Same-sex Dizygotic Twin Pairs Research Articles

The Genetics of Sexuality and Aggression (GSA) Twin Samples in Finland

The Genetics of Sexuality and Aggression (GSA) project was launched at the Abo Akademi University in Turku, Finland in 2005 and has so far undertaken two major population-based data collections involving twins and siblings of twins. To date, it consists of about 14,000 individuals (including 1,147 informative monozygotic twin pairs, 1,042 informative same-sex dizygotic twin pairs, 741 informative opposite-sex dizygotic twin pairs). Participants have been recruited through the Central Population Registry of Finland and were 18-49 years of age at the time of the data collections. Saliva samples for DNA genotyping (n = 4,278) and testosterone analyses (n = 1,168) were collected in 2006. The primary focus of the data collections has been on sexuality (both sexual functioning and sexual behavior) and aggressive behavior. This paper provides an overview of the data collections as well as an outline of the phenotypes and biological data assembled within the project. A detailed overview of publications can be found at the project's Web site: http://www.cebg.fi/.

The Genetic and Environmental Foundation of the Simple View of Reading in Chinese

The Simple View of Reading (SVR) in Chinese was examined in a genetically sensitive design. A total of 270 pairs of Chinese twins (190 pairs of monozygotic twins and 80 pairs of same-sex dizygotic twins) were tested on Chinese vocabulary and word reading at the mean age 7.8 years and reading comprehension of sentences and passages one year later. Results of behavior-genetic analyses showed that both vocabulary and word reading had significant independent genetic influences on reading comprehension, and the two factors together accounted for most but not all of the genetic influences on reading comprehension. In addition, sentence comprehension had a stronger genetic correlation with word reading while passage comprehension showed a trend of stronger genetic overlap with vocabulary. These findings suggest that the genetic foundation of the SVR in Chinese is largely supported in that language comprehension and decoding are two core skills for reading comprehension in nonalphabetic as well as alphabetic written languages.

Heritability of Performance Deficit Accumulation During Acute Sleep Deprivation in Twins

To determine if the large and highly reproducible interindividual differences in rates of performance deficit accumulation during sleep deprivation, as determined by the number of lapses on a sustained reaction time test, the Psychomotor Vigilance Task (PVT), arise from a heritable trait. Prospective, observational cohort study. Academic medical center. There were 59 monozygotic (mean age 29.2 ± 6.8 [SD] yr; 15 male and 44 female pairs) and 41 dizygotic (mean age 26.6 ± 7.6 yr; 15 male and 26 female pairs) same-sex twin pairs with a normal polysomnogram. Thirty-eight hr of monitored, continuous sleep deprivation. Patients performed the 10-min PVT every 2 hr during the sleep deprivation protocol. The primary outcome was change from baseline in square root transformed total lapses (response time ≥ 500 ms) per trial. Patient-specific linear rates of performance deficit accumulation were separated from circadian effects using multiple linear regression. Using the classic approach to assess heritability, the intraclass correlation coefficients for accumulating deficits resulted in a broad sense heritability (h(2)) estimate of 0.834. The mean within-pair and among-pair heritability estimates determined by analysis of variance-based methods was 0.715. When variance components of mixed-effect multilevel models were estimated by maximum likelihood estimation and used to determine the proportions of phenotypic variance explained by genetic and nongenetic factors, 51.1% (standard error = 8.4%, P < 0.0001) of twin variance was attributed to combined additive and dominance genetic effects. Genetic factors explain a large fraction of interindividual variance among rates of performance deficit accumulations on PVT during sleep deprivation.

Externalizing problems, attention regulation, and household chaos: A longitudinal behavioral genetic study

Previous research documented a robust link between difficulties in self-regulation and development of externalizing problems (i.e., aggression and delinquency). In this study, we examined the longitudinal additive and interactive genetic and environmental covariation underlying this well-established link using a twin design. The sample included 131 pairs of monozygotic twins and 173 pairs of same-sex dizygotic twins who participated in three waves of annual assessment. Mothers and fathers provided reports of externalizing problems. Teacher report and observer rating were used to assess twin's attention regulation. The etiology underlying the link between externalizing problems and attention regulation shifted from a common genetic mechanism to a common environmental mechanism in the transition across middle childhood. Household chaos moderated the genetic variance of and covariance between externalizing problems and attention regulation. The genetic influence on individual differences in both externalizing problems and attention regulation was stronger in more chaotic households. However, higher levels of household chaos attenuated the genetic link between externalizing problems and attention regulation.

