DZ Pairs Research Articles

Brain Electrophysiological Endophenotypes for Externalizing Psychopathology: A Multivariate Approach

Abnormalities in electrophysiological measures of stimulus-evoked brain activity (including the P3 event-related potential (ERP) and its associated delta and theta time-frequency (TF) components), and intrinsic, resting state brain activity (including EEG in the beta frequency band) have each been associated with biological vulnerability to a variety of externalizing (EXT) spectrum disorders, such as substance use disorders, conduct disorder, and antisocial behavior. While each of these individual measures has shown promise as an endophenotype for one or more aspects of EXT, we proposed that the power to identify EXT-related genes may be enhanced by using these measures collectively. Thus, we sought to explore a multivariate approach to identifying electrophysiological endophenotypes related to EXT, using measures identified in the literature as promising individual endophenotypes for EXT. Using data from our large twin sample (634 MZ and 335 DZ, male and female same-sex pairs), and fitting multivariate biometric Cholesky models, we found that these measures (1) were heritable, (2) showed significant phenotypic and genetic correlation with a general vulnerability to EXT (which is itself highly heritable), (3) showed modest phenotypic and genetic correlation with each other, and (4) were sensitive to genetic effects that differed as a function of gender. These relationships suggest that these endophenotypes are likely tapping into neurophysiological processes and genes that are both common across them and unique to each-all of which are relevant to a biological vulnerability to EXT psychopathology.

A relationship between migraine and biliary tract disorders: findings in two Swedish samples of elderly twins

To investigate whether there is a relationship between the clinical occurrence of migraine and biliary tract disorders (BTD) and to study whether there is a genetic influence on such an association. The near lifetime morbidity for migraine and BTD was examined in two Swedish twin-samples: OCTO-Twin (149 MZ and 202 DZ pairs; 234 men, 468 women; 80 years of age or older at inclusion), and the GENDER study (249 unlike-sex DZ-pairs; 70-80 years of age at inclusion). The diagnosis of BTD was established by perusal of medical records from the last twenty years. The diagnosis of migraine was based on iterated questionnaires and personal interviews. The odds ratio (OR) of BTD among OCTO-Twin subjects suffering from migraine was 3.5 (1.9-6.7) in monozygotic pairs and 1.7 (1.0-2.9) in dizygotic pairs The corresponding figures among the GENDER unlike-sex DZ-pairs was 2.7 (1.6-4.5). Migraine was associated with female sex and waist circumference. There is a relationship between the occurrence of migraine and BTD, also when controlling for the fact that both disorders are more frequent in women. The association appears to be partly attributable to genetic influences.

A General Factor of Personality: Evidence from the HEXACO Model and a Measure of Trait Emotional Intelligence

The purpose of the present study was to determine if a general factor of personality (GFP) could be extracted from the six dimensions of the HEXACO model and four factors of trait emotional intelligence. Participants were 1,192 pairs of twins (666 MZ pairs, 526 DZ pairs) between the ages of 19 to 86 years, who completed the Trait Emotional Intelligence Questionnaire - Short Form and the HEXACO Personality Inventory - Revised. Principal components analysis yielded a strong GFP accounting for 33% of the variance, on which all variables with the exception of honesty-humility from the HEXACO showed moderate to large loadings. Behavioral genetic (BG) analyses revealed that individual differences in the GFP were entirely attributable to additive genetic and non-shared environmental factors - results that are in accord with previous BG analyses of a GFP. The present study adds to the body of evidence in support of a heritable GFP but an alternative perspective is also discussed.

Long-Term Leisure Time Physical Activity and Properties of Bone: A Twin Study

Effects of physical activity on bone properties, when controlled for genetic effects, are not fully understood. We aimed to study the association between long-term leisure time physical activity (LTPA) and bone properties using twin pairs known to be discordant for leisure time physical activity for at least 30 yr. Volumetric BMD and geometric properties were measured at the tibia shaft and distal end using pQCT in 16 middle-aged (50-74 yr) same-sex twin pairs (seven monozygotic [MZ] and nine dizygotic [DZ] pairs) selected from a population-based cohort. Paired differences between active and inactive co-twins were studied. Active members of MZ twin pairs had larger cortical bone cross-sectional area (intrapair difference: 8%, p = 0.006), thicker cortex (12%, p = 0.003), and greater moment of inertia (I(max), 20%, p = 0.024) at the tibia shaft than their inactive co-twins. At the distal tibia, trabecular BMD (12%, p = 0.050) and compressive strength index (18%, p = 0.038) were also higher in physically active MZ pair members than their inactive co-twins. The trends were similar, but less consistently so, in DZ pairs as in MZ pairs. Our genetically controlled study design shows that LTPA during adulthood strengthens bones in a site-specific manner, that is, the long bone shaft has a thicker cortex, and thus higher bending strength, whereas the distal bone has higher trabecular density and compressive strength. These results suggest that LTPA has a potential causal role in decreasing the long-term risk of osteoporosis and thus preventing osteoporotic fractures.

