Young Adult Twin Pairs Research Articles

Mental, physical, and social well-being and quality of life in healthy young adult twin pairs discordant and concordant for body mass index.

The relationship between obesity and mental health is complex and is moderated by the level of obesity, age, sex, and social and genetic factors. In the current study, we used a unique co-twin control design, with twin pairs discordant for body mass index (BMI), to control for shared genetic and environmental effects between obesity and several dimensions of mental health. We studied 74 monozygotic (MZ) twin pairs, of whom 36 were BMI-discordant (intra-pair difference in BMI ≥ 3 kg/m2), and 77 dizygotic (DZ) twin pairs (46 BMI-discordant). We assessed subjective health, especially mental health and mental well-being (depression, anxiety, self-esteem, health-related quality of life, life satisfaction, and social well-being) through questionnaires. Heavier MZ co-twins from BMI-discordant pairs had poorer general health (58.8±3.0 vs. 72.4±3.8, P = 0.001, FDR = 0.017 on a scale from 0 to 100 where higher scores indicate more positive results), physical functioning (90.3±1.1 vs. 95.5±2.2, P = 0.024, FDR = 0.122), energy levels (55.6±3.4 vs. 66.6±3.3, P = 0.013, FDR = 0.109), and emotional well-being (65.9±3.2 vs. 75.4±2.9, P = 0.031, FDR = 0.122), as well as a tendency for depressive symptoms (8.4±1.3 vs. 5.6±0.9, P = 0.071, FDR = 0.166) compared to their leaner co-twins. Heavier DZ co-twins had poorer total physical well-being (91.6±1.9 vs. 95.6±1.0, P = 0.035, FDR = 0.356) and more depressive symptoms (4.3±0.9 vs. 2.4±0.5, P = 0.016, FDR = 0.345 on a scale from 0 to 63 where lower scores indicate fewer depressive symptoms) than their leaner co-twins. Association analyses, using all twin pairs, confirmed that higher BMI within pairs linked to general health, physical functioning and depressive symptoms. No association was found between BMI and anxiety, self-esteem, life satisfaction, or social well-being. In conclusion, this study underscores the notable association between elevated BMI and physical well-being and to a lesser extent between elevated BMI and depressive symptoms, while revealing no discernible connections with anxiety, self-esteem, life satisfaction, or social well-being.

Genetic and environmental influence on resting state networks in young male and female adults: a cartographer mapping study.

We propose a unique, minimal assumption, approach based on variance analyses (compared with standard approaches) to investigate genetic influence on individual differences on the functional connectivity of the brain using 65 monozygotic and 65 dizygotic healthy young adult twin pairs' low-frequency oscillation resting state functional Magnetic Resonance Imaging (fMRI) data from the Human Connectome Project. Overall, we found high number of genetically-influenced functional (GIF) connections involving posterior to posterior brain regions (occipital/temporal/parietal) implicated in low-level processes such as vision, perception, motion, categorization, dorsal/ventral stream visuospatial, and long-term memory processes, as well as high number across midline brain regions (cingulate) implicated in attentional processes, and emotional responses to pain. We found low number of GIF connections involving anterior to anterior/posterior brain regions (frontofrontal > frontoparietal, frontotemporal, frontooccipital) implicated in high-level processes such as working memory, reasoning, emotional judgment, language, and action planning. We found very low number of GIF connections involving subcortical/noncortical networks such as basal ganglia, thalamus, brainstem, and cerebellum. In terms of sex-specific individual differences, individual differences in males were more genetically influenced while individual differences in females were more environmentally influenced in terms of the interplay of interactions of Task positive networks (brain regions involved in various task-oriented processes and attending to and interacting with environment), extended Default Mode Network (a central brain hub for various processes such as internal monitoring, rumination, and evaluation of self and others), primary sensorimotor systems (vision, audition, somatosensory, and motor systems), and subcortical/noncortical networks. There were >8.5-19.1 times more GIF connections in males than females. These preliminary (young adult cohort-specific) findings suggest that individual differences in the resting state brain may be more genetically influenced in males and more environmentally influenced in females; furthermore, standard approaches may suggest that it is more substantially nonadditive genetics, rather than additive genetics, which contribute to the differences in sex-specific individual differences based on this young adult (male and female) specific cohort. Finally, considering the preliminary cohort-specific results, based on standard approaches, environmental influences on individual differences may be substantially greater than that of genetics, for either sex, frontally and brain-wide. [Correction added on 10 May 2023, after first online publication: added: functional Magnetic Resonance Imaging. Added: individual differences in, twice. Added statement between furthermore … based on standard approaches.].