A Twin Study of Computer Anxiety in Turkish Adolescents

The present study investigated computer anxiety within a sample of Turkish twins aged 10-18. A total of 185 twin-pairs participated in the study. Of the twins, 64 pairs (34.6 percent) were monozygotic (MZ) and 121 pairs (65.4 percent) were dizygotic (DZ). Of the 121 DZ twins, 54 pairs (44.63 percent) were same-sex twins and 67 pairs (55.37 percent) were opposite-sex twins. Computer anxiety was assessed using Computer Anxiety Rating Scale-Turkish Version (CARS-TV), one of the three main scales of "Measuring Technophobia Instruments" developed by Rosen and Weil. The results of paired t test comparisons showed no significant differences in MZ and same-sex DZ twin-pairs' levels of computer anxiety. On the other hand, a significant difference was found in opposite-sex DZ twin-pairs' level of computer anxiety. Interesting enough, males appeared to be more computer anxious than their female co-twins. In the present study, using Falconer's formula, heritability estimate for computer anxiety was derived from correlations based on MZ and DZ twins' mean scores on CARS-TV. The results showed that 57 percent of the variance in computer anxiety was from genetics and 41.5 percent was from nonshared environmental factors. Shared environmental influence, on the other hand, was very small and negligible. Interpretations of results and potential directions for future research are presented.

A Behavioral-Genetic Study of Alexithymia and its Relationships with Trait Emotional Intelligence

The present study is the first to examine relationships between alexithymia and trait emotional intelligence (trait El or trait emotional self-efficacy) at the phenotypic, genetic, and environmental levels. The study was also conducted to resolve inconsistencies in previous twin studies that have provided estimates of the extent to which genetic and environmental factors contribute to individual differences in alexithymia. Participants were 216 monozygotic and 45 dizygotic same-sex twin pairs who completed the Toronto Alexithymia Scale-20. In a pilot study, a sub-sample of 118 MZ and 27 DZ pairs also completed the Trait Emotional Intelligence Questionnaire. Results demonstrated that a combination of genetic and non-shared environmental influences contribute to individual differences in alexithymia. As expected, alexithymia and trait El were negatively correlated at the phenotypic level. Bivariate behavioral genetic analyses showed that that all but one of these correlations was primarily attributable to correlated genetic factors and secondarily to correlated non-shared environmental factors.

Heritability of Parkinson disease in Swedish twins: a longitudinal study

Previous twin studies report no heritability of Parkinson's disease (PD) based on cross sectional information. Here, we apply a longitudinal design and re-evaluate cross sectional data in the population-based Swedish Twin Registry (STR) using clinical as well as hospital discharge and cause of death diagnoses. In the longitudinal analyses (based on 46,436 individuals), we identified 542 twins with PD and 65 twins with Parkinsonism. Concordance rates for PD were 11% for monozygotic and 4% for same-sexed dizygotic twin pairs, with a heritability estimate of 34%. Concordance rates for PD or parkinsonism were 13% for monozygotic and 5% for same-sexed dizygotic twin pairs, with a heritability estimate of 40%. In the cross sectional analyses (based on 49,814 individuals), we identified 287 twins with PD and 79 twins with parkinsonism. Concordance rates for PD were 4% for monozygotic and same-sexed dizygotic twin pairs and 0 for opposite-sexed twin pairs. Concordance rates for PD or parkinsonism were somewhat higher but the heritability estimate was nonsignificant. Our longitudinal analyses demonstrate that PD and parkinsonism are modestly heritable.

Beyond the Big Five: The Dark Triad and the Supernumerary Personality Inventory

The Dark Triad of personality, comprising Machiavellianism, narcissism, and psychopathy, was investigated in relation to the Supernumerary Personality Inventory (SPI) traits, because both sets of variables are predominantly distinct from the Big Five model of personality. Correlational and principal factor analyses were conducted to assess the relations between the Dark Triad and SPI traits. Multivariate behavioral genetic model-fitting analyses were also conducted to determine the correlated genetic and/or environmental underpinnings of the observed phenotypic correlations. Participants were 358 monozygotic and 98 same-sex dizygotic adult twin pairs from North America. As predicted, results revealed significant correlations between the Dark Triad and most SPI traits, and these correlations were primarily attributable to common genetic and non-shared environmental factors, except in the case of Machiavellianism, where shared environmental effects emerged. Three correlated factors were extracted during joint factor analysis of the Dark Triad and SPI traits, as well as a heritable general factor of personality - results that clarified the structure of the Dark Triad construct. It is concluded that the Dark Triad represents an exploitative and antisocial construct that extends beyond the Big Five model and shares a theoretical space with the SPI traits.