Heritability of Risk-Taking in Adolescence: A Longitudinal Twin Study

Adolescents are prone to risk-taking behaviors leading to adverse consequences such as substance abuse, accidents, violence, and victimization. However, little is known about the contribution of genetic and environmental factors to individual differences in the propensity for risk-taking. This study investigated developmental changes, longitudinal stability, and heritability of risk-taking using data from 752 adolescent twins including 169 MZ and 203 DZ pairs. The Balloon Analogue Risk Task (BART), an experimental behavioral measure of risk taking, was administered to the twins at age 12 and then re-administered to a part of this sample at age 14. Risk-taking increased with age, but individual differences showed a significant longitudinal stability. Genetic model fitting showed that at age 12, heritability of risk-taking was modest but significant in both sexes, whereas at age 14, heritability increased to 55% in males and became nonsignificant in females. The findings suggest that propensity for risk-taking as measured by BART can be a useful endophenotype for genetic studies of adolescent externalizing psychopathology, however, the utility of this measure may be limited by sex differences in heritability.

Gene–environment interplay between peer rejection and depressive behavior in children

Genetic risk for depressive behavior may increase the likelihood of exposure to environmental stressors (gene-environment correlation, rGE). By the same token, exposure to environmental stressors may moderate the effect of genes on depressive behavior (gene-environment interaction, GxE). Relating these processes to a peer-related stressor in childhood, the present study examined (1) whether genetic risk for depressive behavior in children is related to higher levels of rejection by the peer group (rGE) and (2) whether peer rejection moderates the effect of genetic factors on children's depressive behavior (GxE). The sample comprised 336 twin pairs (MZ pairs = 196, same-sex DZ pairs = 140) assessed in kindergarten (mean age 72.7 months). Peer acceptance/rejection was measured via peer nominations. Depressive behavior was measured through teacher ratings. Consistent with rGE, a moderate overlap of genetic effects was found between peer acceptance/rejection and depressive behavior. In line with GxE, genetic effects on depressive behavior varied across levels of peer acceptance/rejection. An increased genetic disposition for depressive behavior is related to a higher risk of peer rejection (rGE). However, genes play a lesser role in explaining individual differences in depressive behavior in rejected children than in accepted children (GxE).

Genetic Covariation Between the Author Recognition Test and Reading and Verbal Abilities: What Can We Learn from the Analysis of High Performance?

The Author Recognition Test (ART) measures print exposure and is a unique predictor of phonological and orthographic processes in reading. In a sample of adolescent and young adult twins and siblings (216 MZ/430 DZ pairs, 307 singletons; aged 11-29 years) ART scores were moderately heritable (67%) and correlated with reading and verbal abilities, with genes largely accounting for the covariance. We also examine whether high (and low) (i.e. 1SD above the mean) represents a quantitative extreme of the normal distribution. Heritability for high ART was of similar magnitude to the full sample, but, a specific genetic factor, independent from both low ART performance and high reading ability, accounted for 53-58% of the variance. This suggests a distinct genetic etiology for high ART ability and we speculate that the specific genetic influence is on orthographical processing, a critical factor in developing word recognition skills.

Evidence that self‐reported psychotic experiences represent the transitory developmental expression of genetic liability to psychosis in the general population