Association of lifetime major depressive disorder with enhanced attentional sensitivity measured with P3 response in young adult twins

Major depression is associated with alterations in the auditory P3 event-related potential (ERP). However, the persistence of these abnormalities after recovery from depressive episodes, especially in young adults, is not well known. Furthermore, the potential influence of substance use on this association is poorly understood. Young adult twin pairs (N = 177) from the longitudinal FinnTwin16 study were studied with a psychiatric interview, and P3a and P3b ERPs elicited by task-irrelevant novel sounds and targets, respectively. Dyadic linear mixed-effect models were used to distinguish the effects of lifetime major depressive disorder from familial factors and effects of alcohol problem drinking and tobacco smoking. P3a amplitude was significantly increased and P3b latency decreased, in individuals with a history of lifetime major depression, when controlling the fixed effects of alcohol abuse, tobacco, gender, twins’ birth order, and zygosity. These results suggest that past lifetime major depressive disorder may be associated with enhanced attentional sensitivity.

Patterns and sources of the association between intelligence, party identification, and political orientations

We examined the association between intelligence, party identification, and political orientations using genetically informative data gathered from German twins and their families (n = 9553 individuals including 1524 adolescent and young-adult twin pairs). The results indicated that supporters of the Pirate Party and the Green Party had levels of intelligence that were significantly higher than those of other party identifiers, with the exception of individuals who preferred the Free Democrats. These findings were remarkably similar to results based on British research participants that were reported in a previous study. We also observed a significant, albeit modest, negative correlation between intelligence and political conservatism, and our data indicated that this correlation could be accounted for by common genetic influences on those two variables. The findings of the study were interpreted in light of competing theories on the nature of the relationship between intelligence, party identification, and political orientations.

Eating Behaviors in Healthy Young Adult Twin Pairs Discordant for Body Mass Index

We aimed to study the eating behavioral traits that associate with body mass index (BMI) among BMI-discordant twin pairs. This cross-sectional study examined self-reported eating behaviors in 134 healthy young adult twin pairs (57 monozygotic [MZ] and 77 same-sex dizygotic [DZ]), of whom 29 MZ and 46 DZ pairs were BMI discordant (BMI difference ≥ 3 kg/m2). In both MZ and DZ BMI-discordant pairs, the heavier co-twins reported being less capable of regulating their food intake optimally than their leaner co-twins, mainly due to 'frequent overeating'. Furthermore, the heavier co-twins reported augmented 'disinhibited eating', 'binge-eating scores' and 'body dissatisfaction'. The twins agreed more frequently that the heavier co-twins (rather than the leaner co-twins) ate more food in general, and more fatty food, in particular. No significant behavioral differences emerged in BMI-concordant twin pairs. Overeating - measured by 'frequent overeating', 'disinhibited eating' and 'binge-eating score' - was the main behavioral trait associated with higher BMI, independent of genotype and shared environment.