Progression of lumbar disc degeneration over a decade: a heritability study

ObjectivesLumbar disc degeneration (LDD) is prevalent, age-related and contributes to low back pain. Cross-sectional LDD as determined by MRI scan is known to be highly heritable. The authors postulated that...

An investigation into the relationship between soft tissue body composition and bone mineral density in a young adult twin sample

The purpose of this study was to investigate the relationship of fat mass (FM) and lean mass (LM) with bone mineral density (BMD) independent of genetic effects. We also assessed the extent to which genetic and environmental influences explain the associations between these phenotypes. Body composition and BMD were measured using dual-energy X-ray absorptiometry in 57 monozygotic and 92 same-sex dizygotic twin pairs, aged 23 to 31 years, chosen to represent a wide range of intrapair differences in body mass index (BMI; 0 to 15.2 kg/m2). Heritability estimates were adjusted for height and gender. In multiple linear regression analysis, intrapair differences in both FM and LM were independently associated with intrapair differences in BMD at most skeletal sites after adjustment for gender and differences in height. Within monozygotic and dizygotic pairs, LM was a significantly stronger predictor of whole-body BMD than FM (p < .01). Additive genetic factors explained 87% [95% confidence interval (CI) 80%–91%), 81% (95% CI 70%–88%), and 61% (95% CI 41%–75%) of the variation in whole-body BMD, LM, and FM, respectively. Additive genetic factors also accounted for 69% to 88% of the covariance between LM and BMD and for 42% to 72% of the covariance between FM and BMD depending on the skeletal site. The genetic correlation between LM and whole-body BMD (rg = 0.46, 95% CI 0.32–0.58) was greater than that of FM and whole-body BMD (rg = 0.25, 95% CI 0.05–0.42). In conclusion, our data indicate that peak BMD is influenced by acquired body weight as well as genetic factors. In young adulthood, LM and BMD may have more genes in common than do FM and BMD. © 2011 American Society for Bone and Mineral Research.

The Structure of the Genetic and Environmental Influences on Mental Well-Being

We sought to investigate the structure of the genetic and environmental influences on 3 measures of mental well-being. Analyses focused on the subsample of 349 monozygotic and 321 dizygotic same-sex twin pairs from a nationally representative sample of twins who completed self-report measures of emotional, psychological, and social well-being. The best-fit model contained a common pathway to all 3 measures of well-being, no shared environmental effects, and 1 set of parameters for men and women. Heritability for the latent "mental well-being" factor was high (72%) and best indexed by psychological well-being. Moderate trait-specific genetic effects were seen for emotional and social well-being. Nonshared environmental effects for all measures were mostly trait specific. Genetic influences on the measures of mental well-being reflect a single, highly heritable genetic factor, although some trait-specific genetic influences were seen for emotional and social well-being. Moderate proportions of environmental influences were also shared, but the majority of unique environment was trait-specific.

Does Educational Status Impact Adult Mortality in Denmark? A Twin Approach

To disentangle an independent effect of educational status on mortality risk from direct and indirect selection mechanisms, the authors used a discordant twin pair design, which allowed them to isolate the effect of education by means of adjustment for genetic and environmental confounding per design. The study is based on data from the Danish Twin Registry and Statistics Denmark. Using Cox regression, they estimated hazard ratios for mortality according to the highest attained education among 5,260 monozygotic and 11,088 dizygotic same-sex twin pairs born during 1921–1950 and followed during 1980–2008. Both standard cohort and intrapair analyses were conducted separately for zygosity, gender, and birth cohort. Educational differences in mortality were demonstrated in the standard cohort analyses but attenuated in the intrapair analyses in all subgroups but men born during 1921–1935, and no effect modification by zygosity was observed. Hence, the results are most compatible with an effect of early family environment in explaining the educational inequality in mortality. However, large educational differences were still reflected in mortality risk differences within twin pairs, thus supporting some degree of independent effect of education. In addition, the effect of education may be more pronounced in older cohorts of Danish men.