It has been suggested that self-reported, common, non-clinical psychotic experiences may represent the transitory developmental expression of distributed genetic risk for psychosis. In a sample of female MZ (176 pairs) and DZ twins (113 pairs), cross-twin, cross-trait analyses were conducted to investigate the association between repeated continuous measures of self-reported psychotic experiences (PE-three measures over 18 months), assessed with the CAPE, in one twin and clinical interview categorical measures of psychotic symptoms (PS), assessed with SCID-I, in the other twin. The results showed that in MZ but not DZ pairs (interaction: chi(2) = 7.9, df = 1, P = 0.005), the cross-twin association between PE and PS was large and significant (standardized effect size: 0.26, 95% CI: 0.10-0.42) and of similar magnitude as the within-twin PE-PS association (standardized effect size: 0.28, 95% CI: 0.10-0.45), demonstrating both PE validity and genetic effects. In addition, the cross-twin association between PE and PS was significantly larger (interaction: chi(2) = 20.3, df = 1, P < 0.0001) for younger MZ twins (standardized effect size: 0.67, 95% CI: 0.44-0.90) than older MZ twins (standardized effect size: -0.05, 95% CI: -0.26 to 0.16), demonstrating developmental effects. This study indicates that self-reported psychotic experiences in the general population may represent the developmental expression of population genetic risk for psychosis.

Effects of 32-Year Leisure Time Physical Activity Discordance in Twin Pairs on Health (TWINACTIVE Study): Aims, Design and Results for Physical Fitness

The physically active lifestyle is associated with low future morbidity and mortality, but the causality between physical activity and health is not always clear. As some inherited biological characteristics and childhood experiences may cause selection bias in observational studies, we sought to take them into account by identifying 16 twin pairs (7 MZ, 9 DZ, mean age 60 years) discordant for leisure time physical activity habits for thirty years. We conducted detailed health-related examinations among these twin pairs. Our main aims were to study the effects of physical activity and genes on fitness and body composition, with special reference to body fat compartments, metabolic syndrome components and related diseases and risk factor levels, status of arteries, structure and function of the heart, bone properties, and muscle and fat tissue-related mechanisms linked to physical activity and chronic disease development. Our physical activity assessments showed that inactive co-twins were on average 8.8 MET hours/day less active than their active co-twins through out their midlife (2.2+/-2.3 vs. 11.0+/-4.1 MET h/day, p< .001). Follow-up fitness tests showed that physically inactive co-twins were less fit than their active co-twins (estimated VO(2peak) 26.4+/-4.9 vs. 32.5+/-5.5 ml/kg/min, p< .001). Similar differences were found in both MZ and DZ pairs. On the basis of earlier epidemiological observations on nonrelated individuals, these physical activity and fitness differences are large enough to cause differences in many mechanisms and risk factors related to the development of chronic diseases and to permit future analyses.

Unipolar depressive disorders have a common genotype

Background: The purpose of this study was to estimate the contribution of genetic, common- and unique environmental factors in the aetiology of unipolar major depression (MD), and to investigate whether the unipolar depressive disorders; MD, atypical depression/depression NOS, dysthymia and depressive adjustment disorder can be viewed as various expressions of an underlying genetic commonality. Methods: A sample consisting of same-sexed mono- and dizygotic twins was drawn from in- and outpatient hospital registers ( N = 303). DSM-III-R criteria were assessed by personal interviews. One hundred and forty-three of the probands fulfilled the criteria for one or another unipolar depressive disorder. Cross-tabulations were used to compare concordance rates for MD and different combinations of MD and other unipolar depressive disorders. Correlations in liability and estimations of the heritability ( h 2) with biometrical model fitting were performed. Results: Concordance rates were higher among MZ- than among DZ pairs for both MD and all the different combinations of MD and other unipolar depressive disorders. Cross-concordance between MD and other unipolar disorders was observed. In all instances, except for the situation when MD was considered alone, the correlations in liability among MZ pairs were more than twice the correlations in liability among DZ pairs. The heritability of MD was 0.42, of MD + atypical depression 0.51, of MD + atypical depression + dysthymia 0.45 and of MD +atypical depression + dysthymia + depressive adjustment disorder 0.46. Limitation: Probands were not sampled from the general population. Most often the same person interviewed both twins in a pair. Conclusion: Unipolar MD is moderately heritable without significant shared family environmental effects. Unipolar depressive disorders taken together are moderately heritable without any detectable shared family environmental effects. The tendency is towards higher heritability estimates for the combined groups compared to MD alone. The study suggests that the disorders in the unipolar depressive spectrum may be different manifestations of the same genetic liability.

