Differences In Twin Pairs Research Articles

Stature and Life-Time Labor Market Outcomes: Accounting for Unobserved Differences

We use twin data matched to register-based individual information on earnings and employment to examine the effect of height on life-time labor market outcomes. The use of twin data allows us to remove otherwise unobserved ability and other differences. The twin pair difference estimates from instrumental variables estimation for genetically identical twins reveal a significant height-wage premium for women but not for men. This result implies that cognitive ability explains the effect of height on life-time earnings for men. Additional findings using capital income as the outcome variable suggest that discrimination against short persons may play a role for women.

Pubertal testosterone predicts mental rotation performance of young adult males

Robust sex differences in some spatial abilities that favor males have raised the question of whether testosterone contributes to those differences. There is some evidence for prenatal organizational effects of testosterone on male-favoring spatial abilities, but not much is known about the role of pubertal testosterone levels on adult cognitive abilities. We studied the association between pubertal testosterone (at age 14) and cognitive performance in young adulthood (at age 21-23), assessing male-favoring, female-favoring, and sex-neutral cognitive domains in a population-based sample of 130 male and 178 female twins. Pubertal testosterone was negatively associated with performance in the Mental Rotation Test in young adult men (r=-.27), while among women no significant associations between testosterone and cognitive measures were detected. The significant association among men remained after controlling for pubertal development. Confirmatory within-family comparisons with one-sided significance testing yielded a negative correlation between twin pair differences in testosterone levels and Mental Rotation Test performances in 35 male twin pairs (r=-.32): the twin brother with higher testosterone performed less well on the Mental Rotation Test. That association was evident in 18 pairs of dizygotic male twin pairs (r=-.42; analysis controlling for shared environmental effects). In contrast, the association of differences was not evident among 17 monozygotic male twin pairs (r=-.07; analysis controlling for shared genetic influences). Results suggest that pubertal testosterone levels are related specifically to male-favoring spatial ability and only among men. Within-family analyses implicated possible shared genetic effects between pubertal testosterone and mental rotation ability.

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.

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 19.8years, SD=1.26, range=18-22years). 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.

No Evidence of Genetic Mediation in the Association Between Birthweight and Academic Performance in 2,413 Danish Adolescent Twin Pairs

Evidence of a positive association between birthweight and IQ has been established in several studies. Analyses of within twin pair differences in birthweight and IQ have been used to shed light on the basis of the association. The strength of this approach is the possibility of controlling for both unmeasured common childhood-environmental factors as well as genetic factors shared by the co-twins. Two twin studies suggest the existence of genetic mediation between birthweight and IQ, that is, common genetic factors influence both fetal growth and IQ in childhood, while two other twin studies find no evidence of such mediation. In the present study we use a large population-based national register study of 2,413 Danish twin-pairs from birth cohorts 1986-1990, of which we have zygosity information on 74%. We perform individual level as well as intra-pair analyses of birthweight and school achievements at age 16. For both sexes we observed a monotonic increase in academic performance with increasing percentiles of birthweight. However, we did not find that this association is due to genetic mediation.

X Chromosome Inactivation Pattern is not Associated With Interindividual Variations in Thyroid Volume: A Study of Euthyroid Danish Female Twins

Ahigher frequency of skewed X chromosome inactivation (XCI) is found in patients with autoimmune thyroid disease (AITD) than in controls. Although goitre is often present in AITD, a recent study failed to show an association between XCI and clinically overt nontoxic goitre. However, the etiology of overt goitre is complex, and the mechanisms influencing thyroid volume may involve fewer factors than the mechanisms underlying overt goitre. In order to examine the impact of XCI on thyroid volume in euthyroid females, we studied whether within cohort (n = 138) and within twin pair (n = 69) differences in XCI are correlated with differences in thyroid volume. XCI was determined by PCR analysis of a polymorphic CAG repeat in the first exon of the androgen receptor gene. Thyroid volume was determined by ultrasound. Neither in the within cohort nor in the within twin pair analysis could we demonstrate a statistically significant association between XCI and thyroid volume: Regression coefficient (beta) = 0.023 (95% confidence interval, -0.062-0.108), p = 0.592 and beta = 0.038 (-0.080-0.156), p = 0.521, respectively. Controlling for potential confounders such as zygosity, age, TSH, smoking habits and use of oral contraceptives did not change the findings. In conclusion, in a sample of euthyroid Danish female twins, we found no evidence of a relationship between XCI pattern and thyroid volume.

Do childhood and adult socioeconomic circumstances influence health and physical function in middle-age?