Improving the Accuracy of Self-Reports on Diet and Physical Exercise: The Co-Twin Control Method

The objective was to examine the association between several obesity-related nongenetic behaviors and body mass index (BMI) and waist circumference (WC) in young adult twins using reports from both twins on their similarities and differences. A total of 713 monozygotic (MZ) and 698 same-sex dizygotic (DZ) twin pairs aged 22-28 years filled in structured questionnaires to compare their eating, physical activity and dieting behavior with their co-twin's behavior, and to report their own eating and exercise habits. In both MZ and DZ pairs, the co-twins for whom both twin pair members concordantly answered that this twin eats more, snacks more, eats more fatty foods and sweet and fatty delicacies, chooses less healthy foods, eats faster and exercises less, had significantly higher BMIs (0.6-2.9 kg/m(2)) and WCs (1.5-7.5 cm). Multivariate regression analysis identified co-twin differences in the amount of food consumed as the strongest independent predictor of intrapair differences in BMI (beta = 0.63 and 1.21, for MZ and DZ, respectively, p < .001) and WC (beta = 1.52 and 3.53, for MZ and DZ, respectively, p < .001). Higher leisure-time physical activity and healthier dietary choices clustered in the same subjects. The measurement of habitual dietary intake and physical activity has previously relied on subjective self-reports that are prone to misreporting. By using comparative measures within twin pairs we found that the amount of food consumed is the major contributor to obesity independent of genetic predisposition.

Familial Factors Do not Confound the Association Between Birth Weight and Childhood Asthma

Studies have found associations between low birth weight and asthma. However, this association could be due to familial confounding. Our objective was to investigate whether fetal growth and birth weight affect the risk of asthma in childhood, controlling for gestational age (GA), and shared (familial) environment and genetic factors. Information on asthma, zygosity, birth characteristics, and potential confounders was collected for all 9- and 12-year-old twins through the Swedish Twin Register and Medical Birth Register. To obtain an overall effect of birth weight on risk of asthma, we performed cohort analyses on all twins (N = 10918). To address genetic and shared environmental confounding, we performed a co-twin control analysis by using the 157 monozygotic and 289 dizygotic same-sex twin pairs who were discordant for asthma. The overall rate of asthma ever was 13.7%. In the cohort analysis, the adjusted odds ratio (OR) for asthma in relation to a 1000-g decrease in birth weight was 1.57 (95% confidence interval [CI]: 1.38-1.79), and for each reduced gestational week the OR was 1.10 (95% CI: 1.07-1.13). In the co-twin control analyses, a 1000-g decrease in birth weight corresponded to an OR of 1.25 (95% CI: 0.74-2.10) for dizygotic same-sex twins and 2.42 (95% CI: 1.00-5.88) for monozygotic twins. There is an association between fetal growth and childhood asthma that is independent of GA and shared (familial) environment and genetic factors, which indicates that fetal growth restriction affects lung development, supporting additional studies on the early metabolic and physiologic mechanisms of childhood asthma.

The genetic basis of entrepreneurship: Effects of gender and personality

Extending previous research on the genetic underpinnings of entrepreneurship, we investigate gender differences in the genetic influences on the tendency of people to become entrepreneurs. We also examined two mediating variables through which genetic factors may impact this tendency: extraversion and neuroticism. Based on 1285 pairs of identical twins (449 male and 836 female pairs) and 849 pairs of same-sex fraternal twins (283 male and 566 female pairs), we found that females have a strong genetic influence and zero shared-environmental influences on their tendency to become entrepreneurs. In contrast, males show zero genetic influence, but a large shared-environmental influence. Extraversion and neuroticism mediate the genetic influences on women’s tendency to become entrepreneurs, whereas extraversion mediates shared-environmental influences on men’s tendency to become entrepreneurs. We discuss this sharp difference in genetic influences on entrepreneurship across gender groups and highlight the different challenges that men and women face in their entrepreneurial endeavors.

The nature of individual differences in EEG parameters during homogeneous visual field in 5- and 6-year-old twins: A pilot study

To assess the genetic and environmental influences on the inter-individual variability of spectral EEG parameters under two conditions of reduced visual input in preschoolers. EEG was recorded in 21 monozygotic and 20 same-sex dizygotic twin pairs aged 5-6 years under the Closed eyes and Darkness conditions. The contributions of genetic and environmental influences to individual EEG parameters were estimated using structural equation modeling. The alpha rhythm parameters - both spectral amplitude and frequency - were influenced mainly by genetic factors. Theta spectral amplitude variance was presumably accounted for by both genetic and shared environmental factors. The difference in the nature of population variance between EEG alpha and theta band spectral amplitudes was reproduced for two identical functional loads: Closed eyes and Darkness. The majority of EEG spectral parameters in preschool children are determined by genetic factors. Tentative finding on systematic environmental influences on theta spectral amplitude under the condition of reduced visual input may point to the role of experience in the individual differences in the functioning of theta response system. The findings contribute to the scanty knowledge about the heritability of EEG rhythms in preschool children.