The Effect of Assumptions About Parental Assortative Mating and Genotype–Income Correlation on Estimates of Genotype–Environment Interaction in the National Merit Twin Study

In a previous paper (Harden et al. in Behav Gen 37:273-283, 2007) models of genotype-environment interaction were fitted to data from the National Merit Twin Study, resulting in evidence for an interaction: the heritability of National Merit Qualifying Test scores increased at higher levels of family income. The present paper investigates two assumptions made in the previous modeling. These were a lack of resemblance between parents for cognitive skill, and possible correlations between family income and a child's genes because of the contribution of parental genes to both. The assumptions were found not to seriously affect estimates of the interaction effect-heritability still increased with income-but they did make a difference for other parameter estimates from the modeling. One possible explanation of the observed interaction, decreasing levels of assortative mating at higher income levels, was examined and found not to be consistent with other evidence from the study. Another possible explanation, a greater freedom of members of DZ pairs at higher income levels to follow independent interests, remained plausible.

A Further Note on the Validity of Weinberg's Differential Rule

If Weinberg's Differential Rule were valid, it may be used to estimate the proportion of DZ and monozygotic pairs in a sample of twin pairs in respect of which the only available genetic information is their sex combinations. However, across the intervening years, evidence has accumulated to suggest that (in singletons at any rate), p shows appreciable Poisson variation within, and Lexis variation across, sibships. Since this would nullify one of Weinberg's premises, a problem is posed by the finding that, to a close approximation, Weinberg's Rule is nevertheless valid (Fellman & Eriksson, 2007).

Traumatic dental injuries in twins: Is there a genetic risk for dental injuries?

Traumatic dental injuries are a major cause of morbidity among children. Investigation of risk factors of dental trauma is important to take preventive measures in patients at high risk for tooth accidents. The aim of this work was to conduct a pilot study in twins, assessing concordance of suffered traumatic dental and facial injuries. Comparison of concordances between monozygotic and dizygotic twin pairs was performed in order to investigate possible genetically determined accident-proneness. Standardized interviews were conducted focusing on the frequency of orofacial accidents and their circumstances. In addition, the front teeth of all participating twins were clinically examined. A total of 104 twins (52 twin pairs) were surveyed: 31 of the 52 twin pairs were DZ and 21 pairs were MZ. 36 (34.6%) of the 104 interviewees had already suffered from a tooth injury, whereas 53 (51%) of the participants had experienced a facial injury. Investigation of concordances of suffered traumatic dental injuries revealed that 66.7% of the MZ twin pairs and 58.1% of the DZ pairs were concordant. With respect to facial traumata, 52.4% of the MZ pairs compared to 58.1% of the DZ pairs were concordant. Differences between zygosity were not statistically significant (P >/= 0.05). With the present pilot study, no statistical evidence of genetic risk for dental and facial injuries could be displayed. On the contrary, environmental factors seem to dominate in determining an individual's risk for orofacial traumata.

Discordance of prenatal and neonatal brain development in twins

Background Discordance of birth weight has been observed in twin pairs, though little is known about prenatal and early neonatal discordance of head and brain size, and the role that zygosity and chorionicity play in discordances of early brain development in twins. Aims To compare prenatal and neonatal discordances of head size in monozygotic–monochorionic (MZ–MC), monozygotic–dichorionic (MZ–DC), and same-sex dizygotic–dichorionic twin pairs (DZ). Study design Subjects prospectively had ultrasounds at 22 and 32 weeks gestational age, and magnetic resonance imaging (MRI) of the brain MRI after birth. Subjects 88 twin pairs recruited from two university hospital prenatal diagnostic clinics; 22 MZ–MC, 17 MZ–DC, and 49 same-sex DZ pairs. Outcome measures Discordance of head circumference (HC) and weight at 22 weeks, 32 weeks and birth, as well as intracranial volume (ICV) on neonatal MRI. Results There were no group differences in discordance of head circumference and weight on the 22 or 32 week ultrasounds, or at birth. MZ–MC twins tended to have numerically greater discordances of HC and weight. There was a significant group difference in ICV on neonatal MRI (ANOVA, p = 0.0143), with DZ twins having significantly greater discordance than MZ–MC ( p = 0.028) or MZ–DC ( p = 0.0131) twins. Conclusions This study indicates that zygosity and chorionicity do not contribute to significant discordances of head size in late prenatal development. DZ twins do have significantly greater discordances of ICV on neonatal MRI, suggesting a relatively greater genetic influence on brain growth in the first weeks after birth.