Genetic Contribution to Variation in Risk Taking: A Functional MRI Twin Study of the Balloon Analogue Risk Task

Excessive risk-taking behaviors have been implicated as a potential endophenotype for substance use disorders and psychopathological gambling. However, the genetic and environmental influences on risk taking and the risk-related brain activations remain unclear. This study investigated the heritability of risk taking and the genetic influence on individual variation in risk-related brain activation. The Balloon Analogue Risk Task was used to assess individuals’ risk-taking behavior. In a sample of 244 pairs of young adult twins, we found that there was a moderate heritability (41%) of risk taking. Using voxel-level analysis, we found a moderate genetic influence on risk-related brain activation. We also found a moderate genetic correlation between risk-taking behavior and risk-related brain activation in the left insula, right striatum, and right superior parietal lobule in the active-choice condition. The present study provides important evidence for the genetic correlation between risk-taking behavior and risk-related brain activation.

NO CLEAR LINK BETWEEN G LOADINGS AND HERITABILITY: A TWIN STUDY FROM KOREA.

A fundamental theoretical question in intelligence research is to what extent the g factor and heritability coefficients of the subtests of an IQ battery are correlated. Five studies from Western countries showed modest to strong positive correlations (range = .43-.77), and six studies from a Japanese meta-analysis showed zero to modest correlations (range = -0.01-0.59). The Western and Japanese studies were compared with a Korean sample of 24 monozygotic and 20 dizygotic young adult twin pairs administered the Korean Wechsler Adult Intelligence Scale-Revised (WAIS-R). A univariate twin model was applied to all subscale scores leading to estimates of heritability for all scales. g loadings were also computed using principal-axis factor analysis. Finally, the correlation between the heritabilities and the g loadings of these scales were computed. The correlations of r = -.15 were not in line with previous studies. It could be that Spearman's hypothesis is less strongly confirmed for Northeast Asians, but it is also possible that the study will be an outlier in a future meta-analysis. More primary research and a meta-analysis of all studies are needed.

Developmental Changes in Genetic and Shared Environmental Contributions to Smoking Initiation and Subsequent Smoking Quantity in Adolescence and Young Adulthood.

Few studies examining the genetic architecture of cigarette smoking have focused on adolescents or examined developmental changes in additive genetic, shared environment, and unique environmental influences on liability to initiate cigarette smoking and quantity of cigarettes smoked. The aim of this study was to add to the literature on liability to initiate and use cigarettes during adolescence using a nationally representative sample. Data for this study came from adolescent and young adult twin pairs (aged 14-33 years) from the National Longitudinal Study of Adolescent to Adult Health. We ran a series of developmental causal-contingent-common pathway models to examine whether additive genetic, shared, and unique environmental influences on liability to the initiation of cigarette use are shared with those on smoking quantity, and whether their contributions change across development. We found evidence for a developmental shift in genetic and shared environmental contributions to cigarette use. Early in adolescence, genetic and environmental influences work independently on liability to cigarette smoking initiation and quantity of cigarettes smoked, but liability to these behaviors becomes correlated as individuals age into young adulthood. These findings provide insight into the causal processes underlying the liability to smoke cigarettes. With age, there is greater overlap in the genetic and environmental factors that influence the initiation of cigarette smoking and quantity of cigarettes smoked.

Sex Differences in Genetic and Environmental Influences on Obsessive–Compulsive Symptoms in South Korean Adolescent and Young Adult Twins

Recent molecular genetic studies provide suggestive evidence for sexual dimorphism in genetics of obsessive-compulsive disorder. However, only a few twin studies have addressed the question of sex differences in genetic and environmental contributions to variation of obsessive-compulsive symptoms. The aim of the present study was to estimate genetic and environmental influences on obsessive-compulsive symptoms in South Korean twins, with a special emphasis on sex difference. In total, 751 adolescent and young adult twin pairs (ages: 13-23 years) completed a Korean version of the 30 items of the Maudsley Obsessional - Compulsive Inventory (MOCI) through a mail survey. A sum of the answers for the 30 items was calculated to represent a total score for obsessive-compulsive symptoms (hereafter, the MOCIT). Males had significantly higher variance of the MOCIT than did females. In males, monozygotic (MZ) twin correlation was significantly higher than dizygotic (DZ) twin correlations (.56 vs. .24), whereas in females, MZ and DZ twin correlations were not significantly different from each other (.39 vs. .36). The general sex-limitation model was applied to the twin data. The results of model-fitting analyses indicated that the unstandardized genetic variance as well as heritability estimate (53% vs. 41%) for the MOCIT was higher in males than in females. However, shared environmental influences did not attain statistical significance perhaps due to insufficient statistical power.