Mapping the regional influence of genetics on brain structure variability — A Tensor-Based Morphometry study

Genetic and environmental factors influence brain structure and function profoundly. The search for heritable anatomical features and their influencing genes would be accelerated with detailed 3D maps showing the degree to which brain morphometry is genetically determined. As part of an MRI study that will scan 1150 twins, we applied Tensor-Based Morphometry to compute morphometric differences in 23 pairs of identical twins and 23 pairs of same-sex fraternal twins (mean age: 23.8 ± 1.8 SD years). All 92 twins' 3D brain MRI scans were nonlinearly registered to a common space using a Riemannian fluid-based warping approach to compute volumetric differences across subjects. A multi-template method was used to improve volume quantification. Vector fields driving each subject's anatomy onto the common template were analyzed to create maps of local volumetric excesses and deficits relative to the standard template. Using a new structural equation modeling method, we computed the voxelwise proportion of variance in volumes attributable to additive (A) or dominant (D) genetic factors versus shared environmental (C) or unique environmental factors (E). The method was also applied to various anatomical regions of interest (ROIs). As hypothesized, the overall volumes of the brain, basal ganglia, thalamus, and each lobe were under strong genetic control; local white matter volumes were mostly controlled by common environment. After adjusting for individual differences in overall brain scale, genetic influences were still relatively high in the corpus callosum and in early-maturing brain regions such as the occipital lobes, while environmental influences were greater in frontal brain regions that have a more protracted maturational time-course.

A Behavioral Genetic Study of the Dark Triad of Personality and Moral Development

The present study is the first behavioral genetic investigation of relationships between the Dark Triad of personality--Machiavellianism, narcissism, and subclinical psychopathy--and moral development. Participants were 154 monozygotic twin pairs and 82 same-sex dizygotic twin pairs. Higher scores on Machiavellianism and psychopathy were positively correlated with low levels of moral development; high psychopathy scores also correlated negatively with high levels of moral development. Individual differences in lower levels of moral development were attributable to genetic and nonshared environmental factors but, very interestingly, individual differences in the highest levels of moral development showed no genetic basis but were entirely attributable to shared and nonshared environmental factors. Finally, correlations between the Dark Triad and moral development variables showed no genetic basis while correlations among the moral development variables were variously attributable to correlated genetic and correlated environmental factors.

The ABCs of math: A genetic analysis of mathematics and its links with reading ability and general cognitive ability.

The goal of this first major report from the Western Reserve Reading Project Math component is to explore the etiology of the relationship among tester-administered measures of mathematics ability, reading ability, and general cognitive ability. Data are available on 314 pairs of monozygotic and same-sex dizygotic twins analyzed across 5 waves of assessment. Univariate analyses provide a range of estimates of genetic (h(2) = .00 -.63) and shared (c(2) = .15-.52) environmental influences across math calculation, fluency, and problem solving measures. Multivariate analyses indicate genetic overlap between math problem solving with general cognitive ability and reading decoding, whereas math fluency shares significant genetic overlap with reading fluency and general cognitive ability. Further, math fluency has unique genetic influences. In general, math ability has shared environmental overlap with general cognitive ability and decoding. These results indicate that aspects of math that include problem solving have different genetic and environmental influences than math calculation. Moreover, math fluency, a timed measure of calculation, is the only measured math ability with unique genetic influences.

Geneic and Environmental Influences on Temperament in Adolescence

This study, which is a part of a Moscow longitudinal twin project, aims to explore genetic and environmental contributions to inter-individual variability of temperamental traits in adolescence on the basis of a Russian sample. 85 monozygotic (MZ) and 64 same-sex dizygotic (DZ) twin pairs aged 12 – 14 years completed the children version of Rusalov Structure of Temperament Questionnaire (C-STQ). The results of model-fitting analyses indicate considerable hereditary determination of individual differences in 3 out of the 8 C-STQ dimensions - social tempo, objectrelated emotional sensitivity, and social emotional sensitivity. Non-shared environmental effects explained the rest of the total variance in these dimensions. Individual differences in the other STQ dimensions were due to environmental factors.