Genetic Effects on Bone Loss in Peri- and Postmenopausal Women: A Longitudinal Twin Study

This longitudinal twin study was designed to assess the heritability of bone loss in peri- and postmenopausal women. A sample of 724 female twins was studied. Baseline and repeat BMD measurements were performed. Results of genetic model-fitting analysis indicated genetic effects on bone loss account for approximately 40% of the between-individual variation in bone loss at the lumbar spine, forearm, and whole body. BMD and bone loss are important predictors of fracture risk. Although the heritability of peak BMD is well documented, it is not clear whether bone loss is also under genetic regulation. This study was designed to assess the heritability of bone loss in peri- and postmenopausal women. A sample of 724 female twins (177 monozygotic [MZ] and 185 dizygotic [DZ] pairs), 45-82 yr of age, was studied. Each individual had baseline BMD measurements at the lumbar spine, hip, forearm, and total body by DXA and at least one repeat measure, on average 4.9 yr later. Change in BMD (DeltaBMD) was expressed as percent of gain or loss per year. Intraclass correlation coefficients for DeltaBMD were calculated for MZ and DZ pairs. Genetic model-fitting analysis was conducted to partition the total variance of DeltaBMD into three components: genetic (G), common environment (C), and specific environment, including measurement error (E). The index of heritability was estimated as the ratio of genetic variance over total variance. The mean annual DeltaBMD was -0.37 +/- 1.43% (SD) per year at the lumbar spine, -0.27 +/- 1.32% at the total hip, -0.77 +/- 1.66% at the total forearm, -0.36 +/- 1.56% at the femoral neck, and -0.16 +/- 0.81% at the whole body. Intraclass correlation coefficients were significantly higher in MZ than in DZ twins for all studied parameters, except at the hip sites. Results of genetic model-fitting analysis indicated that the indices of heritability for DeltaBMD were 0.38, 0.49, and 0.44 for the lumbar spine, total forearm, and whole body, respectively. However, the genetic effect on DeltaBMD at all hip sites was not significant. These data suggest that, although genetic effects on bone loss with aging are less pronounced than on peak bone mass, they still account for approximately 40% of the between-individual variation in bone loss for the lumbar spine, total forearm, and whole body in peri- and postmenopausal women. These findings are relevant for studies aimed at identification of genes that are involved in the regulation of bone loss.

Heritability of Body Mass Index: A Comparison Between The Netherlands and Spain

A high body mass index (BMI) is commonly used as an index of overweight and obesity. There is persistent evidence of high heritability for variation in BMI, but the effects of common environment appear inconsistent across different European countries. Our objective was to compare genetic and environmental effects on BMI in a sample of twins from two different European countries with distinct population and cultural backgrounds. We analysed data of adult female twins from the Netherlands Twin Register (222 monozygotic [MZ] and 103 dizygotic [DZ] pairs) and the Murcia Twin Register (Spain; 202 MZ and 235 DZ pairs). BMI was based on self-reported weight and height. Dutch women were taller and heavier, but Spanish women had a significantly higher mean BMI. The age related weight increase was significantly stronger in the Spanish sample. Genetic analyses showed that genetic factors are the main contributors to variation in height, weight, and BMI, within both countries. For height and weight, estimates of genetic variances did not differ, but for height, the estimate for the environmental variance was significantly larger in Spanish women. For BMI, both the genetic and the environmental variance components were larger in Spanish than in Dutch women.

P-163: Genetic and environmental influences on cognitive performances in an elderly Italian twin cohort

The progressive reduction of cognitive ability that emerge with aging is a result of complex interactions of genetic and environmental factors. The nature and the mechanisms of these interactions remain largely unknown. The aim of the present study is to explore, in a sample of twins belonging to the population-based Italian Twin Registry, the relative contributions of genetic and environmental factors in determining inter-individual variation of neurocognitive abilities. Ninety-three twin pairs (mean age ± SD, 67.6 ± 4.7 years; 35 MZ pairs, 58 DZ pairs), resident in Rome city were enrolled. All subjects underwent the following neuropsychological tests: Mini Mental State Examination (MMSE), Attentional matrices, Story Recall test, Raven's Coloured Progressive Matrices (PM 36), Phonological (F-P-L) and Semantic (animals) Verbal Fluency test, Token test, Copying Drawings. Data on socio-demographic characteristics and medical history were also collected. Standard univariate twin modelling based on linear structural equations was used. The model allows for separation of the total phenotypic variance into the following components: additive genetic variance; nonadditive genetic variance; common (shared) environmental variance; unique (unshared) environmental variance. No significant differences between MZ and DZ twins were observed in demographic and social data. The means of each neuropsychological test were very similar for MZ and DZ pairs. However, all the cognitive tests, except for Token, showed a higher correlation in MZ than in DZ pairs, suggesting the presence of moderate to sizeable genetic influences. The best fitting model was a model incorporating additive genetic and unique environmental sources of variance for the following tests: MMSE, Raven, Attentional Matrix, Copying Drawings and Story Recall. For Phonological and Semantic Verbal Fluency, the best explanation of the data was obtained under a model, including nonadditive genetic effects. Number of packs of cigarettes smoked per year was negatively associated (coefficient=−0.0041) with the score of the Phonological Verbal Fluency test. Our results suggest that most of the individual differences in specific cognitive abilities in the elderly show moderately to substantially genetic origin. The environmental factors that are relevant for these abilities appeared to be those causing within-family differences.