A twin study of the common vulnerability between heightened sensitivity to hypercapnia and panic disorder

For unknown reasons the inhalation of CO(2)-enriched air mixtures evokes acute panic-like symptoms in people with panic disorder and in their unaffected relatives. This study was set to determine whether, and to what extent, CO(2)-induced acute anxiety and panic disorder share the same genetic and environmental determinants. Cholesky structural equation models were used to decompose into genetic and environmental elements the correlation between self-assessed anxiety post-35%CO(2)-65%O(2) inhalation and interview-based DSM-IV lifetime diagnoses of panic disorder in 346 young adult twin pairs of the Norwegian Institute of Health Panel, 12% of whom had been invited to take part into the CO(2) study on the basis of self-reported symptoms of anxiety gathered 4-7 years before the provocation challenge. A full model corrected for the partially selective ascertainment showed that the phenotypic correlation between post-CO(2) anxiety and DSM-IV panic was largely due to additive genetic influences, while shared and unique environmental agents concurred to explain a relatively minor proportion of the correlation between these two traits. According to the best-fitting model the genetic correlation between post-CO(2) anxiety and panic was 0.81 (0.50-0.98); a common genetic factor was sufficient to explain the traits' covariation and a further, specific genetic factor was necessary to account for the residual phenotypic variance. The genetic determinants that lead to overreact to a hypercapnic stimulus coincide at a considerable extent with those that influence liability to naturally occurring panic. Environmental factors provide a modest--or no--contribution to the covariation of CO(2)-provoked anxiety with naturally occurring panic.

The heritability of cluster A personality disorders assessed by both personal interview and questionnaire

Personality disorders (PDs) as assessed by questionnaires and personal interviews are heritable. However, we know neither how much unreliability of measurement impacts on heritability estimates nor whether the genetic and environmental risk factors assessed by these two methods are the same. We wish to know whether the same set of PD vulnerability factors are assessed by these two methods. A total of 3334 young adult twin pairs from the Norwegian Institute of Public Health Twin Panel (NIPHTP) completed a questionnaire containing 91 PD items. One to 6 years later, 1386 of these pairs were interviewed with the Structured Interview for DSM-IV Personality (SIDP-IV). Self-report items predicting interview results were selected by regression. Measurement models were fitted using Mx. In the best-fit models, the latent liabilities to paranoid personality disorder (PPD), schizoid personality disorder (SPD) and schizotypal personality disorder (STPD) were all highly heritable with no evidence of shared environmental effects. For PPD and STPD, only unique environmental effects were specific to the interview measure whereas both environmental and genetic effects were found to be specific to the questionnaire assessment. For SPD, the best-fit model contained genetic and environmental effects specific to both forms of assessment. The latent liabilities to the cluster A PDs are highly heritable but are assessed by current methods with only moderate reliability. The personal interviews assessed the genetic risk for the latent trait with excellent specificity for PPD and STPD and good specificity for SPD. However, for all three PDs, the questionnaires were less specific, also indexing an independent set of genetic risk factors.

Genetic and environmental influences on dimensional representations of DSM-IV cluster C personality disorders: a population-based multivariate twin study