This study examines the joint and separate contribution of social class in early and adult life to differences in health and physical function in middle-aged men. We use data from the Metropolit project which includes men born in 1953 in Copenhagen and a study of middle-aged Danish twins (MADT). In total 6292 Metropolit participants in a follow-up survey on health in 2004 were included in the study together with 2198 male twins of which 1294 were part of a male twin pair ( N = 647 pairs). Logistic regression was used to investigate the association between social class in early and adult life, respectively and health in midlife, measured as limitations in running 100 m, poor dental status, poor self-rated health, and fatigue. In both datasets, men with low childhood or adult social class had a higher risk of being unable to run 100 m, having poor dental status, having poor self-rated health and fatigue than men from the highest social classes. When childhood and adult social class were mutually adjusted, the estimates for both measures were attenuated. Adjustment for living without a partner, body mass index (BMI) and smoking in midlife, which were also related to the four outcomes, had marginal effects on the estimates for childhood social class, but attenuated the effect of adult social class somewhat. Among male twin pairs discordant on adult social class, the twin in the lowest class seemed to be unable to run 100 m, rate own health poorer and being fatigued more often than the high class co-twin, while there seemed to be no twin pair difference in dental status. This suggests that the associations of adult social class with functional limitations, poor self-rated health and fatigue may partly be due to causal effects related to adult social class exposures, while social class differences in dental status might be consistent with an effect of factors mainly operating early in life.

Intrapair birthweight discordance in twins

Birthweight differences in twin pairs influence their perinatal outcome as well as growth and development. Growth-discordant twins have not been well characterized in our local environment. To determine the frequency of birthweight discordance in live-born twin pairs and document some factors that may influence it. In this cross-sectional study, 104 live-born twin pairs were recruited and their intrapair birthweight differences were determined. Twin pairs whose birthweight difference was 15% or more, using the larger infant as the growth standard, were designated as discordant. The maternal age, parity, and birth order of the heavier twin were noted. The prevalence of birthweight discordance was 28.8% when a 15% cut-off point was used but dropped to 9.6% when a 25% cut-off point was applied. Grand multiparity was associated with a significantly increased risk of delivery of discordant twins. Same-sex twin pairs accounted for 60% of all cases of birthweight discordance. Birthweight discordance is common in twin gestations in Benin City, especially among grand multiparous women.

The Influence of Twin Pair Permutation on Likelihood-Based-Estimates of Genetic Variance That Require Ordering of Twin-Pairs

Results from analyses of twin data that use models assuming a bivariate distribution of twin values will change when twins within pairs are reordered. We examined the effect of twin pair ordering on additive genetic variance estimates and hypothesis tests, from a bivariate normal model, both via simulation and through examination of real twin data. The simulations generated twin data for varying sample sizes and amounts of additive genetic and common environmental variance. The real data sets had sample sizes of 60 or less per zygosity. The results indicate that for moderate or large size studies, the effects of twin pair ordering are unlikely to greatly change the results of the data analysis; but for small studies the results can be sensitive to twin pair ordering. We therefore suggest that methods, not sensitive to within twin-pair differences be compared to the results obtained from twin-pair ordering. Methods not influenced by twin-pair ordering include least squares methods or covariance matrices approaches as described by Carey (2005, Behav Genet 35:667-670) or Guo and Wang (2002, Behav Genet 32:37-49).

Genotype–Environment Interactions: Cognitive Aging and Social Factors

The possibility of genotype-environment interaction for memory performance and change was examined in 150 monozygotic (MZ) twin pairs from the Swedish Adoption Twin Study of Aging (SATSA). We used an MZ twin pair difference approach to examine the possibility that genotype was associated with intrapair variability and thus suggestive of genotype-nonshared environment interactions. Multiple 'variability genes' were found for longitudinal change in a semantic memory task including candidates coding for apolipoprotein E (APOE) and estrogen receptor alpha (ESR1) as well as serotonin candidates (HTR2A and 5HTT). One candidate also related to variability in change in episodic memory (5HTT). Of the significant associations observed, generally results indicated that MZ pairs who carry putative risk alleles were less variable than noncarriers, suggesting that noncarriers may be more sensitive to environmental contexts. We sought to 'contextualize' the possible nonshared environmental influences for found gene-environment (G x E) effects by considering intrapair differences in measured social and stress factors, including social support, life events and depressive symptoms. Results suggested that nonshared environmental influences associated with depressive symptoms may moderate the G x E relationship observed for ESR1 and APOE and longitudinal semantic memory change whereby noncarriers of putative risk alleles may be relatively more sensitive to depressionevoking environmental contexts than carriers of the risk allele. Thus, the contexts that facilitate or reduce depressive symptoms may affect semantic memory resiliency dependent on genotype. Further work ought to consider larger sample sizes as well as consider additional social and contextual factors.

