathways model with sex limitation. Variance in the latent psychometric resilience factor was largely explained by additive genetic factors (77% in boys, 70% in girls), with the remaining variance (23 and 30%) attributable to non-shared environmental factors. Additive genetic sources explained more than 50% of the informant specific variation in mothers and fathers scores. In twins, additive and non-additive genetic factors together explained 40% and non-shared environmental factor the remaining 60% of variation. In the mothers’ scores, the additive genetic effect was larger for boys than for girls. The non-additive genetic factor found in the twins’ self ratings was larger in boys than in girls. The remaining sex differences in the specific factors were small. Trait resilience is largely genetically determined. Estimates based on several informants rather than single informants approaches are recommended.     

Genotype by Environment Interaction in Adolescents’ Cognitive Aptitude

In a replication of Turkheimer, Haley, Waldron, D’Onofrio, Gottesman II (2003, Socioeconomic status modifies heritability of IQ in young children. Psychological Science, 14:623-628), we investigate genotype–environment (G × E) interaction in the cognitive aptitude of 839 twin pairs who completed the National Merit Scholastic Qualifying Test in 1962. Shared environmental influences were stronger for adolescents from poorer homes, while genetic influences were stronger for adolescents from more affluent homes. No significant differences were found between parental income and parental education interaction effects. Results suggest that environmental differences between middle- to upper-class families influence the expression of genetic potential for intelligence, as has previously been suggested by Bronfenbrenner and Ceci’s (1994, Nature-nurture reconceptualized in developmental perspective: a bioecological model Psychological Review, 101:568-586) bioecological model. Edited by Stacey Cherny     

Molecular characterization of hard and soft ticks from Romania by sequences of the internal transcribed spacers of ribosomal DNA

In the present study, four hard tick species and one soft tick species, namely, Dermacentor marginatus, Haemaphysalis punctata, Haemaphysalis parva, Ixodes ricinus, and Dermanyssus gallinae, from south-western Romania were characterized genetically by the first (ITS-1) and second (ITS-2) internal transcribed spacers (ITS) of nuclear ribosomal DNA (rDNA), using a hard tick, Haemaphysalis longicornis, from China for comparative purposes. The ITS rDNA was amplified by polymerase chain reaction (PCR) and sequenced from individual ticks. The lengths of the ITS-1 sequences were 238–1819 bp, and the lengths of ITS-2 were 137–1695 bp, respectively, for all ticks sequenced. While sequence variation within a hard tick species was 0–1.5%, nucleotide differences between hard tick species ranged 2–25.2%, indicating that ITS rDNA sequences provide genetic markers for the differentiation of hard ticks from Romania. Hence, a PCR-linked restriction fragment length polymorphism approach was developed for their unequivocal differentiation based on ITS-1 rDNA. This is the first characterization of ticks from Romania using a genetic approach, which provides the foundation for further studies on ticks in Romania and has implications for studying the population genetic structure of the Romanian ticks and for identification and differentiation of closely related ticks. An erratum to this article can be found at     

Phenotypic and Genetic Analyses of the Wisconsin Card Sort

The present study assessed the factor structure and etiology of traditional perseverative and nonperseverative errors, and six narrowly defined errors that occur during the Wisconsin Card Sorting Task (WCST). A computer-administered version of the WCST, designed to maximize the variance in a nonclinical sample, was used. Phenotypic factor analysis and twin models were used to examine the structure and genetic and environmental etiology in 191 monozygotic and 165 dizygotic adolescent twin pairs. Factor analysis did not support the traditional division of errors into perseverative and nonperseverative errors. Heritability of individual indices was small to moderate (a² = 0.10 – 0.42), with varying significance. Estimates of shared environment (c² = 0.00 – 0.14) were not significant. The best fitting multivariate genetic model had one genetic factor, with specific variance and covariance due to nonshared environmental influences. These results suggest that there are common underlying genetic influences on WCST indices, along with index-specific environmental variance that does not correspond to the traditional division between perseverative and nonperseverative errors.     

Association Between Polymorphisms of the Dopamine Receptor D2 and Catechol-o-Methyl Transferase Genes and Cognitive Function

The dopaminergic neurotransmitter system of the brain is involved in working memory and other cognitive functions. Studies suggest an important role for dopamine synthesis and uptake in modulation of human cognitive processes. We studied the association between polymorphisms in the catechol-o-methyl transferase (COMT) and dopamine receptor D2 (DRD2) genes and general cognitive ability in a secondary analysis of 2091 men and women, aged 55–80 years living in Scotland. General cognitive ability ‘g’ was derived from five cognitive tests of different domains. COMT was not associated with cognitive ability in this population. The DRD2 C:C genotype of rs6277 was associated with decreased general cognitive ability ‘g’ (p = 0.003), and DRD2 rs1800497 heterozygotes had lowest mean general cognitive ability ‘g’ (p = 0.007). There was an indication of a potential interaction between the DRD2 SNPs.