Subclinical psychotic experiences and cognitive functioning as a bivariate phenotype for genetic studies in the general population

Objective Cognitive deficits may be vulnerability markers for the development of schizophrenia. This study examined whether cognitive deficits are related to specific dimensions of subclinical psychotic experiences and whether associations between these variables are caused by additive genetic, common environmental and/or individual-specific environmental factors. Method A general population sample of 298 female twin pairs completed the Community Assessment of Psychic Experiences and a neuropsychological test battery. Associations between subclinical positive and negative psychotic dimensions and neuropsychological factors (episodic memory and information processing speed) were examined. Univariate correlation and structural equation analyses were performed to explore the role of genetic and environmental factors in the phenotypes separately. Bivariate correlation and structural equation analyses were applied to examine the causes of association. Results There were significant correlations between information processing speed and both the positive ( r = .11; p < .05) and the negative dimension ( r = .10; p < .05). For the negative dimension and for speed of processing, the data suggested a model that included genetic factors. The observed phenotypic correlation between the negative dimension and information processing speed could be solely explained in terms of additive genetic factors. Although the comparison of the correlations for MZ and DZ pairs did not give a clear indication as to the underlying causes of the association, structural equation modelling suggested that the observed phenotypic correlation between the negative dimension and information processing speed could be solely explained in terms of additive genetic factors. Conclusion Negative symptoms and information processing speed are associated at the subclinical level and this association appears to be influenced by genetic factors exclusively. Bivariate psychosis phenotypes may represent suitable candidates for molecular genetic studies in the general population.

Rated personality and measured intelligence in young twin children

Phenotypic, genetic, and environmental correlations between measured intelligence and caregiver-provided ratings of personality were examined in a sample of 4- to 6-year-old twin children ( N = 680 individuals). Personality ratings were factor analyzed. Five factors were extracted and labelled: agreeableness, extraversion, neuroticism, conscientiousness, and psychoticism. Univariate genetic analyses conducted on the same-sex pairs (101 MZ pairs and 132 same-sex DZ pairs) demonstrated that all of the personality factors had heritable components (range = 31–86%). Performance and full-scale intelligence were also found to have heritable components, but verbal intelligence was better explained by environmental factors. At the phenotypic level, agreeableness and conscientiousness correlated positively with intelligence and neuroticism and psychoticism correlated negatively with intelligence. Multivariate genetic analyses revealed that some of the observed phenotypic correlations could be explained by common genetic factors.

Genetic Analysis of Anger: Genetic Dominance or Competitive Sibling Interaction

The knowledge of the causes and development of anger is still scarce. Previous studies on the sources of variance on Type A Behavior Pattern (TABP) related measures found variable heritability estimates ranging from 0.12 to 0.68, and large differences between MZ and DZ correlations. Some authors considered dominance genetic effects, competitive sibling interaction and sex differences as possible mechanisms to explain the results, but most studies lacked power. The present study uses a large sample of more than 2,500 families, with longitudinal data from MZ and DZ pairs as well as their parents, to disentangle the sources of variance on anger. Model Fitting results showed that the sources of variance differ across sexes. For males 23% of the variance is due to additive genetic effects, and 26% to dominance genetic effects. For females 34% of the variance is due to additive genetic effects, and no dominance effects are found. There was no consistent evidence to confirm the presence of competitive sibling interaction as an alternative explanation for the low correlations in DZ males. The focus of research on the prediction of coronary heart disease (CHD) risk through psychological characteristics has recently changed from the multidimensional TABP to its emotional component: Anger. Understanding the sources of individual differences on anger can help to clarify the mechanisms that link it with CHD and its possible implications for treatment and prevention.