Leptin, insulin, insulin‐like growth factors and their binding proteins in cord serum: insight into fetal growth and discordancy

Singleton infants with intrauterine growth restriction have an adaptive hormonal profile characterized by decreased levels of IGF-1, IGF-2, IGFBP-3 and insulin and elevated levels of IGFBP-1 and IGFBP-2. This study examined the association between cord serum levels of six growth factors and anthropometric features at birth in twins in order to determine the intrauterine growth factor interactions and to characterize the specific hormonal profile of small discordant twins. Prevalent case-control study. Twenty pairs of discordant twins (5 monozygotic, 15 dizygotic) and 20 pairs of concordant twins (6 monozygotic, 14 dizygotic) matched for gestational age. Cord blood levels of IGF-1, IGF-2, IGFBP-1, IGFBP-3, insulin, leptin and anthropometric measurements at birth. Intra- and inter-pair differences and correlation coefficients were calculated, and the data were fitted to multivariate regression models. In both discordant and concordant groups, the smaller twins had a significantly lower level of IGF-1 (P < 0.03) and significantly higher level of IGFBP-1 (P < 0.02) than their larger cotwins. IGFBP-1 was inversely correlated with IGF-1 (P < 0.05). Insulin levels were significantly higher in the smaller discordant than the smaller concordant twins (P < 0.001) and in the larger discordant than the larger concordant twins (P < 0.004). Among the monozygotic twins, the leptin level was significantly higher in the larger discordant than the larger concordant twins (P < 0.025). Percentage birth weight discordancy was statistically correlated with twin-pair differences in IGF-1 and IGFBP-1. Of the six factors studied, IGF-1 appears to be the main indicator of intrauterine growth. Twin discordancy may involve compensatory rather than adaptive mechanisms or a multihormone relative resistance syndrome.

Epigenetic influences may explain dental differences in monozygotic twin pairs

Comparisons between monozygotic (MZ) co-twins have tended to focus on the similarities between their dentitions rather than differences. The aim of this study was to determine the prevalence of discordant expression for simple hypodontia and supernumerary teeth in MZ twin pairs and to explain how phenotypic differences might occur despite their similar genotypes. Records of 278 pairs of MZ twins, including dental casts and radiographs, were examined and the prevalences of discordant expression for missing upper lateral incisors (ULI) or second premolars (PM2), and of mesiodentes, were determined. Zygosities were confirmed by comparisons of blood markers and DNA. There was evidence of at least one missing ULI or PM2 in 24 of the 278 MZ pairs (8.6 per cent), with 21 of these 24 pairs (87.5 per cent) showing discordant expression. Nine of the 278 MZ pairs (3.2 per cent) displayed evidence of mesiodentes, with eight of these nine pairs (88.9 per cent) being discordant. Our findings show that differences in the expression of missing or extra teeth occur often between MZ co-twins whose genetic make-up predisposes them to simple hypodontia or mesiodentes. We postulate that minor variations in epigenetic events during odontogenesis may account for these distinct differences.

Neuroanatomic variation in monozygotic twin pairs discordant for the narrow phenotype for autism.

The broader autism phenotype includes relatives of individuals with autism who display social and language deficits that are qualitatively similar to those of autism but less severe. In previous studies of monozygotic twins discordant for autism, more than 75% of the twins without autism displayed the broader phenotype. Differences in neuroanatomy between discordant monozygotic twins might be associated with the narrow and broader behavioral phenotypes. The authors examined the relationship of twin pair differences in clinical phenotype to differences in neuroanatomic phenotype. The subjects were 16 monozygotic twin pairs between the ages of 5 and 14 years and 16 matched singleton comparison subjects. Seven twin pairs were clinically concordant and nine twin pairs were clinically discordant for strictly defined autism. After magnetic resonance imaging, a semiautomated procedure was applied to images in which the brain tissue was subdivided into neurofunctional regions and segmented into gray, white, and ventricular compartments. Both the concordant and discordant twin pairs exhibited concordance in cerebral gray and white matter volumes. However, only the clinically concordant pairs exhibited concordance in cerebellar gray and white matter volumes. Within the discordant twin pairs, both the twins with autism and their co-twins exhibited frontal, temporal, and occipital white matter volumes that were lower than those of the comparison subjects. These findings support the role and the limits of genetic liability in autism. Continuing to clarify the neuroanatomic pathways in autistic spectrum disorders could illuminate the etiology of autism and, ultimately, contribute to treatments.