The DSM-IV cluster C Axis II disorders include avoidant (AVPD), dependent (DEPD) and obsessive-compulsive (OCPD) personality disorders. We aimed to estimate the genetic and environmental influences on dimensional representations of these disorders and examine the validity of the cluster C construct by determining to what extent common familial factors influence the individual PDs. PDs were assessed using the Structured Interview for DSM-IV Personality (SIDP-IV) in a sample of 1386 young adult twin pairs from the Norwegian Institute of Public Health Twin Panel (NIPHTP). A single-factor independent pathway multivariate model was applied to the number of endorsed criteria for the three cluster C disorders, using the statistical modeling program Mx. The best-fitting model included genetic and unique environmental factors only, and equated parameters for males and females. Heritability ranged from 27% to 35%. The proportion of genetic variance explained by a common factor was 83, 48 and 15% respectively for AVPD, DEPD and OCPD. Common genetic and environmental factors accounted for 54% and 64% respectively of the variance in AVPD and DEPD but only 11% of the variance in OCPD. Cluster C PDs are moderately heritable. No evidence was found for shared environmental or sex effects. Common genetic and individual environmental factors account for a substantial proportion of the variance in AVPD and DEPD. However, OCPD appears to be largely etiologically distinct from the other two PDs. The results do not support the validity of the DSM-IV cluster C construct in its present form.

Dimensional representations of DSM-IV Cluster A personality disorders in a population-based sample of Norwegian twins: a multivariate study

The 'odd' or 'Cluster A' personality disorders (PDs) - paranoid, schizoid and schizotypal PDs - were created in DSM-III with little empirical foundation. We have examined the relationship between the genetic and environmental risk factors for dimensional representations of these three personality disorders. These personality disorders were assessed using the Structured Interview for DSM-IV Personality (SIDP-IV) in 1386 young adult twin pairs from the Norwegian Institute of Public Health Twin Panel. Using Mx, a single-factor independent pathway twin model was fitted to the number of endorsed criteria for the three disorders. The best-fit model included genetic and unique environmental common factors and genetic and unique environmental effects specific to each personality disorder. Total heritability was modest for these personality disorders and ranged from 21% to 28%. Loadings on the common genetic and unique environmental factors were substantially higher for schizotypal than for paranoid or schizoid PD. The proportion of genetic liability shared with all Cluster A disorders was estimated at 100, 43 and 26% respectively for schizotypal, paranoid and schizoid PDs. In support of the validity of the Cluster A construct, dimensional representations of schizotypal, paranoid and schizoid PD are all modestly heritable and share a portion of their genetic and environmental risk factors. No evidence was found for shared environmental or sex effects for these PDs. Schizotypal PD most closely reflects the genetic and environmental liability common to all three Cluster A disorders. These results should be interpreted in the context of the limited power of this sample.

Illicit psychoactive substance use, abuse and dependence in a population-based sample of Norwegian twins

Prior population-based twin studies from two Anglophonic countries with relatively high rates of drug use - the USA and Australia - suggest that genetic factors contribute substantially to individual differences in the use, abuse and dependence of illicit psychoactive substances. Would these results replicate in Norway, a Nordic country with a low prevalence of illicit drug use? Lifetime use, abuse and dependence of five illicit drug categories (cannabis, stimulants, opiates, cocaine and psychedelics) were assessed at personal interview in 1386 complete young adult twin pairs ascertained from the Norwegian Institute of Public Health Twin Panel. Twin model fitting was performed using the Mx statistical package on three phenotypes: any lifetime use, endorsement of at least one DSM-IV symptom of abuse or dependence, and meeting DSM-IV criteria for abuse or dependence. Significant lifetime use of illicit substances (defined as use 10 or more times) was reported by only 6.4% of the sample. Meaningful analyses were possible for use of any substance and each of the five substances individually, but for symptoms or a diagnosis of abuse/dependence meaningful analyses were possible only for any substance and cannabis. Full twin models uniformly found twin resemblance to be due largely or entirely to genetic factors. Best-fit models for all analyses included only genetic and individual-specific environmental effects with heritability estimates ranging from 58% to 81%. In accord with prior results from the USA and Australia, genetic factors appear to play an important role in the etiology of use and abuse/dependence of illicit drugs in Norway.