Mechanisms for differences in monozygous twins.

Monozygous (MZ) twins are often described as being physically and genetically identical. Clinical determination of zygosity relies on the assumption that any physical differences between a pair of twins imply they are dizygous. Most twin research relies on the assumption that dizygous twins share approximately 50% of the same genes, whereas monozygous twins share 100%. There is, however, increasing evidence to challenge both these assumptions. In this review, we describe a number of intrauterine effects and genetic mechanisms that may result in phenotypic, genotypic, and epigenetic differences between monozygous twins. Newer molecular techniques are resulting in such differences being increasingly commonly recognised. The potential for differences in monozygotic twin pairs is an important consideration for both clinicians and researchers involved in twin work.

Effects of chorion type on neonatal temperament differences in monozygotic twin pairs.

Monozygotic (MZ) twins with a known chorion type were assessed in the neonatal period to determine if placentation was related to twin similarity in early temperament development. The sample included 48 pairs with monochorionic placentas and 29 pairs with dichorionic placentas. The assessment focused on irritability, resistance to soothing, activity while awake, activity during sleep, reactivity, and reinforcement value. There were no differences between the monochorionic and the dichorionic twins in any of the temperament ratings. Chorion type was not related to cotwin similarity on any of the temperament ratings. It was concluded that differences in prenatal environment reflected in placentation type did not affect early temperament for MZ twins.

Birth weight in opposite sex twins as compared to same sex dizygotic twins

The question addressed in the present report is whether the large birth weight differences in dizygotic twin pairs of opposite sex (DZos), especially in ‘male first’ couples — observed by Blickstein and Weissman (Blickstein I, Weissman A. Birth weight discordancy in male-first and female-first pairs of unlike-sexed twins. Am J Obstet Gynecol 1990;162:661–663) and replicated in the present study — can be explained by two general influences on birth weight, viz. sex and birth order, or whether some specific effect on fetal growth has to be assumed that is present only in twin pairs of differing sex. The associated enhanced health risk would hit the female twin (from a male first-female second couple) in the first place. If the hypothesis is correct, then one may expect that birth weight of twins is somehow dependent on the sex of the co-twin. This was studied in 3069 twin pairs born in The Netherlands since the end of 1986. Results show that among DZ twins, birth weight is not affected by the sex of the co-twin. Therefore, birth weight differences in DZos pairs have to be ascribed to the general effects of sex and birth order. There is no effect that is specific to DZos pairs only.

Power and validity of methods to identify variability genes

A variability gene model [Magnus et al., Clin Genet 19:67-70, 1981] hypothesizes that environmental influences on the expression of additive genes for a quantitative trait such as cholesterol are under the control of alleles at a separate nonadditive locus. They suggest identifying such genes using an analysis of variance to compare absolute intrapair monozygotic twin trait differences between the genotypes of the postulated variability locus. However, quantitative traits such as cholesterol often have skewed distributions with a long right tail; what are the effects of such non-normality on the procedure suggested by Magnus et al. [1981]? We show that their method is a special case of the Levene tests, robust tests for variability differences. We introduce a statistical model representing sources of variability in twin pair differences and demonstrate with simulation studies that although the Levene tests have robust Type I error, power is enhanced when nonnormal data are transformed before analysis, and the apparent presence and degree of variability differences are dependent on the scale of analysis. These findings indicate the importance of appropriate transformation of the trait before analysis. Analysis of a well-characterized twin data set illustrates these conclusions.

Twin-Twin Transfusion Syndrome during the 2nd Trimester Is Associated with Small Intertwin Hemoglobin Differences

In four twin pregnancies presenting with acute 2nd-trimester polyhydramnios due to presumed twin-twin transfusion syndrome, the fetal hemoglobin concentration was measured in samples obtained by cordocentesis. In no instance did the twin pair difference in the hemoglobin concentration exceed 2.7 g/dl.

A multivariate twin analysis of within-family environmental influences in infants' social responsiveness

Observational measures of social responsiveness were obtained for twin infants. The structure of within-family environmental influences was assessed by correlating identical twin pair differences for 13 behaviors in seven social settings. Factor analysis of the difference score matrix yielded one interpretable factor. This factor indicated that within-family environmental influences were not completely random but may exert a systematic effect on social behavior in several situations.

Sources of information for discriminating MZ and DZ twins by dermatoglyphic patterns.

It is shown that the within twin-pair difference is the most important source of information in determination of twin zygosity using dermatoglyphic data. Inclusion of a measure of among-pair variability provides added discrimination beyond that found with the within-pair difference alone.