Risk factors for asthma in young adults: a co‐twin control study

The liability to asthma is influenced both by genetic and environmental factors. The objective of this study was to identify risk factors for asthma in young adult twin pairs during an 8-year period. From the birth cohorts 1953-1982 of the Danish Twin Registry, 6,090 twin pairs who were initially unaffected with respect to asthma at a nationwide questionnaire-based study in 1994 participated in a similar follow-up study in 2002. Subjects were regarded incident asthma cases when responding affirmatively to the question 'Do you have, or have you ever had asthma'? in 2002. Pairs in which only one twin developed asthma -- discordant pairs -- were identified and conditional logistic regression was applied to detect effects of risk factors. A total of 126 monozygotic (MZ) and 273 dizygotic (DZ) discordant twin pairs were identified. In MZ twins hay fever (OR = 3.16, 95% CI: 1.29-7.73, P = 0.007) and exercise (OR for inactivity = 0.35, 95% CI: 0.13-0.91, P = 0.023) were significantly associated with asthma, whereas in DZ twins, hay fever (OR = 2.44, 95% CI: 1.44-4.13, P = 0.001), eczema (OR = 1.96, 95% CI: 1.02-3.78, P = 0.040), female sex (OR between males and females = 0.54, 95% CI: 0.36-0.80, P = 0.002), and increasing levels of body mass index (BMI; OR per unit = 1.11, 95% CI: 1.02-1.20, P = 0.009) were significant predictors of asthma. Hay fever, eczema, female sex, exercise and increasing levels of BMI were risk factors for asthma in young adults. The different risk profile observed in MZ twins compared with DZ twins may reflect an underlying genetic vulnerability shared between those risk factors and asthma.

Emotional and Behavioral Problems Among Female Twins: An Evaluation of the Equal Environments Assumption

To evaluate the equal environments assumption (EEA) of the twin method for mother-reported symptoms of child and adolescent emotional and behavioral problems. Four emotional and behavioral problem scales (symptoms of separation anxiety disorder, attention-deficit/hyperactivity disorder, oppositional defiant disorder, and conduct disorder) and four environmental similarity measures (sharing friends, sharing classes, dressing alike, and perceived zygosity) were assessed by telephone interviews with the biological mothers of 1,948 female adolescent and young adult twin pairs. The effect of environmental similarity on the magnitude of the monozygotic and dizygotic twin correlations and on the parameter estimates from genetic model-fitting was examined for each symptom scale. The measures of environmental similarity were not strongly or consistently related to the similarity of twins for mother-reported emotional and behavioral problems. On average, controlling for environmental similarity did not substantially affect estimates of genetic and environmental influences. These results lend support for the validity of the EEA and suggest that estimates of genetic and environmental influences obtained from twin studies of mother-reported child and adolescent emotional and behavioral problems are not unduly biased by the greater environmental similarity of monozygotic than dizygotic twins.

Genetic and environmental influences on the size and number of cells in the blood.

The heritabilities of human blood cell characteristics were estimated in a study of 206 pairs of young adult twins, male and female. White cell numbers, indices related to circulating red cell mass (haemoglobin, red cell count, and haematocrit), and platelet numbers and size all appeared to be accounted for by genetic and nonshared environmental influences only. Mean cell volume (of the erythrocytes) appeared to be influenced by environmental factors shared by siblings as well as the other two sources of variation. Correlation between red cell count and haemoglobin is modulated by both genetic and environmental factors, but the negative correlation between red cell numbers and size is due mainly to genetic factors independent of those influencing haemoglobin. A significant negative correlation also exists between platelet numbers and size. In males, alcohol consumption increased mean cell volume, and genetic factors influencing alcohol consumption are partly responsible for the correlation between them.

Relationships in Sleep Characteristics of Identical and Fraternal Twins

We recorded EEGs for 14 identical and 14 fraternal young adult twin pairs during one night of sleep in our laboratory. They were instructed to sleep as long as possible. The extended sleep of the identical and fraternal twins was correlated. The structural measures of sleep of the fraternal twins were not correlated. The onset latencies, awakening measures, stage changes, and rapid eye movement amounts were significantly correlated in the identical twins. The nature of the measures used supports an interpretation of hereditary aspects of sleep length and certain structural components of sleep.